I have a customer who has a fresh longblock 572. When he decels from a plane (its one of two in a 36' cig. gladiator), he says it backfires; and blows off the oil cap. The back of the boat is covered in black soit. It has demon 1050s? It sounds to me like it has too much crankcase pressure- but why would it only blow out the cap on decel? The carb hasnt been touched since the last 572 was in. Maybe the needle isnt seating on one of bowls? The base timing is set at 24*. There is only 1.5 hours on the engine so far. Any thoughts??

 

Think Ive got it- Main jets were waaaaay to big. Gas was passing the rings and exploding in the crankcase. The boat was jetted rich @ sea level. We are at 6500ft.

 

Had a Chevelle BBC that I HAD to get running in the middle of a Wisconsin winter. Guess I had too much gas in the crankcase, it backfired and blew a valvecover right off! I wasn't laughing at the time, 20 years later and now it's funny.

 

Went down 8 jet sizes- per demon. See what happens.