TheSamba.com :: View topic - Dual IDF Tuning

Sun Feb 19, 2017 7:48 am

Did some more measurements on the linkage.

Measuring from the tip of the idle speed screw to the linkage stop, there is 1mm difference. I took measurements at a couple different locations and always about 1mm different.

Obviously at idle they are the same, and at full throttle all butterflies are vertical.

Does the spitting indicate a lean condition? It would appear so with the recommendation to increase the idle jet.

Thanks again

Sun Feb 19, 2017 9:00 am

Hello.
I never saw that "Alfa1750" modification, - luckily I think. It doesnt sound very good.
In the matter of the OP, if those carbs have been exposewd to that I tend to agree with Modok, that you should search for an untouched set. They are still out there.
With the jetting. a couple of things.
The rule of thumb with venturi size that Modok refers to is not entirely accurate. Weber actually has a formula that you can use that brings you in the ball park. RpmXdispacemenrt/50 brings you close to what is needed.
In the OP´s case I am pretty sure he has fallen into the trap most do when they are rookies. He did not check the CR. Unless something special has been done such a 1200 with Okrasa heads will result in a CR of maybe 6,6-1. On top of that the cam has been cut to 102-15/16 specs. That results in even less DCR which again means less strength in the vacum signal to the carbs. It all adds up.
The linkage set up is not the best, I agree, but that is not the issue here.

Lets go back the particular engine and look at the needed venturi size for a decently matched engine. We assume that the engine will peak at approx 5000 rpm. So the math will be like this: 300 x 5000 / 50 = 30. So the optimum venturi size would be 30 mm.
Now we "know" that the engine is somewhat low on DCR, so we take that into account, and the 27 mm venturies installed should not be that far off. Next, we know from experience that engines with low vacum signal need higher jet sizes. The high CR Alfa 1500 wanted 46-47,5 idles. The 36 hp is low DCR so it needs much more. I would start with ,55 and see where that brings you.
Once the flow through the carburettor is established we need to adress the midrange and transition. The engine is small, so I am thinking F55/57 or even F7 to help mask the weak vacum signal. Cut it has to be trial & error. Can´t say for sure. If you get the idle jet right, then the F11 might work too.
My guess for a starting point would be something like 0,552-0,55 idle, 115main, F11 E tube and 180 main air..

T

Sun Feb 19, 2017 9:52 am

Thank you for the response.

Static compression is 7.1-1...reduced chamber and .040 deck

I will definitely begin with changing the idle jets. Would 26mm vents help? Could always swap those as well.

Alfa1750 has always been quick to respond and so I don't want this post reflect negatively on him. I emailed him about the progression hole issue. His response was that all "VW" Webers had 4 progression holes, not 5 like Alfas. I still don't understand why he would modify 5 hole Alfa carbs to 4 just because they are going on a vw

Sun Feb 19, 2017 9:57 am

as you mentioned txoval, it would definitely better if you got the later model push/push style of linkage and accompanying throttle shaft arms.

Sun Feb 19, 2017 10:08 am

txoval wrote: Thank you for the response.

Static compression is 7.1-1...reduced chamber and .040 deck

I will definitely begin with changing the idle jets. Would 26mm vents help? Could always swap those as well.

Alfa1750 has always been quick to respond and so I don't want this post reflect negatively on him. I emailed him about the progression hole issue. His response was that all "VW" Webers had 4 progression holes, not 5 like Alfas. I still don't understand why he would modify 5 hole Alfa carbs to 4 just because they are going on a vw
OK. 7-1 is still low for the cam, which is designed for 9 tgo90 9,5-1.
The IDF push pull will not do much with the agressive angle on the rods. Maybe the center part needs to be custom made to level out the rods..
I will admit that my memory is not quite clear on the Alfa 36és regarding progression holes. but i can check tonight since I have some in the shop.
No. 26 mm venturies will not solve your problem.

T

Sun Feb 19, 2017 10:09 am

for the most part, the more progression holes the better for a street engine. It's very easy to delay the main circuit (if you have too much progression/main overlap). To bring the mains in earlier because of not enough overlap, is more challenging, especially on smaller engines.

Sun Feb 19, 2017 12:06 pm

txoval wrote: Did some more measurements on the linkage.

Measuring from the tip of the idle speed screw to the linkage stop, there is 1mm difference. I took measurements at a couple different locations and always about 1mm different.

Obviously at idle they are the same, and at full throttle all butterflies are vertical.

Does the spitting indicate a lean condition? It would appear so with the recommendation to increase the idle jet.

Thanks again

And Yes, the splitting would be it not getting enough fuel on that side that falls behind? I was thinking that you could shift the balance to compensate but, that would not really work because it's running lean on on side. So I am going out on a limb here and guess there is some little thing different about these two carbs. It could just be the gas level in the bowels is different!

Sun Feb 19, 2017 12:12 pm

The spitting is occurring on both carbs...

Sun Feb 19, 2017 12:29 pm

Oh, your saying they spit I thought your were saying one was separating not pulling the load for it's side. But Yes still Spitting is what they do when cold due to a lack of fuel as the butterfly opens a lttle. Too much air not enough fuel!

I am playing with a set of 36mm dell's as we speak on a 1600 and yes there is spitting going on It was better with 28mm venturi's than with 30's I have in it now. So reduce the air or increase the fuel, These little jewels have about 100 ways to do things but can get really out of sorts if your not right with the basic's

That 36 horse is not breathing strong enough for the air your giving them, probably start with the idle jets? Richer. You may have too much air correction or something going on inside the idle or main circuit that is messing up the air correction.

One more thought! Your probably using thick paper or some kind of insulating manifold gaskets with them and that 36 horse is not going to warm them up at all. One idea would be to make copper manifold gaskets for them.
See my 1679cc link below for what jet's I am playing with.

Sun Feb 19, 2017 1:13 pm

I modified the linkage arms so that the ball ends are flat (parallel to one another) on each carb. Checked the measurements again and they are now equal.

I need a few other unique parts for these carbs, so I'll get the jets from Dellorto UK as the are the only ones that sell new aux. vents for 36IDFs.

I'll get a set of larger mains as well

The carbs can be made to work, just troubleshooting and learning.

Hot VW's used 40IDF's on their 36hp Okrasa engine. Need to find that issue and see what jetting they used.

Any other rebuilders out there? Need to call Blackline

Sun Feb 19, 2017 3:46 pm

aux vent is same as 40 idf, very common, the new carbs you will buy will have plenty of spares too.

I tried not to think about it, but I did anyway, transfer ports are hard to think about. I am not surprised Alstrup thinks my "math is off", as I hardly understand it myself, but the next day things tend to be clearer.
Check my logic-
If you transfer the hole pattern of a 40Idf-70 INTO a 36 idf, you make a smaller version of it.
If the angle at which the throttle has revealed all transfer ports is the same, yet the throttle is smaller, then they are all-in at a lower airflow.
Changing the transfer ports as he has done, will require a smaller venturi all else equal.

VW folks are well aware how dellortos use one or two sizes larger venturi in the same carburetor bore size, compared to weber. Now do you see why? It is because dellortos have extended transfer ports.

As usual, Torben is probably right. I am glad he said it. I did not want to insult anything more than necessary to get you pointed the right direction.
The weber venturi chart does say a 30mm vent can work but it does not indicate you you what carburetor body to put that in.
For one thing, the chart is for divorced idle racing model carbs.
It might be interesting to BUILD a 40 IDF with divorced idle but I've never seen that done.

Sun Feb 19, 2017 3:55 pm

One other thing to look at and perhaps to talk to the Brits about is the Auxiliary Venturis, I have noticed some of mine (There Dellorto's though) are of different lengths, in other words the small tube extends down to a different position within it's venturi. With that little 27mm venturi you should get a richer mix with this just right, It's somewhere there (2mm maybe?) into the choke that the Auxiliary Venturi needs to end up. I would be curious to know what they say at Eurocarb about it.

Sun Feb 19, 2017 4:17 pm

Another way to look at it would be this:
You could have a 2000cc 90hp engine, or a 1200cc 90hp engine.
If venturi size is a function of HP, they can both use the same size vent (not 100% true but plausible)
Which engine will need more transfer ports? The tiny one. Answer is clear.

Sun Feb 19, 2017 6:52 pm

I made new linkage arms today to ensure each end was parallel to each in both directions...up/down and side to side. Also fixed the orientation of the center mount bell crank.

Once the idles come in I'll report back. If it doesn't work, I guess I'll look at those used 36IDFs and get them rebuilt or move on to a set of 36DRLAs.

Who else sells rebuilt DRLAs besides Alfa?

Sun Feb 19, 2017 7:01 pm

There are a couple of guy here on the samba rebuilding them. I have been rebuilding my own and now have several, But I am just a beginner at this point!
Say I have an idea, Now don't try this at home! But can you plug the air corrections on a IDF? You know stick tooth picks in them or something? Just to see what happens, I did that on a Solex PBIC 32 I had as a single on a single port bug and that was the ticket!

Sun Feb 19, 2017 11:35 pm

Morning. I just realized that I goofed up on the math last night. I woke up and had that notion that something was wrong. So I checked, and sure enough, I was too fast on the trigger. - The formula for venturi size is rpm x displacement, take the square root and divide by 50. That brings some totally different numbers. This also brings Modoks thumb close back to normal size. :roll: :D
Sorry about that. :oops:
But you should still up the idle jet.

T

Mon Feb 20, 2017 6:30 am

Alstrup wrote: Morning. I just realized...

Thanks Alstrup. Now it makes sense! :lol: Al

Mon Feb 20, 2017 9:50 am

With that formula...24.5mm vents would be ideal, but are not available.

If I'm understanding this right, my 27mm vents are providing too much air, causing a lean condition, which leads to increasing the jet sizes to compensate???

Thanks again

Mon Feb 20, 2017 11:33 am

ON a small displacement engine that does not draw the volume of air of a larger engine the venturi size must be small enough for the air velocity to be high enough to draw the fuel through the fuel circuits in the carb. Air correction lowers the volume of fuel that can be drawn through the jets, jet size increases the amount of fuel or decreases if you go smaller. Making Air correction jets smaller will increase fuel.
These all effect each other and a wideband can provide the information that shows just what is going on in the carb.
Too big of Idle jets may hide too large of Main Air correction. Big pump jets can help the engine to rev even though the Idle jets may be too small.
Another thing that happens with these 1432 firing order engines is they tend to carry the load on just 2 cylinders, one on each side. I am trying to figure this out with mine. My manifolds are different temperatures, on both sides of the engine I have one warm and one cold intake on the same manifold. It's simple to touch them and see it. This could be a valve adjust thing or a compression thing or a spark timing thing, something other than the carb though.
One nice thing about your carbs is they have the little vacuum monitor ports, So do my dells, I found it real easy to adjust manifold vacuums to all be the same at idle but my engine still run stronger on one cylinder on each side.

Mon Feb 20, 2017 3:38 pm

For your linkage: bring the engine ip to 1200- 1500 rpm or so,lock the accelerator in place (rig something up with vise-grips or something, acting as the cable would pull it), and check you sync again, this is the true measure to see how well the linkage is working.

As for jetting: do what they say, and maybe consider a rebuild with higher compression? I don't know how dedicated you are.