sounds like your engine has excessive crank pressure, or blow by (combustion gas leaking past the piston rings). Rent a compression tester from a parts shop and check compression on every cylinder. Whether the number is high or low doesn't really tell you much... what you need to look for is inconsistency. See if one or two cylinders are low on compression more than the others.
If a simple test doesn't reveal any major inconsistencies, rent a leak-down tester. This tool puts air in the cylinders via a regular shop air compressor, then measures the amount of air leaking out. This can reveal valve seal leaks, head gasket leaks, or piston ring leaks. If your piston rings are worn, you will also hear hissing air inside the crankcase as it leaks past the rings.
Another simple test you can try is a engine vacuum test. Hook up a vacuum gauge to full manifold vacuum on your carb or intake, and what the needle. If it is not smooth, if it's sticking or jumping, you can suspect a combustion loss in one or a couple cylinders.
here's a handy guide
http://www.youtube.com/watch?v=NrnzD7PgoHI
You should also check for vacuum leaks through your intake system (carb, charcoal canister, vacuum hoses and lines, etc...). Clogged lines can cause weird vacuum pressures inside, causing excessive gas venting from your crankcase