I have a 98 Mariah Z202 with a 5.7 Mercruiser. I am having a problem once the engine reaches temperature. The boat will accelerate up to 3k RPM's, then the tachometer will peg out to 6K and the engine will backfire. Think of accelerating like you are pulling a knee board or kids on the tube that don't want to go fast. If you advance the throttle slightly the engine will eventually operate normally and the tachometer will come back to an accurate reading. Once the boat has planed out you can place it at half throttle with no issues. If you accelerate to full throttle when trying to pull a wake boarder no problem... Again this only happens when the boat gets up to temperature and if you try to slowly accelerate.
So far I have replaced spark plugs, distributor and cap, coil, ignition module, and throttle sensor. I have disconnected the tachometer and still have the same issue. I have checked all the plug wires and the ground tree on the stern in the engine bay. I talked to a tech at the dealership and he didn't have any good idea of what it might be. Any advice will be greatly appreciated.