What is nice about having two engines is that you can switch one component at a time from one engine to the other and see if the problem jumps over to the other engine. From his posts I guessed he doesn't know what engine is doing it yet.

Mike, you've probably got Rochester Quadrajets for carbs. Check a couple of plugs that are reasonably easy to get to and check to see if they are running lean. I'm guessing it's somewhere in the ignition system.

I tend to agree with the coil going bad idea. It might be just starting to break down a bit as it gets hot.

Except for wet sanding on the transom, got Outlaw finished up this afternoon. Gonna crank the motors over tomorrow on the hose and double check everything then try to drop it in at the local ramp for a little test run.

I would agree with Russ on this as well, check for the simple things first, plug wires, ignition wires, hell even a bad ground can cause a backfire. If your losing spark, and then reconnecting its lighting too much fuel. Especially if you just repalced the motor, go over it and check all your connections. I think we all tend to think worse case scenario right off the bat.

I had a situation on my dually where both my low beam lights went out at the same time, I was convinced it was something serious, as the bulbs looked fine, drove for a month blinding people, then while at one of my GM dealers, had them check it, sure enough, both bulbs were bad. I quoted Homer Simpson...DOH!!!

Check the timing on the offending engine when you find out which one it is. Check the distributer cap for carbon tracks. Also check your coil for carbon spots from the negative terminal to the coil wire. 4500 RPM is kind of vanilla to float a valve so I don't think this is the problem. As mentioned in a post above if it is too lean it could cause a backfire thru the carb. if all checks okay and the problem still persist hook up a vacuum gauge and make sure things are smooth. If you have a burnt valve it will show up here.

I had the exact same thing happen years ago to my 1979 Coupe de Ville. Maybe we are slamming our hoods closed too hard?

[QUOTE=offshore-drillin] I think we all tend to think worse case scenario right off the bat.

It's a boat. It's because of all our experiences that we COMPLETELY AND TOTALLY PANIC WHEN SOMETHING GOES WRONG. All we hear is "ka-ching, ka-ching, ka-ching" and then the mechanic telling us, "it's worse than we originally thought".

The first thing I think of with a backfire is lean fuel mixture. It can be caused by several problems. Check the easiest first, filters, collapsed lines, etc.

As far as a coil being bad, I couldn't tell you how many times I've convinced myself I had a bad coil, bought a new one, and it has never yet fixed the problem. I'm sure I have a couple spare coils laying around.

Sounds like a personal problem, and without question more information than I needed to know.

Same here; it either works or it doesn't - No in between for me. I have also never had a fuel filter restrict my fuel flow enough to cause a low pressure problem.

Roby

Had a similar problem, one of my engines "backfired" thru the exhaust at 4000 rpm. It was the the electrical connector (socket) on the engine that was bad and caused the engine to shut down just enough to get fuel/air mixture into the exhust and in the next rev it fired with a pop in the exhuast. I cleaned the connector and separeted the pins going into the socket and then it worked