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Join Date: Jan 2011
Location: Central Illinois U.S.A.
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The vacuum line goes to the air cut valve on the side of the carb. The air cut valve shuts off air to the idle circuit during deceleration (enrichening the mixture) to prevent backfiring in the exhaust. The valve is spring operated and relies on vacuum upon throttle closing to close. If the air cut valve holds vacuum, it is ok. If the air cut valve does not hold vacuum, then it is open and may cause a rich mixture at idle.
Also, I presume that although you removed the air filter you have the top on the air box? If the crankcase has filled with gas (may be caused by not closing the petcock when not in use) the crankcase vent line will supply fuel rich air to the air box causing poor idling and plug fouling. The problem may worsen as the oil heats and the fuel vaporizes out of the oil at an accelerated rate.
Don't forget that a tight exhaust valve can cause an appearent over rich carb problem.
Sometimes the carb just needs to be replaced.