Dual Weber IDF 40s, Carb Rookie question... Options

[malcolm2]

Where are your idle mixture screws? If I recall correctly they should be in the range of 1 1/2 turns out from seated. If less the idle jets are too large, if more the idle jets are too small. PLEASE check the previous statement. It's one way or the other

Don't throw new jets in her just for the heck of it

1 is 1/2 a turn +1/8 a turn (0.625 turns)

2 is 1/2 a turn + 1/8 a turn (0.625 turns)

3 is 1-1/2 turns + 1/8 a turn (1.625 turns)

4 is 1 turn (1 turn)

after checking, i started it up and checked the flow..... all were very close to the same

[r_towle]

You are using the idle bypass screw to make up the difference in throttle plate positioning….at least it seems to be that way

What throttle linkage do you have again?
I recall totally giving up on cross bar linkage and going to dual cable controls by Foley as the only way to tune the carbs at lower rpm’s.
The linkage geometry is really hard to get right at low rpm’s, but you can get it to work 2500-6500 rpms

[malcolm2]

You are using the idle bypass screw to make up the difference in throttle plate positioning….at least it seems to be that way

What throttle linkage do you have again?
I recall totally giving up on cross bar linkage and going to dual cable controls by Foley as the only way to tune the carbs at lower rpm’s.
The linkage geometry is really hard to get right at low rpm’s, but you can get it to work 2500-6500 rpms

Yes i used the air bypass as discussed above. One barrel on each side is used to match the flow on that side.

I have the cable type sync-link.

[r_towle]

3&4 are one carb?
Not sure how you are numbering

[r_towle]

I may be missing something, but when I start from the beginning, I do this.
Start car, run till warm
Set idle bypass screws exactly the same
Set idle arm mechanism and throttle plates exactly the same
Verify throttle plate position by measuring from top to plate with a micrometer (be accurate)

Turn it off
Disconnect throttle linkage
Start car
Adjust each barrel to match with idle bypass screw

Very gently, with no pulling or repositioning, adjust and attach throttle linkage.

[malcolm2]

Question from 930 Cabman was for mixture screws. The larger screws with the springs on them.

Your question was for Bypass screws, right? They were adjusted as stated and the last step: only adjusting one on each carb to match flow in each barrel on that side. Leaving one on each side closed. I don’t remember the turns there. Maybe 1-1/2 on one barrel on each side.

Those screws are smaller and have a lock nut on them.

[930cabman]

Question from 930 Cabman was for mixture screws. The larger screws with the springs on them.

Your question was for Bypass screws, right? They were adjusted as stated and the last step: only adjusting one on each carb to match flow in each barrel on that side. Leaving one on each side closed. I don’t remember the turns there. Maybe 1-1/2 on one barrel on each side.

Those screws are smaller and have a lick nut on them.

There are 2 idle mixture screws per carb near the bottom and 1 idle speed screw per carb.

[malcolm2]

I may be missing something, but when I start from the beginning, I do this.
Start car, run till warm
Set idle bypass screws exactly the same
Set idle arm mechanism and throttle plates exactly the same
Verify throttle plate position by measuring from top to plate with a micrometer (be accurate)

Turn it off
Disconnect throttle linkage
Start car
Adjust each barrel to match with idle bypass screw

Very gently, with no pulling or repositioning, adjust and attach throttle linkage.

Yes bypass screws were set the same. All were closed. I did not measure the throttle plate. As it was not part of weber’s Lean Idle process. Other than that, and adding the mixture screw adjustment, that is what i did.

[malcolm2]

Question from 930 Cabman was for mixture screws. The larger screws with the springs on them.

Your question was for Bypass screws, right? They were adjusted as stated and the last step: only adjusting one on each carb to match flow in each barrel on that side. Leaving one on each side closed. I don’t remember the turns there. Maybe 1-1/2 on one barrel on each side.

Those screws are smaller and have a lick nut on them.

There are 2 idle mixture screws per carb near the bottom and 1 idle speed screw per carb.

Yes. Speed screw is the one on the shaft that opens the plates. Just one per carb. 1 per barrel of each: bypass screw and mixture screw. I will add a picture with all that labeled. That drawing is an idf. I have idfs.

[malcolm2]

top 1/2 of PAGE ONE of http://www.redlineweber.com/html/Tech/idf_...nt_controls.htm

I will add your suggestion on the throttle plate measurement. In that case, how do you adjust one if the measurements don't match on one carb?





This is a strange document..... page one and page 2 say pretty much the same thing. But this one does have the diagram.

[930cabman]

I have always closed the air bypass screws and never used them for adjustments.

[malcolm2]

I found these posts from other forums.... this is how i set the bypass screws.

[930cabman]

Personally the twin Webers I have worked with have been close from barrel to barrel. I'm not sure how exact this needs to be? Within 1/2 on a snail gauge works for me.

[malcolm2]

3&4 are one carb?
Not sure how you are numbering

the cylinders are numbered on the tin.

1 carb is on the 1-2 cylinder side, one carb is on the 3-4 side. each cylinder has a barrel in the carb, and bypass and mixture screws.

[malcolm2]

as for this statement about JETS..... i reported the mixture screw settings and 3 seems wonky. 4 is only a 1/4 turn more than 1 and 2.

This would not be suggesting that you use different jets on different barrels, right?


I am thinking I should start the whole process again as per the Best Lean Idle doc. I have done it about 4 times, I am getting good at it, or at least I thought I was, until I see these mixture screw results.

[Superhawk996]

You’ve got something a bit wonky. All 4 idle mix should be pretty close to each within about 1/4 turn to the others assuming all cylinders are pulling equivalent airflow and one of the air bypass screws hasn’t been opened too much.

Side 3/4 is different enough from 1/2 that I’d suspect both sides might not be closing the butterfly plates the same or possibly that you just have the idle speed screw turned in just a little bit too far (back to that 1.3mm delta between screws).

Over tightening the nuts that hold the end linkages to the throttle shafts can cause binding. Again, for me, this usually this gets checked on initial bench setup. I know your carbs are brand new and probably OK but I’m starting to lean toward recommending you pull them again for a good bench check now that you have a couple other things you’ve learned and can check for.

Before you do - I’m with you - go ahead and do another best idle mix. Practice makes perfect. I wish there were a magic way to tune carbs but in all honesty, you’re right on course as a beginner. I takes a while to get the hang of it. It’s all about iteration, knowing each adjustment affects the others.

Also I should have asked but assumed all 4 plugs were pretty similar to the one you posted?

See where you end up after another go-round. You made a good call ordering the other idle jets - very easy to swap and even if you ultimately end up back at .50’s, the experience of trying them, observing the changes, and how idle tuning affects drive ability is worth the price of admission into the mythical realm of carb wizardry (IMG:style_emoticons/default/laugh.gif)

[malcolm2]

I did not pull the other plugs to see how they looked. I will do that.

On the linkage, remember, I have the cable type with pulleys.... SYNC-LINK.

https://lnengineering.com/sync-link-throttl...arburetors.html

I did notice that if the nut holding the pulley is tight, the shaft does not rotate easily and smoothly. There is a tab type lock washer under that nut. So i get the nut snug and check the rotation. Snug a bit more, check again. Once I feel it affecting the throttle plate movement, I back off the nut and bend the tab. So I feel that the pulleys are snug, but not tight.

[Superhawk996]

A note of encouragement.

Weber’s are among the most tunable carbs in the world. There are so many things that can be changed - venturis, idle jets, main jets, emulsion tubes, air correction jets, accelerator pump metering, accelerator pump nozzles! Not to mention the air bleed screw, mixture screws, and the idle speed.

This makes them highly adaptable to almost any engine and it’s why they were used so widely back in the day by Porsche, Ferrari, Alpha, Lamborghini and so many legendary cars!

The down side is that they can be overwhelming at first. Hang in there - you’re doing great as a beginner!

[Superhawk996]

I did notice that if the nut holding the pulley is tight, the shaft does not rotate easily and smoothly. There is a tab type lock washer under that nut. So i get the nut snug and check the rotation. Snug a bit more, check again. Once I feel it affecting the throttle plate movement, I back off the nut and bend the tab. So I feel that the pulleys are snug, but not tight.

Perfect. You’re aware of it and managing it!

[r_towle]

3&4 are one carb?
Not sure how you are numbering

the cylinders are numbered on the tin.

1 carb is on the 1-2 cylinder side, one carb is on the 3-4 side. each cylinder has a barrel in the carb, and bypass and mixture screws.

3/4 carb setup does not match 1/2 carb
Thus the wierd idle and off idle issue