Volvo Penta AQ131D 4cyl. Last October was the last time this boat was running, at that time the only issue it has was a backfire when I throttled the engine up slowly, if I did it quick no problem. Well that last time out, I took a few waves which ended up getting into the gas tank which was apparently full of holes on top in the back end. Got towed back because the water entered the tank. I vaccumed the water out of the intake manifold, and pulled the plugs, cap, rotor, and replaced all. The boat sat since Oct. I had the starter rebuilt while it sat. I started the boat with a new above deck tank I installed last week. It turned over ok, but did not fire. Today, I got it to fire by opening the throttle and turning the key, it ran smooth and steady, when I put the throttle back of course it backfired all over the place and died, restarted with no problem and idled ok, what I noticed was the outdrive vibrating like crazy, but the motor looked fine. I hooked the timing light to it and it was at 6 degrees at idle. Cannot figure out why the drive is vibrating so bad, it smooths out considerably when I move the throttle forward and let it run at a higher RPM. I need help with what I should do to, 1, fix the backfire, and 2 stop the vibration. Could the vibration be caused by a blocked exhaust manifold? I had water coming out the exhaust, but when I opened the plug on the manifold, there was no water coming out, just black rust.
