Weber mixture screws Options

[dumbjim]

Hi all, I spent yesterday fooling around with my weber idf 40's. by following mrbubbleheads instructions. With my snail, I got all four barrels pulling the same by adjusting the air bleed screws. I then moved on to the mixture screw. The mixture screws have absolutely no effect on how the engine is running. I can back them out or screw them in and there's not change in the idle. Is there something else going on here? In general, after adjusting the bleed screws, the engine is running pretty smooth. Any ideas? Thanks for reading.

[johnhora]

Have these been rebuilt recently?

[rfinegan]

sounds like your idle speed is high and off the mixture circuits. Reduce the speed and try the screws again

[rhodyguy]

How much are the ABS backed out? CB book on pre-run setup states "Set them 1/2 turn off the bottom". So, reset. Set the idle speed screws achieving .004 clearance between the leading/forward butterfly and Venturi. Then the idle mix screws at 3 turns out from gently seated. This will more than likely result in a high idle. Don't trust your linkage. Back the pass side idle speed screw just off the stop. Now measure flow. Adj linkage as nessassary.

[Ansbacher]

Be careful adjusting those air bleeds to get the barrels sucking even. You might balance things out per the snail, but if too much, it will cause leanouts and erratic, popping, bucking performance. Use them minimally as a last fine adjustment.

Ansbacher

[Charles Freeborn]

There are a LOT of factors in proper jetting, synchronizing and adjusting Webers. As said, adjusting air bleed screws can be masking other issues. Also as said, you could be above the idle circuit's range so the mixture screws are ineffective.
As far balancing the flow(s) first check the top end is healthy with a leak down test. If one or more cyl's has an issue it will draw less. Then look for vacuum leaks in the manifolds, carb bases, etc. . Any leaks below the butterflies will throw off readings. This can also be butterfly shafts, etc.
Double check fuel pressure. Too high can overdrive the needle valves and cause the floats to over fill / spill / over rich. For webers it should be between 2.5 lbs and 3lbs.
Then double check the idle jets themselves. They are the smallest jets in the carbs and plug easily. Then do a proper sync with linkage removed. Then and only then adjust air bleeds. One more item - the "mixture screw" is actually a misnomer. It adjusts the volume of air/fuel to the idle circuit, not the rich/lean ratio.

[dumbjim]

sounds like your idle speed is high and off the mixture circuits. Reduce the speed and try the screws again

I had the idle at 800 rpm

[914043]

I don't know if the 40 idf weber are set up the same as 6cyl idta but the air correction screws are interchangeable with the fuel mixture screws rendering the mixture screws non adjustable. Just something to check ask me how I know.

[Charles Freeborn]

sounds like your idle speed is high and off the mixture circuits. Reduce the speed and try the screws again

I had the idle at 800 rpm

Take it to a proper mechanic. This is above your pay grade.

[rhodyguy]

I don't get the 800rpm idle thing at all.

[Ansbacher]

Take it to a proper mechanic. This is above your pay grade.
[/quote]


Bad advice!!! You will never learn anything and in all probability it will not be fixed correctly anyway. Dig in, scour this site and the rest of the internet and you will find you can do this stuff yourself. It doesn't take a Werner von Braun rocket scientist.

Ansbacher

[dumbjim]

Thanks for your comment Ansbacher. How am I gonna learn if I don't try and do it myself. If I screw it up, I can always pay an "expert".

[Charles Freeborn]

Thanks for your comment Ansbacher. How am I gonna learn if I don't try and do it myself. If I screw it up, I can always pay an "expert".

Okay, here's an "expert" sequence. Do each of these in sequence or all bets are off.

1. Fully understand the design of a Weber carbeurator. I suggest this book:
https://vwparts.aircooled.net/Weber-Tech-Ma...tech-manual.htm . It is by no means the last word, but it is a good starting point. There are 3 basic circuits in a Weber. Idle, mid and WOT (wide open throttle). Each of them has jets to control the fuel mixture in each stage. They overlap and must be in synch or the system does not work to spec.

2. Confirm top end health (that's rings, valves, etc) with a leak down and compression test. (as said in my previous post ). Much more important than compression is leak down as it can inform you of where ( rings, valves) the problem is.

3. Confirm you don't have vacuum leaks anywhere else such as intake manifolds, carburetor gaskets to manifold gaskets, etc (as said in my previous post )

4. Make sure the carburetors are properly clean and adjusted - jets clean, fuel pump pressure correct, etc .(as said in my previous post ).

5. If the problem (idle mixture still non-op) persists then go back through the previous steps and find the fault.

6. If you still can't find the problem you need to consult someone who has more knowledge than you. If your idle mixture screws have no effect then there is a problem. In a healthy engine /carb set up if you seat the idle mixture screws (all the way in) the engine will not run. Period.

I can't sugar coat it any more than that. If you have tried all of the above steps, I applaud you for trying, but maybe you're lacking sufficient understanding to trouble shoot this issue.

[IronHillRestorations]

I’m thinking @914043 could be on the right track. Having swapped mixture and air balance screws in my very first carb reassembly 35 years ago, it can happen

[rhodyguy]

One has a jam nut and one doesn't. How could they get mixed up? Fwiw, if running the idle air mix screw in to gently seated with no effect on the idle there may be a problem with the idle jet for that Venturi. Every time you make a adj you need to wait for a bit. The change will not be immediate.