here's the cumulative affect, this is on a gcv190 honda mower. exhaust valve in cylinder caked up and wouldn't seal, got flame out the exhaust on hot restart and died. Then wouldn't restart until cold. And then if i got it started it was with the carb mix screw out 4 turns and engine would only idle for 20 seconds until it got hot, could not give it any throttle or would die. had to pull entire engine apart on the gcv type to access the valve seats. the exhaust valve itself was mostly clean, the problem was the cylinder not the seat face but the outer lip just past the 30° or 45° angled seat where too much carbon built up preventing the valve from seating and sealing, the light valve springs in this kind of engine probably does not help either. less than 3/4 gallon of gas with hp ultra oil at 40:1 was used, which was then mixed again with regular gas in mower tank to make about 70:1 to 100:1 in the mower, never any smoke engine ran fine until the last restart.