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Small Engines (Lawn Mowers, etc.)/Engine backfire and shearing flywheel key


I have a Craftsman 46"cut lawn tractor model #917.28852 powered by a 21 horsepower Briggs & Stratton single cylinder OHV engine model #331877, type #2371-G5. It ran and mowed fine the first year and for several months this year. The last time I mowed it was running fine when I parked it in the barn, but when I tried to start it about a week later to mow again the engine wouldn't turn over. I had the battery tested and it was OK. I took it to a repair shop and he found that the valves were out of adjustment and it had sheared the flywheel key. He adjusted valves and replaced flywheel key.  It started and ran fine, but when he turned it off, it backfired and sheared the flywheel key again. He repeated this procedure several times with the same result. What could be causing this?

Bent push rod(s)?  You can remove each push rod, one at a time and noting which end goes inside the engine, and then roll each one on a piece of glass to see if the rod(s) is bent.

Excessive carbon build up in the valves?

While rare to shear flywheel keys on rider engines, I would use a little lapping compound and lap the flywheel to the crankshaft.

Do you know what the valves are being set to (the gap)?


Small Engines (Lawn Mowers, etc.)

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Eric A. Jones


Lawnmower Repair . Certified Master Service Technician from B&S. Have 23 years experience on B&S, Lawn Chief, Weed Eater, Echo, Peerless, Wheel Horse, Snapper, Atlas, MTD, McCulloch, Homelite and many other numerous brands. Specialize in electrical repair.


Born and raised in the midwest. Started tinkering with engines when I was about 14 on my Suzuki RM-80. I began lawn mower repair at a small hardware store. I knew absolutely nothing. I read lots of repair manuals and met an older fellow who taught me many lessons. I continued working on small engines through high school and paid my way through college working on mowers at the same hardware store. Decided to get away from the midwest and mower repair so I joined the Air Force. I repaired air traffic control electronic equipment and ended up in Hawaii where I got a part time job at Small Engine Clinic. I gained a lot of experience from the Small Engine Clinic and had a blast repairing small engines. I then took the Briggs and Stratton Master Service Technician test and earned my MST. I then traveled to Wisconsin where I attended the factory update training seminar and received formal training. Continued working on mowers part time as I completed 20 years of military. Retired from the military on a Friday and continued in the lawn and garden industry the next Monday.

MAS Aerospace Operations BA Mathematics AAS Electronic Communications AAS Electronic Technology

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