Nasty crankcase backfire

[upnorth4951]

Here's a one I've never heard of before. Went to fire up the M-600 with the VH4D engine after using it yesterday. Cranked it no choke for 2 seconds and didn't fire up so choked it and she proceeded to blow out the dipstick and blew the oil filler cap clean off. Sounded like a firecracker lol. Could it be a timing problem or the weird crank case breather gong to the carb intake elbow? Have owned this machine for a couple of weeks and has been starting fine. Any ideas ? Thanks!

 

[rodbuilder]

Sounds like you have a SERIOUS compression loss in one of the cylinders. How else would the fuel/air mixture get in the crankcase.......................BUT it that where the combustion took place? As a former engine builder I'd say NO, because the spark plug is obviously in the head! Which brings me back to having a really bad set of rings on one cylinder, or even a hole in a piston??. Stick a fork in me - I'm done!!!!

 

[upnorth4951]

Sounds like you have a SERIOUS compression loss in one of the cylinders. How else would the fuel/air mixture get in the crankcase.......................BUT it that where the combustion took place? As a former engine builder I'd say NO, because the spark plug is obviously in the head! Which brings me back to having a really bad set of rings on one cylinder, or even a hole in a piston??. Stick a fork in me - I'm done!!!!

Thanks for the reply rodbuilder! That makes sense but I'm hoping it has something to do with the flooding problem I have had. The guy I bought this old 600 from said he put in a carb kit because it flooded so bad and changed the oil before he sold it to me. Since then I have pulled the carb twice cleaned er up and reinstalled it hoping the new gas filter and hose will eventually fix the flooding problem. The crankcase vents directly from oil fill tube to the carb intake elbow so may have had some crazy ignition in the oil fill tube. Anyway I'm hoping I wont have to yank the engine.