Maybe the choke is taking it's time opening up ... or jamming or ... maybe
*tie* the choke open after a brief warmup, see if that helps, if so ...

After the engine
warms up running at 3500 rpm the tach returns to normal and the noise
(backfiring) goes away. If I stop the boat and sit fro 30 minutes the
process repeats. I've done the following:
1. Changed plugs
2. Changed rotor/ditributor
3. Changed plug wires
4. Replaced fuel filter
5. Cleaned carburetor

Any help would be appreciated. Thanks, Dave