I bought a 1987 Sea Ray Monaco 210 with a 454 in it two years ago. When I bought it, it had a light backfire in the carb at anything over about 3400 rpm. Ran great, just had this light continuous popping in the carb. Sucked down 50 gallons a fuel in about three clock hours though, last trip out that summer. I found that the needle was screwed out about 8 1/2 turns, actually about to fall out. Reset it to around 3 1/2, was hoping that would also help with the backfiring. Well the first trip out last summer, I managed to hydro-lock the motor(to many teenagers on the swim deck while filling tubes), by the time I got the right parts to replace the water shutters, it was time to winterize. Took it out Sunday, still ran great, fuel consumption seemed better, we weren't out that long, but still has the backfiring. Plugs were great when I was fixing the hydro-lock, so I'm wondering, is it timing? Any other ideas?
