Hello,
Sound like a ignition/timing issue. You've probably replaced the points, and condenser with the new distributor along with setting dwell and timing.
So then an easy check would be to see if the dampener pulley has slipped on the harmonic balancer, changing the timing. You can do this by bringing #1 piston to TDC and checking if timing mark is on zero. If all looks good here are a couple more Items to check.
Make sure your not dropping coil voltage at that point.
Try disconnecting gray tachometer lead from coil negative.
You may also be experiencing some weak valve springs. To check this try isolating which cylinders are backfiring.
Please let me know if theres more I can do.
The distributor is a brand new MSD electronic marine distributor and has a new coil with it. Timing marks on balancer are accurate and initial timing set to 10 degrees BTDC. Total advance was set to 30 degrees at 3800 rpm. I know it sounds like ignition or timing, but the engine runs exactly the same with the new distributor as with the old one.
I will try disconecting the tach lead as it is possible that it is creating a problem. I will also check the valve springs as I already have the valve covers off. (I was checking to make sure that the cam didn't have a worn lobe)
I believe your on the right track. Sometines those boken valve springs are hard to see!
If you don't see anything with valve covers off, you might try to isolate the cylinder/s by pulling a wire of the dist cap each run till you find the cylinder/s misfiring.
Marine Mechanic
26 years certified marine mechanic and past state certified teaching degree