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donald.lemay
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PostPosted: Sun Aug 08, 2021 11:19 am    Post subject: Re: Wideband Results Reply with quote

That’s encouraging news.
Thanks for the reply
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PostPosted: Sun Aug 08, 2021 11:24 am    Post subject: Re: Wideband Results Reply with quote

donald.lemay wrote:
Is anyone getting good readings with vintage speed exhaust- reading through I get mixed reviews. Not gonna go down this rabbit hole if not. Kinda bummed if not. Not looking to super dialed in just want to see where I’m at and adjust accordingly.

From what I read, 4 into 1 header, is still the best.

Get 4 O2 sensors setup wideband, if you really what correct results with VS exhaust.
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Clatter
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PostPosted: Fri Sep 03, 2021 2:14 pm    Post subject: Re: Wideband Results Reply with quote

Just stopping back in here again for the unpteenth time..

What a great thread.
Big thanks to John for taking so much time to answer questions (over and over.. Laughing )

Whenever i do a set of carbs it's great because the answer is in here somewhere for sure.

Trawling again i found the link to Mark Harney's great old site expired and broken now,
So i used the Wayback machine and harvested his writing to copy here for posterity.
This great info needs to be saved.
Let me know if I'm violating any copyrights.

He really puts transition tuning down very well: Cool




IDF Transition and you...

Sometimes you can use different main and air jet combinations to get your transition straight, and to help with issues related to WFO mixture problems. For instance.. suppose you had some problems with weak transition, and you were running 11mm floats, and you felt pretty sure that the floats were dead on, and that fuel pressure and delivery were behaving. What can you do to make the mains come in sooner to meet the idle at the right time?

Look down the throat of the carb at the auxiliary venturi. There is a tube with a nozzle in it, and there is a port that feeds the nozzle. The nose end of that port is further out at the top, and two things are happening.. one is that the difference in pressure of the air flowing down through there (the inherent drop due to air acceleration) PLUS the fact that the nose end is further out at the top, makes the pressure at the port opening lower as you increase air velocity through the carb throat, right? Right. They're designed so that when the relative amount of low pressure area around the idle circuit starts to taper off, the port is developing enough vacuum under the nose end to start to suck emulsified fuel up the well and draw it into the throat of the carb. When sufficient vacuum work happens, the emulsified fuel mixture will start to flow out of the port and into the throat.

So what's the deal with the fuel in the well, the emulsion tubes, and the jet combinations? I'm not going to go into a lot of detail about different emulsion tubes. It suffices to say that the emulsion tube's job is to control the supply of fuel in a way that makes sense for what the engine needs under different conditions. There are constants, like the throttle plate size, the geometry of the carburetor body, etc., and there are variables, each of which serves distinct purposes. The variables in question are emulsion tubes, float height, and jet sizes. On dual IDFs, F11 is generally the solution for gas, other tubes can work better for other fuels. On single IDFs, F7 is generally best. On DRLAs, 9146.2 are the popular tube, as it does the best for our application.
Float height is important, period. It is important because it sets the height of the fuel in the float bowl, and hence in the jet wells. This level is critical for the things that happen with the emulsion tubes. They must happen at the right time, and there is a narrow window of float heights in which that works best.

Float height is important as a first thing to look at in getting good transition. Don't be fooled into thinking that raising the floats will solve all your problems. It might, depending on where your floats are currently set. If they are too low, it will be weak in transition and there will not be a lot you can do to fix it except set the floats right. The correct height for IDFs is 10-11mm and the correct height for DRLAs is 5-6mm, in both cases as measured from the cover to the furthest edge of the float assembly.

Too high is a bad thing too.. you start to defeat the purposes of the emulsion tubes, may even delay transition, or cause a lot of extra fuel spillover. I would never recommend to someone that they raise the float level to less than about 9mm.

How does transition actually start? Being familiar with the anatomy of IDF or DRLA carbs helps with understanding this. You have an idle circuit that is comprised of a mixture screw port, and a set of progression ports along the wall of the barrel. They are all exposed by the time you have pressed the gas pedal a small fraction of it's total travel. This gives you a smooth low speed operation and happens while the throttle is open enough to rush air past the progression ports, causing an amount of fuel to be delivered to the engine. The idle jet size determines this mixture strength once you crack the throttle, until the point that you have exposed all the progression ports.

By the time you have exposed all the ports, you have reached a throttle position that is open enough that two things are starting to occur:

1. When the throttle plates are just opening.. the majority of the air flowing past them is passing by the walls of the cylinder. The first progression port is exposed in a situation where a lot of air is flowing right by the progression hole, and the resulting turbulence pulls fuel from the ports. The more you open the throttle, the more ports are exposed, BUT, at these angles, more and more of the air is passing far away from the port holes. This means that more air is now doing less work to the progression ports to pull fuel out. Somewhere in this effect, you will no longer be able to support the needed air-fuel mixture on the idle circuits, so you need to get more fuel some other way.

2. As the plates open more and more, you increase the airflow through the carburetor, and the increased rate of airflow eventually reaches a point that it does substantial work on the venturi. The venturi is basically a mild restrictor, and the result is that as air is pulled down with a specific amount of force, the air will be accelerated through the venturi. As this happens, the pressure of the air develops a gradient along the length of the venturi.. such that there is the relative high pressure region (above the venturi) and a relative low pressure region (typically somewhere right under (downstream of) the minimum diameter region of the venturi. Note the height position of the auxiliary venturi tube. The top of it is above any region that would have a venturi effect, and the bottom edge of it is close the the minimum diameter region of the venturi. The auxiliary venturi experiences a net flow of air through it, as a result of the inside of the tube being isolated from the area where the pressure gradient is changing the most. The pressure difference in turn pulls a suction on the port in the auxiliary venturi, sucking fuel out.

What we hope for is that the main circuit is there to cover the area where the idle circuits are starting to taper off in effectiveness.

If you pull out a jet stack and shine a light down the hole (i.e. the fuel wells), you will see gas in there. If the float bowl is full, that fuel should be about 4mm below the port in the jet well that leads out to the auxiliary venturi. That gas is at the same height as the gas that is sitting in the float bowl. It takes a certain amount of work to draw the fuel from it's resting level in the well to the height of the aux vent port. No matter what main jets you have in there within reason, the air jets, vent size, engine speed and the throttle position largely control when the main circuit wants to come on, for a given float height.

The air jets, main jets, and emulsion tubes form a system that introduces air into the fuel in the jet well. Liquids have a surface tension property, and one of the effects that can be observed is one that causes the liquid to attach and pull itself up along the walls of a container. If you've ever put water in a test tube and read the quantity of the liquid by way of the meniscus, you know that the liquid does not sit level in the tube. This is also true in the jet wells. But the introduction of an inner tube inside the jet well (emulsion tube) can make the fuel leech up between the emulsion tube and the jet well by about 2mm or so. The fuel that is resting in this area is pretty resistant to going anywhere due to the same surface tension that has drawn it up into the area. But it's closer to the aux vent port when the tube is in there, and so half the work is done. The other half of the work has be be done ON the fuel, by the auxiliary venturi's inlet port.

Since the fuel is resistant to being pulled up any higher than the surface tension has affected, we have to do something that will "mobilize" the fuel. The early action of the auxiliary venturi's inlet port isn't enough to physically do the work on the fuel that is necessary to pull it up the well and into the port.

AHA! Well, news flash...the fuel is mobilized by way of the air jet and holes in the emulsion tube. The air drawn down through the jet stacks, into the emulsion tubes is used as a control to mobilize and to put on the brakes for fuel consumption through the full range of the main circuit. The AIR jet size affects how easily and how much air is fed to the emulsion tube.

When the fuel mixture STARTS flowing at low velocity (when the main circuit is just kicking in) the amount of flow through the main jet does not really depend a whole lot on the size of the jet.. but as there is a higher flow demand, it does matter. As the flow rate goes up, relieving pressure difference (fighting for equilibrium) in the port becomes a contribution between the air jet and the fuel jet. Their sizes RELATIVE to each other will start to matter more as the flow increases. But when the mains JUST start to kick in, the amount of vacuum seen at the port is the same, no matter what jets you have in there, until flow increases and the dynamics of that duo of jets really starts working.

The fuel level in the well, in conjunction with what emulsion tubes you use, will control transition. If you are having problems getting transition to happen correctly, you can tune it out by changing the air jets until the transition starts to smooth out. Then you can adjust your mains to keep the right mixture on higher RPM.

In order to bring the mains in sooner you need a LARGER air jet. Seems sort of backwards doesn't it? Well I learned my lesson on this the hard way. Why does it work this way instead of sucking more fuel when you reduce the air jet? Because the fuel that is reaching the inlet to the secondary venturi is fuel that has been introduced into the two phase flow (emulsified). The larger air jet pulls air down into the fuel well easier and so it takes less to emulsify the fuel, break the surface tension, and pull it into the inlet. Prove it to yourself. Put smaller air jets in there and watch your transition get weak. Put larger ones in and watch it smooth out.

What you want is the smallest air jet that will allow you to get transition with no dead spot. It's more of a struggle with larger venturis due to net airflow through the 4.5mm aux vent compared to the amount of air that goes through the whole main venturi. With larger vents and especially with larger butterflies, you will start to move enough air, far enough away from the progression ports, that the idle circuit will die out before a lot of velocity and vacuum reaches the aux vent horn, so you need to make it more sensitive. With larger air jets, you can do that.

There is a point at which an emulsion tube swap starts to make sense too. With a 40 vent on a 48 IDF, it's a good time to put in an F2 emulsion tube in place of an F11. With the F2, your mains and airs will be a little smaller to get similar effects.

Differences in emulsion tubes will be discussed more as I have time to write about them. They don't seem to be widely discussed in books, and the diagrams require some understanding about what's happening to interpret. Even a lot of Weber gurus don't bother trying to understand the dynamics.. just knowing when to change to a different one depending on what's going on seems sufficient for most of us.

The other thing you can do to help is raise the float levels but only do it a millimeter at a time, or even less. You want the floats about as close to 11mm area as you can get them and still have good transition. As the float levels increase, the distance from the fuel to the aux vent port will decrease, and less work will be required to get it up and out the port. This means transition can come earlier. But there is a limit and that limit is dependent on tolerances and carburetor application. A lot of buggy guys keep the floats lower to keep slosh down, and just run fat ass idle jets and main jets. Some just deal with lean transition as a result, and some deal with some richness under certain conditions.

So you have a few things at your disposal to do your tuning.. the problem here is that now that you have the idea you need a way to read your results. Save the pennies and get a wideband air-fuel meter. It's worth it. Once you have it there is no looking back. You'll have your carb tops off so many times your hardware will all shine like new. You'll have your jet stacks out so many times you can do a jet change in 2 minutes or less. It's annoying but at the same time, fun.
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Clatter
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PostPosted: Fri Sep 03, 2021 2:28 pm    Post subject: Re: Wideband Results Reply with quote

More wisdom from Mark Harney;
Sure do wish he was still active in the hobby..

Notice he sets LBI a bit differently than John; More of an LBI LBI.. Wink



Tuning Dual Weber IDF Carbs on the VW Type 1 Engine

A fact - Weber carburetors never come out of the box clean or jetted perfectly for your engine. If you order a set, they may come to you in the ballpark, but every engine is different, even two engines built with the same parts. You're going to be needing some different jet sizes. No way around it, if you want to get it right. Buying jets is just part of it and you have to be ok with that if you want to go to the trouble to tune things well. Jets are expensive, so if you know someone that has a stockpile of them to lend you, get them and start tuning away. If not, you'll end up with a few sets that you can probably sell off for close to what you have in them.

"Ok, well, I'm smart.. and I am handy with the drill bits.." Well, good for you, sport. For the most part, stick to correctly sized and calibrated idle jets. The holes are SMALL, and small defects in your drillings will add up to big discrepancies in fuel delivery. If you want to get handy with the main or air correction jets, go for it, they're much bigger and will not suffer so badly because you sneezed.

First things first. If you do not have an understanding of the otto cycle, go read about it so you know that what the engine does with intake and exhaust are not separate events from each other. The exhaust cycle contributes to the intake cycle. Given this fact and the carburetor's dependency on the health of the otto cycle, you must make sure that your engine is healthy before you spend any time tuning your carburetors.

Your engine's health - Are your valves correctly adjusted? Is your timing set correctly? Are you SURE? If not, find out. If you don't know what the right timing is, FIND OUT. Don't guess.

Are your carbs CLEAN? Are your floats set correctly? Clean the carbs and adjust the floats. Here is a cleaning procedure that should clean out most carbs, especially new or fairly new ones. Older ones may need more attention. If you slack on this step, the rest will be meaningless and frustrating.

After you adjust the floats, you need to make sure your fuel pressure is about 3 to 3.5 psi. If it's too high or too low you will have poor results. Don't ASSUME anything about the fuel pressure. A stock pump under the wrong conditions can supply 10 psi of fuel pressure. I use an internally regulated rotary pump that runs about 3.5 psi. If your mechanical pump is pumping too much pressure you can add gaskets under it to get it right, or you can put a regulator in place. You also have to make sure you have the right pump and fuel pump rod combination. That is beyond the scope of this article. If you suspect there is a problem, deal with it and come back to this. I like the rotary pumps because you have instant fuel pressure, and your float bowls will be filled properly when you turn the key on.

Venturi selection - For engines that rev pretty high (6000-7000 rpm), you are best off choosing vents that are about 2 to 4mm smaller than your intake valve size.

For engines that are set up for torque, like bus engines, and other torque-happy applications, you can go a little smaller on the vents and get better low end response. For these, I'd recommend 4 to 6mm smaller than your intake valve size.

So what happens if your vents are too big? You'll have reduced air-speed at low RPMs, which makes tuning harder. Air speed is one of the things that tells your different fuel circuits when to start to come on. It will be soggy down low.

So what happens if your vents are too small? The undersized vents will restrict your engine's ability to breathe at higher RPMs.

How critical is it? Not terribly, but if you're way off it will matter, and you will feel it. Putting 40 vents in a set of 44 IDFs with 40mm intake valves will be pretty soggy down low. Sticking with the 36's would be about right. Putting 28 vents into a set of 40 IDFs on an engine with 40mm intake valves will limit your top end noticeably as well. A set of 44 IDFs, for example, can me made to work on many engine sizes with the standard 36 vents, but you might find that you could optimize them better if you are willing to spend the money and try some different vents that would be ideal for your application.

A few notes about jets

More recently made jets are a little different in shape from the standard ones made by Weber, and there are some characteristics of them that could use a little attention. The key thing to watch out for when installing new jets is blockage and burrs. Don't assume that they're clean out of the package. Many have little bits of crap in them that will cause you misery if you just install them. Clean them out with carb spray and look in them to make sure you have them clean. Compressed air is very handy too for blowing them out after you spray them. Get them clean and dry before you install them. Many have burrs along the split areas where they compress to fit into your jet stack or idle jet holder. Inspect for this and get it cleaned off before you install them, or the burrs can chip off and fall into the stack or holder, causing blockage. Check it all. I like to clean them, install them in the holders or stacks, and then pull them out and clean it again, to be sure.. Then you know if anything was going to chip off it did, and has been cleaned out.

It's wise to invest in a set of jet gauges to inspect the jets you use, to make sure they have not been changed. If you trust the stamped or screened writing, you might end up with results you do not expect. I've had them come to me sized wrong a couple times. While I don't like to ream jets, I think having a reamer/sizer is handy for checking them, and sizing them in a pinch. I personally think it is a wise (albeit expensive) choice to have an arsenal of correctly sized jets of all increments around the range that you need to use, but that's something you will likely accrue if you build a few different sized engines with some different combinations of carbs.

Idle jetting

Once you've settled on your vents and your carbs are adjusted and cleaned up, you can choose your idle jet size. I've noted a formula in the books (basically 1.6 * vent size) for idle jet sizes, and I have to say that vent size has little to do with idle jets, on the vw engine at least. More efficient engines can benefit from leaner idle jets, and less efficient engines probably need a little more. 50-60 is the range, and there is a trend toward larger jets for larger carb bodies. The throttle plate size is a key factor.

Idle jets are usually 45-65 in size, mostly in the 55 range. The rule is typically to multiply the vent size x 1.6 to arrive at your rough idle jet size, but what I've seen is that 50 to 55 is mainstream for about any 40 or 44 IDF. 55's will make sure you're not leaning out when you are tuning, so I start with those.

Before you do any more, MAKE SURE the air bypass screws are CLOSED. These are the little screws and lock nuts that are next to the mixture screws. If they are not closed, you'll probably never get it jetted right.

Now. This article assumes that your engine is in top notch condition before you go about tuning the carbs. That means general engine health, timing, dwell, fuel delivery, fuel quality, all the way down the list. And all THAT's another subject.

After installing your idle jets, you close the idle mixture screws and unscrew them all about the same amount, maybe about 1.5 turns out. Then, with your linkage disconnected, unscrew the idle speed screws (the ones the throttle arms rest on) until they are no longer touching the arms, and then turn them back until they are JUST touching. Now turn them in another half turn or so. Start the engine, and after you get it warmed up a little, see if it will idle ok. Now get your synchrometer (Unisyns suck, get a snail if you don't have one) and use the idle speed screws to even it out, and get you to a reasonable idle speed. Not too fast, keep the idle speed down for now, and get it even. If not then turn each mixture screw out another turn. If it STILL will not idle well, then your idle jets could be too small. Now get your synchrometer and use the idle speed screws to even it out, and get you to a reasonable idle speed. Not too fast, keep the idle speed down for now, and get it even using the snail. The snail readings at a proper idle should be between 4 and 6 or so.

Now go to each mixture screw one at a time and slowly turn it in (by slowly, turn it about 1/8 to 1/4 turn at a time and stop and wait for the response of the engine for a few seconds) until the engine begins to slow down (you'll be leaning out a cylinder). Now slowly back the screw back out (same method you used to screw it in) until that cylinder starts to pick up again and run smooth. This is the critical point in the mixture adjustment. Slowly open the screw a little more at a time until you reach a maximum idle speed. Don't go any further! If you're not sure, stop there, screw in back in until the idle drops, and then start to back it out again until you feel confident you have unscrewed the minimum amount that you have to in order to get max idle speed from that screw. Move on to the next cylinder. Do this until you have done all 4 cylinders.

Now back to the synchrometer. Get them as close to the same as you can, and at the target idle speed you want. 800-900 is a reasonable range. Lower you can get and live with it, the better IMO.

You're getting close now. Repeat the idle mixture procedure and when that is done, if your idle is right, you're done with the idle settings for now. If your idle is now off, go back to the synchrometer and idle speed screws.

These iterations help you trim it out - you start low on idle, get your mixture right, and then move the idle up, and check mixture again, and if you have to adjust your mixture to the point that it messes up your idle, then you adjust that, and mixtures one more time and you should have it. If you don't have it by now, you are probably doing something wrong.

So what do you do if two barrels on one of your carbs are not flowing the same? If it's only about quarter point or less, don't sweat it - it's most likely to affect your idle more than anything, maybe a weaker or more eager barrel at idle. If it is more than that, your carbs need attention, namely that the throttle shaft may be bent, or there may be a problem with the butterfly plate or its alignment, or something else causing different air flow. Some people recommend using the air bypass screws to adjust it, and this does help, but if it's more than a point off, it will cause problems that the air bypass can't account for. They need to be FIXED if this is the case. I never use the air bypass screws for more than about a point of difference.

Main jetting

Now with the idle jetting done, main jets are next in line. Assuming F11 emulsion tubes, your starting point for main jets is about 4.1 to 4.3 x the vent size. So for a 36 vent, your main jet should be roughly 145 to 155 to start. 150's a good choice to start with unless you're feeling lucky, in which case you can start with a 145. With a 145-150 main jet, you'll be pretty close with a 36 vent. Similar rules work for smaller and larger vents, though once you get past 40mm vents, the rules don't seem to apply so well.

Air jetting

Air jets are more empirical, but a good starting point for them is about 200 if you have 145 mains. If your mains are considerably smaller, like 135 or 115 or something, a 160 to 180 air jet is a better place to start. Start there and don't mess with them until you know the other jets are right.

Synchronization

This is a topic that does not get enough attention. Synchronization is EVERYTHING after the jetting is done. Synchronization will such a difference in the way it runs you just can't believe it when they are right. Unfortunately all the linkage that is out there has certain characteristics that keep your synchronization from being perfect all the time, so all you can do is get it close and keep after it every once in a while.

The biggest problem with synchronization is keeping the geometry correct. This is hard to understand without illustrations of why, but if you just trust that it's important, you can avoid all the rest.

Linkage

There are different kinds of linkage out there, and a lot of people use crossbar linkage, while some others use bellcrank type linkage. Each one has its application, but I prefer bellcrank when I can use it and can get good linkage.

Crossbar linkage: How to ensure that your downrods have the right geometry With your downrods, if you can position the crossbar arms so that your downrods are vertical (left to right), that will help matters, and complicate the situation less. Once you have achieved that, you can work on making sure that they are both at the same angle (leaning from front to back of the car). Use an angle finder, and read what the rods are, and use washers to get them the same. There shouldn't be a lot of difference to start with, if you have the right linkage for your setup. Different intake manifold types (offset versus straight) will use different linkage setups. If your crossbar is sitting at an angle with respect to your fanshroud, you have the wrong linkage.

Once your downrods are at the same angle side for side, then your synchronization will be easier, because the throttles should now be offsetting the same amount on each side throughout the full range.

Now, to synch your carbs, loosen the nuts on one downrod, and use the rod's opposed threads to set the carbs so they are opening at the same time. Be careful when doing this (I do it with the engine off) so that you don't open them a lot too many times.. you'll dump a bunch of fuel into the engine if you do. When you test, make sure that you are using the point where the cable connects to the throttle arm in the middle of the crossbar. If you twist at one of the outer arms, you will not get accurate results. Just push on the arm at the point where the cable meets it, so that you are applying the same force that the cable does when it pulls. If you put your hand on the crossbar and twist it using your thumb or something on the middle arm, you are still applying a force that the crossbar will not experience under normal conditions.

Watch the throttles, and compare what both sides are doing. They should be opening at the same exact moment. Realize that as the engine warms up the geometry changes slightly so there may be some difference between hot and cold engine. I like to warm mine up before I do this. After you get them synched right, tighten down the nuts so that both the heim joints are centered to their positions. I rotate the joints so that they are both resting against the position they would be pulled to when you tighten the nuts, and then I tighten them. Be sure to not let the rod twist when you position or tighten them. You may find that tightening the nuts changes the geometry slightly, so you may have to compensate a little bit for this on the rod.

After you get the nuts tight, check it again, and repeat if necessary.

Why this matters so much: Once you get them synched perfect, you will FEEL how much better it runs, especially on low throttle lower RPM. If you have a head temperature gauge, you will see that when the linkage is not right, one side will run warmer than the other. If the left side throttle opens first, the left side will run warmer, especially at low throttle low RPMs.

The last thing you should do if you have never done it: Loosen the throttle cable from the center arm and have someone push the gas pedal to the floor. Pull the cable tight, and then snug the connector. This will ensure that you do not put undue stress on the throttle shafts on the carbs at full throttle. If you put too much pressure on them you can twist the throttle shafts too much and bend them. This is especially true of setups that have the return spring and stop on the FRONT of one of the carbs, which is the way most Weber setups are out of the box. I use the CB Weblink kit to put the springs both on the throttle linkage side, to help with synchronization.

This with some practice and a good feel for what you are doing will help a lot with making sure your engine runs smooth.

Drive it

Something you should understand about tuning from this point. Your accelerator pumps are going to try to fool you unless you understand their purpose and function.

Accelerator pumps are there to compensate for some physics. The fact is that fuel is heavier than air, and it takes longer for the fuel to pick up speed in the circuits than it does for the air to pick up speed in the throats. So when you stomp the gas, the air starts moving faster a lot sooner than the fuel does. The accelerator pumps are there to provide a little extra fuel during that short time it takes for the fuel to catch up. Keep this in mind when you are tuning the jets, and avoid rapid pedal movements for now.

It really helps to have a tach, and an air fuel gauge, and I would say that a tach is next to necessary, while an air fuel gauge is a luxury that you can do without unless you are FINE tuning, and going for near perfect in terms of mileage and power.

Take the car out and drive it. Pay attention to what's going on at about 1500 to 2000 RPM, and at 2000 to 3000 RPM, and from 3000 to 4000 RPM. While the RPM bands will vary depending on what size carbs, what vents, and what engine combo you have, these are pretty safe ranges to look at.

1500 to 2000 RPM is almost purely idle jets in action. Keep the pedal steady here, in 3rd gear so you can see what it does with a slight load. If this area is running ok, move on. If it feels weak or soggy, you might need different idle jets. Unfortunately, it's sometimes hard to tell if it's rich or lean unless you are experienced here. This is where an air fuel meter will help you out. What you can try is opening up your mixture screws about half turn and see if it feels better. If it doesn't, then it might be too rich. If it does, then your idle jets may be too lean. For those of you with air-fuel gauges, about 13-13.5 or so is what I like. Move on to the next range.

2000 to 3000 RPM is the "transition" stage. This is the part where the main circuit starts to take over. Your idle jets start to matter less here, and your air corrector jets are the progressively deciding factor for why your engine is running the way it is. With the pedal steady, make note of how it feels in here, and move on.

3000 to 4000 RPM is where your main jets are really doing their job. With the pedal steady, if it feels lean here, (sort of like it is running out of fuel) then you might want to try going up a main jet size. If not, try going down one and see how it runs. Go down until it starts to feel lean (weaker with maybe some popping). When you feel it go lean, move back up a size and you are done with the mains. For those of you with air-fuel gauges, about 13.0 - 13.5 or so is what I like.

Now, back to the transition stage. If it felt lean there before you did anything, and the main circuit felt lean, determine of moving the main jet up helped the transition stage. When the idles and mains are both right, the transition should be pretty good too. If your mains and your idles feel good, but your transition does not, try going up a couple sizes on the air jet and see if that helps. If not, try to go up a couple sizes more and see. By now you should have it cleaned up. If you STILL have problems with transition, something else may be wrong. Read the article to the left about transition.

Accelerator pump settings

Once everything else feels right at steady state, then you can start playing with the accelerator pumps. While there are different pump jets and bypass valves, you can usually get what you need out of the ones that come on the carbs out of the box. First, make sure that they are adjusted the same. Drive it, and see what happens when you give it gas more quickly. If when you are in second or third gear, and you push the pedal from cruise to WOT over the course of about a half second, and it bogs, try unscrewing each accelerator pump nut about 3 turns and try it again. If it's worse, then they were too lean, and you should go back the other way. Try 3 turns at a time until they are right. You shouldn't need more than about 1/2" of rod sticking out of the nut. If you do, you might have some timing issues you need to deal with. For those of you with an air-fuel meter, your meter's response to mashing the pedal should be as close to steady as possible, but that's not realistic to expect. If your engine falls on its face, and the meter goes lean, screw the nuts in about three turns on each side, and try again. Same rules for if it goes rich for any period of time, like two seconds or so. What I like to do is back off the screws until it falls on its face, and then start screwing them back in until it doesn't anymore. Too much fuel from the accelerator pumps is going to cause the excess fuel to wash oil off the cylinders, and cause you poor mileage.

Common mistakes

Lots of folks are driving around thinking their state of tune is good, when in fact, they've covered up symptoms with incorrect jetting. The only ONLY way to know for sure is to have an accurate air/fuel meter, but you can avoid a lot of it by just paying close attention to what the different circuits are doing. For dialing the jets, paying attention to what happens as you mash the pedal or right afterwards is wrong. The accelerator pump is screwing with things when you do that. Jetting requires paying attention to what's happening at steady state, during fixed RPM ranges, under different conditions.

What happens under WOT is not an ideal way to assess your jetting, at least under heavy load and at lower RPM ranges. Things will go rich there, and should due to what your engine needs. If you have an Air/Fuel meter and it is dropping down to the 11's or something when you nail it, worry about some of that when you have the rest tuned. Don't shoot for getting that right until you get the rest right. That part may never be perfect, but you can get it closer after you do the rest.

If you think you have it tuned, drive it for a while, a distance, a few days, whatever, and see how you feel about it. If you try to jet an engine that has been running really rich for a while, it may take a few stages of leaning it before you get it right, due to carbon buildup, and may take some new plugs to be sure. Plugs are another factor that has to be paid sufficient heed to get things right. That's another topic, but about 0.025" for stock, more like 0.040" for CDI and such.

New engines: Never try to tune a new engine perfectly. It's going to change. I tend to suggest running them a little rich in the beginning to keep temps down and make sure you don't lean it out while your rings are trying to seat.

What kind of mileage should you expect? Depends on how you drive. If you drive it like you're sick of the high gas prices you should get close to 20 if everything is right, maybe even more. I get about 25. You can get very good mileage if you lean things out close to 14 across the board, reduce your accelerator pump action to the very minimum, with just a hint of hesitation when you punch it, and use a vacuum advance distributor (SVDA) with the ports on the carbs (many have vacuum ports you can attach to a T fitting and then to an SVDA distributor that is tuned right for use with dual carbs. Not all cam/head setups are going to work well with an SVDA distributor, but many will do fine.

If you drive with a heavy foot, then face facts. Your mileage is going to suck.

I hope this helps you. There was a point in time that it sure would have helped me.
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MikeyM73
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PostPosted: Tue Sep 14, 2021 4:29 pm    Post subject: Re: Wideband Results Reply with quote

Thanks Clatter, that article was awesome. I've been tuning my carbs similarly but wasn't sure about the specific circuit RPM ranges or other small pointers but this helps a ton.

So questoin - If you install larger air corrector jets, that makes the mains come in sooner but also run leaner, is that correct?

Thanks,
Mikey
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PostPosted: Wed Sep 15, 2021 9:50 am    Post subject: Re: Wideband Results Reply with quote

MikeyM73 wrote:
Thanks Clatter, that article was awesome. I've been tuning my carbs similarly but wasn't sure about the specific circuit RPM ranges or other small pointers but this helps a ton.

So questoin - If you install larger air corrector jets, that makes the mains come in sooner but also run leaner, is that correct?

Thanks,
Mikey


Correct.
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PostPosted: Sat Sep 25, 2021 3:01 am    Post subject: Re: Wideband Results Reply with quote

Hi folks , been studying this thread for a few weeks now in readiness for my final tune, now that my engine is broken in.
So my engine spec
2109cc 9.75/1cr
Fk8 cam 1.4 aa rockers
Scat super d heads 42 x 37
Billet pertronix cent only distributor (20 deg total advance, 1 silver 1 copper spring)
Hvac coil 6al msd
Bug pack 1 5/8”merged with hideaway
Twin 44 idfs early cam type squirters
36 vents
F11 emulsion tubes
52 idles , 135 mains and 160 airs
Innovate mtx afr gauge
I’m fairly close to 13.1 all over (as per the advice here) but after a good run round the idle mix sits at 11.5 dropping to high 12’s if I tickle the throttle,tried to redo lean best idle but that’s where it’s sat at best ( I set lbi by ear that’s just what the gauge reads when I’m done)
idle to progression is 12.9-13.2
Lean hole on gauge just past progression is 13.8-14.3 (can’t feel it though) then 13.2 all the way up to finally 12.5 at full revs
I’m just waiting for some 170 airs to arrive which I think will help flatten out the lean hole and weaken the mix up top it feels very close.
Car drives great and nice and cool .
Now my question is does the af at idle indicate too big an idle jet (or some other issue? Maybe the huge ports causing low velocity at tick over? ), I’ve tried 50’s and the car pops and complains . Timing is now at 8.5 btdc at idle (lowered from 9+ to bring idle down to 850 rpm)
The main and air sizes do seem small compared with the usual advice? but I’m now noticing a lot better fuel consumption , am I missing something or has my 3 weeks of trials found my engine sweet spot?
And I just checked my elevation is 128 feet above sea level.
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PostPosted: Thu Oct 07, 2021 11:28 am    Post subject: Re: Wideband Results Reply with quote

As a follow up from this I fitted the 170 airs and it was loads better giving me closer to the 12.9-13.1 at top revs and settled out the transition, but now my idles were showing signs of being rich mid 11’s to low 12’s . So I retried my 50’s and it was weak and popping, the difference was too wide and made me think my 52’s may have been messed with , so another set was ordered and fitted which has now put me bang on 12.9-13.1 Smile I need to do some high speed tests but it feels great now thanks to everyone who has posted help on here. ( shows that old mystery jets can throw you off the scent)
So my final jetting ended up at
52 idles
135 mains
170 airs
36 vents
44 Weber’s
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PostPosted: Thu Nov 11, 2021 6:58 pm    Post subject: Jetting Help Reply with quote

I'm running a 2161 w/ Scat c45 cam, super-d p/p 42 x 37.5 heads and dual 44 idf's running at sea level. Engine runs great. D6EA plugs on the black side. When cruising at 2500 afr is in lower 13's.

I have 135 mains and 175 air jets on hand.

Jetting:

Venturi's 36mm
Main 150
Idle 50
Air 200

AFR readings: Rolling on throttle in 3rd gear to 5000+

Idle 900-950 13.5
1000 - 2000 12.9
2000 - 5000 11.03 average

Recommendations?
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PostPosted: Thu Nov 11, 2021 8:54 pm    Post subject: Re: Wideband Results Reply with quote

your fat on the top end, I would go down 1 or 2 on the main.

What is your timing at?
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PostPosted: Thu Nov 11, 2021 9:38 pm    Post subject: Re: Wideband Results Reply with quote

ivkings4 wrote:
your fat on the top end, I would go down 1 or 2 on the main.

What is your timing at?


Thanks ivkings4, that's what John said. 2 pro's with the same answer!

Using a 019 with 13 initial 30 max. Just got ahold of John (aircooled.net) and he recommends lowering the main from 150 to 140.

Here's a pic of my portable AFR Box. Bought a AEM and the O2 sensor that came with it lasted a whole 30 seconds. Bought this O2 sensor from Amazon and it works great. Comes with a long wire also.
https://www.amazon.com/dp/B07K1SMQYH?psc=1&ref=ppx_yo2_dt_b_product_details
Cost $43.12 a bargain with lifetime warranty.

The wires on the AEM are real long allowing me to run them from the exhaust bung to the passenger seat and have my son-in-law film the readings with my iphone while i hit the pedal. When done, i just disconnect 3 wires (tach, Hot & ground) and the O2 sensor and put it away. Also found that i can run the dash tach and the O2 box tach at the same time.


Image may have been reduced in size. Click image to view fullscreen.
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PostPosted: Fri Nov 12, 2021 3:50 am    Post subject: Re: Wideband Results Reply with quote

Cool set up.
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PostPosted: Fri Nov 12, 2021 9:22 am    Post subject: Re: Wideband Results Reply with quote

Anybody have any suggestions here on a good starting point for my setup?

1776cc engine

Steve Tims unported 37x32 "Super Stock" heads
stock heater boxes
Vintage speed Sport "extreme lowered" muffler (155-733-05200 part number, no provision for an O2 sensor)
Cb2239 cam
Offset manifolds/CSP linkage
40IDF clones that take 44IDF sized vents, similar to HPMX. Currently have 52 idles, 115 and 120 mains, as shipped 28mm vents. I may initially run 28 vents with 120 mains if the idles run well.

For reference I currently run Kadrons with 30mm vents, 55 idles, and 140 mains. May step down to a 137.5 main, but power is good and fairly smooth for Kadrons. MY top end with these vents is great, with a minor typical Kadron transition hiccup associated with the larger vents. I'm reasonably happy with the setup but would like the improved idle and tuning ITB type carbs offer, so bought these 40IDF clones from CIP1 last year.


I was planning to go with 30mm vents, and trying to make an informed starting point for the main jet sizing. Following the 4.2x venturi size rule of thumb for mains I've seen thrown around, it looks like 126 comes up as a suggested main jet size. All our fuel is 10 percent ethanol, so I was thinking of going up to a 127.5 main. Would this be a decent sized main to try once the idle circuit is dialed in? I'm going to be running an SVDA and eventually a Daytona TCS-1 timing control.

At this time I do not have an O2 sensor bung in the exhaust, and I'm told they don't work all that well in these mufflers at lower RPM so not sure how to proceed. Either way I want to order jets when I order vents to at least have a good starting point.
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PostPosted: Mon Nov 29, 2021 8:33 pm    Post subject: Re: Wideband Results Reply with quote

I'm currently rebuilding/cleaning some Weber 44 IDF carbs I am going to use on a 2276 engine with a Weber 86A+3 cam and Tims Stage 2 heads with 42x37 valves. I have skimmed through this topic some and read the Weber Tech manual by Bob Tomlinson. I have attached a photo below from the book that is meant to help in selecting the correct venturi size. The book says to divide the displacement of the engine by the number of cylinders so in my case 2276/4=569. So tracing that up to intersect the 6,000 RPM curve it seems to look like about 38-39mm venturi
Image may have been reduced in size. Click image to view fullscreen.


And from the post above that says
"Venturi selection - For engines that rev pretty high (6000-7000 rpm), you are best off choosing vents that are about 2 to 4mm smaller than your intake valve size.

For engines that are set up for torque, like bus engines, and other torque-happy applications, you can go a little smaller on the vents and get better low end response. For these, I'd recommend 4 to 6mm smaller than your intake valve size.

So what happens if your vents are too big? You'll have reduced air-speed at low RPMs, which makes tuning harder. Air speed is one of the things that tells your different fuel circuits when to start to come on. It will be soggy down low.

So what happens if your vents are too small? The undersized vents will restrict your engine's ability to breathe at higher RPMs.
"

So I f I go in the middle with 4mm less than the intake is 42-4= 38mm

It seems most people use the stock 36mm Venturi with the 44 IDF's but based on these two sources it seems it may possibly work better with the 38 for my application if I can get the rest of the setup correct.

So I was wondering if anyone had experience with experimenting with the 38 vents on the 44 IDF carbs?
Thgank!
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PostPosted: Mon Nov 29, 2021 8:36 pm    Post subject: Re: Wideband Results Reply with quote

The information in that book is not very good just like the Dellorto one from CB.
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PostPosted: Sun May 15, 2022 3:25 pm    Post subject: Re: Wideband Results Reply with quote

Started tuning a pair of Spanish weber 40 idfs with a wideband. If I stomp the throttle, the gauge goes way lean and there's a chirping or whistling sound. If I screw the accelerator pump nut all the way in this is lessened. The gauge goes lean for a second, minor whistle but can't feel it. Is there likely a vacuum leak somewhere? I checked all gaskets. Do I need a larger pump jet?
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PostPosted: Sun May 15, 2022 5:17 pm    Post subject: Re: Wideband Results Reply with quote

what is your set-up what engine and what jetting, you might need more main jet, your timing might be off. too many variables. Most cars do not need larger pump jet
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PostPosted: Sun May 15, 2022 5:19 pm    Post subject: Re: Wideband Results Reply with quote

1776, CB 2239
Timing 9 deg idle at 850rpm, 28 deg all in at 3200rpm
50 idles (these are slightly rich - waiting on 47s)
120 mains (slightly lean - waiting on 122s)
200 airs
F11s
28 vents
0 bypass
50 pumps
2.5 psi fuel pressure
12mm float height to gasket

Do you adjust accelerator pump for AFR or just so you don't feel a hesitation and not worry about a momentary lean reading as long as it can't be felt?
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PostPosted: Sun May 15, 2022 5:39 pm    Post subject: Re: Wideband Results Reply with quote

try to drive your car and don't stab it, roll into it very slow so you don't use the acc pump at all. that way you can tune it for the correct jetting. (Jim Martin taught me this)

the pump jet rod should only be adjusted after the rest is on point, then go out and stab it and see what it does, it is just filling that void for that instant it needs it.
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PostPosted: Sun May 15, 2022 5:41 pm    Post subject: Re: Wideband Results Reply with quote

ivkings4 wrote:
try to drive your car and don't stab it, roll into it very slow so you don't use the acc pump at all. that way you can tune it for the correct jetting. (Jim Martin taught me this)

the pump jet rod should only be adjusted after the rest is on point, then go out and stab it and see what it does, it is just filling that void for that instant it needs it.


So just enough pump adjustment to not feel a hesitation?
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