i have a 190cc 6hp flathead briggs quantum .for a long time i thought i had a exhaust valve going bad or a faulty ignition module.i really thoght the spark plug wire was worn and jumping to ground occasionally.i replaced the ignition module with no improvement in performance.then one day it ran perfect for my entire mowing.i always have fresh gas treated with stabil.gas is always a no ethanol blend and fresh.i now wonder if i had a small air leak on the intake side somewhere.my guess is the o ring between the carb and the block is faulty. (hard or brittle). the mower is ten years old with low hours.is this o ring ever a issue with anyone else? any other possibilities? thanks for any input.