Reply To: Backfire/misfire thru carb

[Don]

OK, well you have hit on another possible cause for a funky idle. The two piece throttle advance lever stays rigid until the sensor stop hits the rubber WOT timing stop. Once this happens, the lower portion of the arm is able to continue its travel forward, under spring tension, to advance the carb butterflies to their WOT position. Perhaps the two piece throttle arm is binding where the two pieces are joined together and swivel, there should be a plastic bushing between the upper and lower throttle arms. If the two pieces are binding and not able to rotate freely/independently (except for the spring pressure which is great), the symptoms will be:
The engine will idle and accelerate normally, the problem develops when the throttle is retarded from WOT. Because the pieces are binding together, the timing will retard first even though the carb butterfles are still supposed to start closing first. Once the throttle is reduced, the timing will be retarded to idle even though the throttles are partially open. The engine will spit lean and probably die. This issue will not cause the engine to surge though, I’m guessing you have a partially plugged high speed jet or an ignition issue.
I like the idea of mixing at 32:1, this jet application is a quite a load. Using the premium fuel is a great idea also, that old 140hp powerhead is relatively high compression, and probably won’t last long on 87 octane…