Small Engines (Lawn Mowers, etc.): 318 John Deere lawm mower, 318 john deere, piston rings

Question
Why are you getting blow bye on a onan 18 horse engine with a  new overhaul and good compression after it is warmed up?

Answer
Hi Nick,

The definition of Blow-by is occurring when:

Unburned  fuel & other elements of combustion are allowed to escape past piston rings & into the crankcase at the last point of the combustion stroke.

This in turn forms a type of sludge inside the crankcase, and it corrodes and hastens wear of the pistons, piston rings, valves, bearings & assorted internal parts of the engine.
Unburned fuel is also carried into the crankcase.

Premature oil leaks can also develop.    Excessive blowby gases are caused by worn rings or scored cylinders.

I would check with the shop that overhauled your engine, to see what they have to say.

Good luck,

Howard