Oil’s running down the carburetor

Question:

I’m restoring a 1966 Chevy C20 pickup truck with a 283 cid engine with a two barrel carburetor and an oil bath air cleaner. The engine was rebuilt nine years ago and has gone 7000 miles but it has a breathing problem. Since the rebuild, the engine pushes oil from the air cleaner and it runs down the front of the carburetor from where the air cleaner connects at the top of the carb. The PCV valve and hose are in good condition.

The air cleaner has no leaks. However, during quick acceleration from 15 to 30 mph the engine air intake is not steady.

I’ve been told the problem is with the intake manifold gasket. If so, how can I verify it?

Answer:

Oil coming out of the carburetor most often indicates that there’s excessive pressure inside the engine.

This excessive pressure is usually caused by compression ring blow-by. When the ring-to-cylinder-wall seal is poor compression pressure escapes on each compression stroke and pressurizes the crankcase. This causes oil-laden air to find every means of escape from inside the engine.

Your diagnosis should begin with a basic evaluation of the engine, including a cylinder leak down test and measurement of manifold vacuum.