Here it goes. Bought this new to me motor and fired it up on the test tank. I'm getting a backfire that seem to be coming between the flywheel and block. Any Ideas or am I screwed. Its not constant but every 15 seconds or so. 1991 merc 9.9.
I guess it could be a bad upper crankshaft seal. Are you seeing some sort of sign that it is coming out underneath the flywheel? It could be your oil recirculating hose that has a crack in it, or even blown off it's fitting. Your idle needle may need to be reset or even the timing advance and throttle pick-up may be out of sync.
