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ranwell1, Small Engine Technician
Category: Small Engine
Satisfied Customers: 10304
Experience:  Owner of Large Small engine repair business. Over 39 years. Certified both 2-cycle and 4-cycle engines
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My engine(###) ###-####E1 stopped and now has a knock. Turning

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My engine(###) ###-####E1 stopped and now has a knock. Turning it by hand leads to resistance once at the same point in each revolution. This can be overcome by hand but becomes a knock if the engine is turned by the starter. The knock seems to come from the bottom of the engine, but there is no play on the output pulley. The compression is 30-35 psi. Before the engine stopped it seemed to be running faster than usual; I wonder if the governor mechanism is causing the trouble. Any ideas?
The governor will not cause this problem.
YOu need much more compression than 35 lbs/ It should be at least 85 to 90 lbs.
You may have carbon build-up on the piston causing the knock. You could have a connecting rod that is loose. This could have been caused buy the engine not having enough oil at some time in the past and caused the engine to die and be damaged.
In any case, this doe not sound like it is a very simple issue.
This may be something that a good mower repair shop may have to look at.
Please let me know if there is anything else I can do to help.
Please give me a good rating as this is the only way that this site pays me for my time.
ranwell1 and other Small Engine Specialists are ready to help you
Customer: replied 2 years ago.

I dismantled the engine and found that two small screws on the throttle butterfly had broken free and had been sucked into the engine. One then became embedded in the piston, and this caused the knock. The absence of the screws was presumably responsible for the over-fast running. The low compression might be due to the use of a 50-60 year old gauge! Anyway, the engine is running quietly again.

Well, it wasn't carbon hitting the piston,as I suggested, but the screw will do it too. You are lucky that it didn't break the piston.
AT least you found the problem.
Good luck and thanks. Feel free to contact me again if you need more help.