It could be carbon caked on the piston tops either glowing or causing the compression ratio to go way up.

Rev the engine up and slowly drizzle some water down the intake. When the engine slows down, stop and let it catch it's breath. Do this with about a beer can full of water and it should knock a lot of the carbon loose. DON'T do it all at once or you can bend a rod, just drizzle the water in slowly.

If you have that much carbon, you can bet that the catalytic converter is plugging up and you have oil control problems.