Carburetor problem?

[barryh]

I use a lean idle mixture setting and have no problem with hang up but it's true that excessively lean idle mixtures will cause the problem if you then compensate with a a larger throttle opening on the throttle stops. What that does is bring the edge of the butterfly too close to the transition ports which then start to flow mixture when they shouldn't and that causes the idle hang up. The object is always minimum possible throttle stop settings for a given idle speed. i.e adjust the mixture screws for max. idle speed and then lower to desired target with the throttle stops.

Engines with hot cams can suffer from this problem. Because they are so wasteful of mixture at idle due to more valve overlap they need a larger throttle opening which again can bring in the transitions ports. The solution is to bypass some air around the butterfly by drilling a small hole through it. That allows a smaller throttle opening which moves the edge away from the transition ports. Not suggesting anyone does that but it helps to explain the mechanism at work when carbs are the problem. It follows that you are marginally less likely to suffer the problem if you tend towards larger valve clearances which will reduce overlap resulting in a reduction in mixture requirements at idle.

[boman]

Thanks for all your help and effort. It seems to be an enormous knowledge in this forum.

I have fixed the "advance mechanism". The springs where OK but it was quite dirty. Since I have to remove the whole "cup" (pointers and stuff) because the advance mechanism sits behind the pointers I have to readjust the ignition and also adjust the carbs... But anyway the idle seems quite steady now. What I need now is a strobo light to set the ignition.

Again, thanks a lot and I hope to help someone else here at some time.

Kjell-Ake
Sweden

[Major Softie]

Yay!

[ME 109]

Bowman, do you know of my website, W6REC.com? Google is your friend. Had you just used Google, you would have had the answer.

Duane, I just googled it and got you telling Boman to google it.