Small Engines (Lawn Mowers, etc.): 99 craftmans 10.5 ridingmower, shaft seals, throttle shaft

Question
Mower starts & runs fine when blade is engaged  but when it sits & idles it revs up & down & backfirers.Is this a sign of a dirty or plugged carburator.                                       Thank you.

Answer
Tim, sounds like you have a leaky carburator!  This could be no more than the bolts holding the carb to the intake on the engine are loose (the gasket here is very sturdy and is likely ok)  or the carb could be worn at the throttle plate shaft.  You can check the throttle shaft just by wiggling it it really shouldn't have more than 1/32" play (find the choke plate shaft at the entrance to the carb, the throttle shaft is between the choke and the engine intake).  To much play means that at the idle position it is sucking air through the shaft seals in the body of the carb and leaning out your mixture, causing engine surging.  This often occurs on engines that are left idling for long periods of time. A simple test is to manually apply grease around the outside of the carb, when the surging stops you found your leak!  Hope this helps!