Mercruiser 454 Magnum w/ Alpha One -- Constant popping starting at 1650 RPMs

Hello-
I just purchased an '86 boat, which had a rebuilt 454 installed 5/2010 (engine from US Engine Inc). Had it on the water yesterday, and got a constant popping (assuming a backfire) starting at 1650 RPMs. I played with the throttle a bit, just to get a feel for what affected the sound. At 1650, the pop is fairly quiet, and by 2000 rpms, the pop is much louder. I didn't push it any further than that to avoid any damage. It is a rhythmic sound, seeming to be coming from a single cylinder every time it fires. The only thing I have done since I bought the boat was to clean the flame arrestor, which was nearly completely plugged with black buildup. I got home after dark last night, and it has been raining all day to day, so I haven't had a chance to get under the motor cover to investigate. Based on the research I've done in this and other forums, it seems the likely culprit may be one of the following: crossed spark plug wires, bad plugs, bad timing, too lean fuel mix, bad water separator. Any insight or help with where to begin or anything to check that I missed would be much appreciated.

Thanks in advance.

Re: Mercruiser 454 Magnum w/ Alpha One -- Constant popping starting at 1650 RPMs

Start with plug wires and timing, also might want to check for a vacuum leak. Does it miss when idling? Is this a recent purchase or did you change the engine? Is it overheating? Is it like this all the time or after an hour of run time?