Sandahl's answer sounds about right

Since it needs it anyway, you might give it a fresh oil change and see if that makes a difference. However, at the same time have a look inside the valve covers for foamy brown stuff indicating water intrusion, which could mean you have a water leak somewhere. I just had a very similar problem on one of two fresh 540ci engines, luckily traced it to a leaky header before anything disasterous happened.

Clean up and separate the plug wires. I doubt that it has anything to do with your problem but it could be one in the future!
I would also suggest changing the rubber fuel hose over to stainless tube.

Being that the problem is occuring at random ie: not when powering up to go over a wave or something like that, and you say that your plugs have good coloring, as mentioned I would take a good look at the valve train. Maybe a broken valve spring on one of the intake valves?

ok here is what you do, get out your vacuum gauge, hook it up to a full time vac. port, watch the gauge with eng. running at idle, the needle should be steady, if it is bouncing around {a lot}you have a problem in the valve train {burned valve, broke valve spring ect.}
if the reading is steady{if you have a big cam you will have a lower than normal vac. reading, normal} the problem is in ignition or fuel, had a problem like yours a while back on a friends boat, it had a flat lobe on the cam shaft, good luck

Guys, please don't tell me I have to tear into the motor. Pulling the valve covers would require removing the exhaust manifold (Imco powerflow). Would I need to replace the gaskets if I did that? Where is a full time vacume port? Is that what the black boot in the carb picture covers?

One thing I didn't mention was I can duplicate the backfire if I punch it from idle. I have to ease into it. No way it could be a carb problem?

If it ended up being valve train would I need to pull the motor? Thanks for the help so far. I know some of these questions must sound stupid. I only know enough to make me dangerous.;)

Summit Filters
 

You can get those from Summit for $4.95. The part number is SUM-G3121. I usually order a few, and replace them often to keep from going lean...

Good luck.

PS- Occasional? I would look at the carb and ignition first. I like the idea of checking the wires (ohmmeter). Also, I would think that carb needs a powervalve now...might as well get it rebuilt...

on the vacuum fitting, the one with the black boot is most of the time ported vacuum {no vacuum at idle, but have vacuum when you open the throttle} you need one that has vacuum on it all the time, pull the boot off with eng. running put your finger on it, does it have vacuum on it? if not plug it back off and look at the front or back of the carb. down low on the throttle plate there should be a vacuum fitting{ might be plugged also, on a car it would be for brake booster or PCV} pull the line or plug there and check for vacuum with the eng. running, the vacuum gauge test works very well I have used it for testing on cars and boats for many years now, good luck

Well I did an oil change yesterday and took the boat out. I wanted to burn off most of the suspect gas in the tank at the same time. I got a backfire about 20 min into the trip. As I was heading back in and about 500 yards from the marina the boat started backfiring and died. I was showing fluctuations on the fuel pressure gauge and I thought I toasted the pump. I got a tow in by a fellow OSO'er and it went right on the rack. In retrospect I think I actually ran out of gas and my fuel gauge must be off. I had never run it that low before. I'll fill it next week and see if I get pressure again. I'm wondering if its not a fuel delivery problem now. I pulled the carb off and am going to rebuild it. The backfire sounded the same when I ran out of gas as it has previously. if it was a spring or a cam lobe, wouldn't it happen every on every revolution of the motor? Any thoughts?

You can get a backfire thru carb when motor is 2 lean.