IDA tuning help - Lean spike

[Gaz64]

Originally Posted by markha

OK... So I call me crazy, but I could not resist trying some 44 mm chokes today. I know... major changes. But the idle circuit mixture did not change much. A little leaner... I assume that is do to flowing more volume which seems a little strange at idle. I tried leaving the mains out and running on the idle stack. Everything looked pretty good. Right about 13 - 14 AFR. Then I through in the mains, and I tried a bunch of different main jets but could never get the thing to even run on the main circuit. Always way to lean. Any ideas for a starting point for 44 mm chokes for mains and air correctors?

Thanks,
Mark

Mark,

Temptation is hard to resist sometimes,

A venturi change would not affect the airflow your engine uses from idle to 2000 rpm.

Jumping from 37mm to 44mm will certainly affect the main system startup hence your lean condition.

I wouldn't be staying with a choke of 44mm personally, you may find that you'll never get around a lean condition between the end of transition to start of main, irrespective of what emulsion tube/main/air corrector you use.

A 2mm jump in venturi diameter would be considered the norm, then 1mm steps to zero in on optimum diameter for driving style.

Here is my suggestion to you:

60F10 idle, 100 holder
55 pump jet
35-50 pump bleed
F7 emulsion tube
160 Main
180 - 190 Air
38 - 40mm choke.

What's your rev range?

What's your timing at idle and 3000 rpm?

If you block off a choke completely to one cylinder does it still idle ok? or does that cylinder go away? (overrich condition)

In other words, is that cylinder receiving more than 50% of its air from the "plenum"?