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Curtis B.
Curtis B., Technician
Category: Small Engine
Satisfied Customers: 28367
Experience:  have worked on and around most engine models for 35 yrs./Polaris ATV expert
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My 8 hp Briggs and Stratton engine in a Simplicity riding

Customer Question

My 8 hp Briggs and Stratton engine in a Simplicity riding mower lost compression and
performance overnight while unused. Compression became 35 psi. Max speed was reduced to about 900 rpm with insufficient torque to run PTO and blades.
I replaced the head gasket and spark plug. I noted that the valves and valve seats appeared to be in good condition and operating normally when the flywheel was rotated manually.
Carburator jets were cleaned and settings verified. Throttle and choke plates
appear to be operating normally.
Can you suggest probable cause of failure and remedy?
Submitted: 1 year ago.
Category: Small Engine
Expert:  Curtis B. replied 1 year ago.

You did'nt check the valve clearances when you had the head off? The valves are not adjustable without grinding off the tips. Clearance needs to be around .006 inch.

Expert:  Curtis B. replied 1 year ago.

Compression is a result of piston rings and valves seating, and head gasket!

Customer: replied 1 year ago.
Curtis: Your response is logical but does not explain the failure occurring overnight. What could happen during
that brief period of inactivity to account for such a drastic change in performance? -- Bruce
Expert:  Curtis B. replied 1 year ago.

Valve may be sticking and is not fully closing. That would be my thought.

Customer: replied 1 year ago.
I can understand that a failure can happen at any time, including when a machine is stopped or started, but what
is the probability?
Expert:  Curtis B. replied 1 year ago.

Not that unusual!

Expert:  Curtis B. replied 1 year ago.

did you find the problem?