OK, well your first hunch is the best idea, try to find another carb to try….You must have something similar laying around the shop…Even an old carb with two needles…I know the linkages won’t be right, but who cares just to try it at idle….
What is the history on this engine? What did customer report when he brought it in? Was it running OK, then stopped idling? Or, hasn’t it been used in awhile….
You mentioned something about altering the cam/throttle pick up….Not quite sure I understood which direction you altered it….First, make sure throttle plate is completely closed at idle…This is a very confusing topic, did you adjust the cam to start opening the throttle sooner than normal? If so, you actually made the engine run leaner off idle…I know, I know, doesn’t seem to make sense, but is true. So, in order to make the mixture richer off idle, you would actually delay/retard the throttle cam/roller pick up from its normal setting. You mentioned the possibility of the carb casting being "worn out", well this is the area that would wear out…A worn throttle shaft allowing air to sneak by, perhaps a bent/binding throttle plate that is not closing properly….The only way to evaluate this condition is to remove the carb and look in through the back to see if the plate is closing completely…Wobble the throttle shaft to check for excess wear.
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