engine races

[jesmed]

OK, here's the final answer. LM, I tried every possible adjustment on both left and right carbs (as I have done several times before). No matter what, the "racing" problem persisted unless I shut down the left idle air screw completely, and even then it didn't entirely go away.

So I took the left carb off, stripped it of ALL the removeable parts (jets, float, slide, etc.) and installed them in another used carb I just bought on eBay. And to test my theory of air leaking past the slide, I checked both carb bodies by shining a flashlight through from behind. I could see light leaking around both sides of the slide in the old carb body, but when I put the same slide in the "new" carb body, no light leaked around the sides.

When I installed the "new" carb (with all the same components from the previous carb, so at this point only the casting is "new"), the racing problem disappeared and I was able to set the idle air mix screw to a normal setting with no adverse effects.

So there's no other possible conclusion but that the carb body itself was causing the air leak because of excessive clearance at the sides of the slide, and I proved it to myself by the flashlight test. The original carb body allowed light to "leak" around the slide when lit from behind; the "new" carb body did not allow light to leak around the SAME slide, and that new carb body eliminated the racing problem.

Thank you all for your thoughts on this perplexing problem. I never want to see the insides of another carburetor again! ---Jesse