Huge blowby, but good compression?

[Piginabag]

I have a 1863 from the early 90s; she’s always worked hard, and I’ve broken one of everything - except the bulletproof Koehler Comnand engine.

Till now. It has a massive amount of blow-by. Unlike anything I’ve ever seen before - inside the air filter is coated, plugs foul, smokes, runs rough. So bad I figured I broke a ring or piston.

But, both cylinders test 120 psi compression.

I wondered if a clogged air filter could suck oil through the breather with negative pressure - but I replaced the filter, and “negative”.

So how can an engine with good compression, that otherwise runs well, just GUSH oil through the crankcase breather?

Thanks!

 

[mgonitzke]

A leakdown test would tell you whether you have a massive amount of blowby or not. If not, I'd suspect something in the breather reed valve is malfunctioning.

 

[lelder]

Pronblem is breather gaskets, if i had motor break down could tell ya what, "CSR" had sane issue.

 

[lelder]

Think there's a one way valve in block where tube goes to air cleaner, my cc had a brigs motor but same problem, buy separate from gasket kit.

 

[Piginabag]

Thank you guys for your replies! Ive discovered a few more things, and I feel a little embarrassed that I didn’t notice them earlier.

Turns out the oil level was a little high, and it was a little thin. Figured out that one coil was sparking weakly intermittently - when it wasn’t sparking, of course it was leaking gasoline past the rings into the crankcase.

So it wasn’t just oil blowby making the engine run rough, it was a bad coil (I hope) making the engine run rough, which dumped gas into the crankcase, which made more oil blow-by, which then made the engine run even worse!

I didn’t know about the breather reed valve till you guys told me. I’m going to start with coils, plugs, and an oil change. If there’s still an issue, I’ll dig into that breather. Thanks again!

 

[jstich]

Save a lot of time and money, start with the breather, replace the broken reed, been there done that!!!!

 

[Jeff Albring]

I also have an 1863 cub that I want to inspect breather filter and gasket. Is this located directly beneath the carburetor?