Sounds like detonation. Try to purchase gas where they don't use an ethanol additive, or use the highest octane rating. Remove the spark plug and inspect it. Look for cracks on the insulator.
Thank you. I had changed everything on the lawnmower, and was having problems with my lawnmower stalling. I had adjusted the idle speed screw and the idle mixture valve; but nothing worked. Then I read the post about the gas cap. Wow! The light turned on. I had lost my gas cap. When I took my lawnmower apart, I found my gas cap hid under the dash. I had started using my gas cap again. I immediately went outside and removed the gas cap. I mowed our front lawn without the mower stalling. Thank you! My next option was to call a mechanic. I had about decided to buy a new lawn mower. I can not believe it was so simple.
After reading another post about stalling in here, I think I found the answer. Mine (16 hp) would stall the same way, after about 20 minutes. I loosened the gas cap and it kept going. I think the vents on the cap are clogged, which prevents the gas from going through the line. I will clean those or get a new cap.
I have the exact problem! Old craftsman which I get about 20 mins of mowing and then its starts stalling. I increase the throttle and it will normally go another 30 secs but will stall. I have to let cool down for hrs before mowing again. I have been able to increase the mowing time by using the lowest possible throttle position but will eventually stall.
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