I have a 318 john deere lawn tractor with new pistons installed. After engine runs @ 15 to 20 min under load oil comes back out breather and goes into carb. Causing engine to choke out. Unit has new pistons .010 over sized

Hi,
Just going to make a guess here! First let me say that what you have going on is an imbalance of crankcase pressure that is forcing the oil into the carb through the breather. The crancase is pressurizing. Your engine is an opposed twin valve-in-block engine. Because of the time you say that it takes for the problem to appear, I am going to guess that the valves are adjusted too tight and the heat that develops over that running time causes one or more to not quite shut or do it incompletely and causing your crankcase pressure problem.
Peace,
Clarke

Hi,
Just gonna make a guess here! First let me say that what you have going on is an imbalance of crankcase pressure that is forcing the oil into the carb through the breather. The crancase is pressurizing. Your engine is an opposed twin valve-in-block engine. Because of the time you say that it takes for the problem to appear, I am going to guess that the valves are adjusted too tight and the heat that develops over that running time causes one or more to not quite shut or do it incompletely and causing your crankcase pressure problem.
Peace,
Clarke

Thanks will go back and check, valves were set to spec after putting in new pistons but could have adjusted to tight. Also could the idle speed have anything to do with problem also?

Hi,
Spr\eed will usually only throw a rod.
Peace,
Clarke