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Fri Sep 30, 2005 1:16 am

I purchased the carb with no manual.
Does anyone know of a thread or web page with step by step instructions?
Thanks in advance!! :)

Fri Sep 30, 2005 8:24 am

This was a nice write up done by another Samba member very informative.

Quote: mharney
Samba Contributor

Joined: 01 Jun 2002
Posts: 607

Posted: Wed Sep 14, 2005 5:27 am Post subject:

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Here's a little writeup I have done on Webers. I am sure it applies to the HPMX carbs too.

Tuning Dual Weber Carbs on the VW Type 1 Engine

As many times as this topic comes up, I decided to try and come up with a document that should answer a lot of the common questions.

A lot of what I learned I have learned from John Connolly at Aircooled.net, so I am giving credit where credit is due.

The fact of tuning - you're going to be needing some different jet sizes. No way around it, if you want to get it right. Don't start howling at me about having to buy jets. It's 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.

First things first. Clean the carbs and adjust the floats. Here is a simple cleaning procedure that should clean out most carbs, especially new or fairly new ones. Older ones will need more attention. http://bouncewithme.bounceme.net:81/vwpage/idf/idfsetup.htm

If you slack on this step, the rest will be meaningless and frustrating.

Venturi selection
There are different schools of thought on what vents to choose for your application. What you choose should reflect what you want to tune for.

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.

Idle jetting
Once you've settled on your vents and your carbs are adjusted and cleaned up, you can choose your idle jet size. Idle jets are USUALLY 50-55 in size on most setups I have seen, though I have seen some engines that benefit from using a 57 idle jet. 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.

After installing your idle jets, you close the idle mixture screws and unscrew them all about the same amount, maybe about 1 to 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, see if it will idle ok. 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. If it does idle ok, then 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. Open the screw about 1/2 turn more. Move on to the next cylinder. Do this until you have done all 4 cylinders.

Now.. with all 4 cylinders set on mixture, get your synchrometer (Unisyns suck, get a snail), and your dwell meter/tach. Don't have one? Well, now it's time to get one, unless you have an ACCURATE tach in your instruments. Adjust the carbs to have the same flow, and have about the right RPM, which will vary, but about 800-900 is usually do-able on an engine that is not radical.

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.

So what do you do if two barrels on one of your carbs are not flowing the same? If it's only about half a point or less, don't sweat it. If it is more than that, your carbs need attention. That's another subject. Some people recommend using the air bypass screws to adjust it, but if it's more than a half 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.

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.

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.

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 14:1 +/- 0.3 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 jets start to take over. Your idle jets start to matter less here, and your main 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, and your air jets aren't really affecting things much yet. 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 13.5 +/- 0.3 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 on one size on the main jet and see if that helps. If not, then go up one size on the idle and see. By now you should have it cleaned up. If you STILL have problems with transition, then it's likely that your floats are wrong. You can subtract about a millimeter at a time to richen up transition, if you need to. I find it rare for transition to be too rich, especially with your floats close to right and your jets close to right. For those of you with air-fuel gauges, if you get the points right at the low and high rpm, this stage should fall into place. If it doesn't, then you may need to adjust your float levels a little. About a mm at a time will do plenty to change the numbers here. You should start close to 10-11mm on the float levels. I would say if you have to go to a float setting that is less than 10mm you might need to go up on one jet or the other. Your transition stage should be in the 13.5 to 14 area. You may have to fudge a little on the idle or main circuit to get transition close to that if you can't get it with a float level as small as 10mm.

Air jets
These are a little easier when you have the rest done and set up right. Usually about 60 or so larger than your final jet size is close. If you find that the higher RPMs are leaning out, you need to go DOWN about 15-20 on your air jet and try it again. Same rule for soggy, go UP a about 15-20 on air jets to see if that cleans it up. Air jets meter AIR, not fuel, so that's why it's backwards from idles and mains.

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.

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.

Hope this helps you.

Fri Sep 30, 2005 10:02 am

Thanks very much!
This outta help alot :D