25 gdlt.

Symptoms - bogs down and backfires when under load at 1/2 throttle and above. Only happens after running for about 1 mile at wide open. Symptoms persist until back on trailer. On trailer the engine runs perfect.

What I've done - Replaced spark plugs.
Pulled plug wires one at a time while running to determine if it has dropped a cylinder (both cylinders are running).
Rebuilt carb about 2 months ago
Replaced fuel/water seperator about 2 months ago.

I know its hard to diagnose ignition problems while on the trailer, but can ya'll help me figure out what's going on? I'd rather not throw parts at it if i can help it. Anything i can do with the boat on the trailer to diagnose the problem? I'm thinking its either the spark advance module or the coils. How can i discipher what parts i need?

later,
mm

check your air filter... (and your fuel filter if not replaced when you did seperator)

Also ck your whole fuel system for air leaks (check all your hoses and connections) and make sure your tank vent is open and clear...

I have seen issues with all the above cause the problem you are describing except the backfire part....