Re: Carburetor problem?
Posted: Wed Nov 11, 2015 5:26 pm
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.
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.
