I have a new problem with my boat. After about an hour of use, the boat develops a high rpm miss/ breakup. Seems to be most notable between 3500 and 4000 rpm. Boat starts beautifully and runs great at lower rpms. Problem is repeatable day after day. Has occurred the last 3 times I have used the boat. This is a new problem, this boating season.

The boat is a 1990 Chaparral 2300sx with a mercruiser 5.7 liter V8 with the Thunderbolt IV ignition. Approximately 4 years ago, a new Mercruiser factory rebuilt motor was installed. At the same time, new exhaust manifolds, new risers, new edlebrock marine intake manifold, new edlebrock 750 cfm marine carb, and new mercury distributor installed. At the same time, the motor was converted to the mercury electric fuel pump setup.

So far I have replaced the cap, rotor, plug wires, spark plugs, coil, and fuel filter, added seafoam to the tank, added 35 gallons of fresh 93 octane fuel, and verified solid fuel flow at the carb. I have tested for a bad tach by disconnecting it from the coil. I have verified that timing at idle is at 8 degrees BTC and it seems to advance properly as the rpms increase. Nothing has fixed it.

I am considering replacing the ICM mounted on the exhaust manifold as it is the only part of the ignition system that hasn’t been replaced. My reservation is that I don’t want to spend the $500 on a new one and find out that wasn’t the issue.

I have read about the anti-siphon valve, but seams to show itself at lower rpms. I have also considered going into the carb, but I am religious about adding stable during fall boating to get it ready for winter storage.

Any thoughts/ help would be appreciated