A simple check is whether you have too much oil. That would cause your problem.
Piston rings/head gasket/valves can be diagnosed collectively with a compression test. The hillbilly test is simple: pull a spark plug and block the hole with your thumb. Good compression will blow your thumb off the hole when the engine spins.
If all else works as it should, you've probably got a clogged crankcase breather. Without knowing the exact engine, I can't tell you for sure where it is: 18 horse probably connects to the air filter and runs down to the engine block/valve cover.
Ben W. in VA