Backfire and running rough

I’ve got a 1990 supra sun sport with the PCM 351. I’ve almost bought for roughly 5 years and have never had a problem or a hiccup. Yesterday we took the boat out to water Ski in the board, but ran a tight 35 miles an hour for slalom skiing, switched over to knee boarding for 25 minutes with two people on board around 15 to 18 miles an hour. The boat started acting up with hesitation and some backfire. We cut our skiing short and headed back for sure the boat ran super rough and was popping and banging because of the misfire. Any idea or suggestions? I have not changed plugs or wires since I’ve owned this boat. I change in empeller and oil yearly. I pulled a couple plugs last year and they looked great. I do not think it’s fuel related as I put in premium (no ethanol)every time.

First thing I would do is verify timing, then if timing is good start looking at the rest of the ignition. Check the coil, then plugs then cap and rotor.

H2O-ski,
I’ll check timing hopefully today. On idle (in neutral) it revs fine. It’s when it’s under load that it sounds like it’s missing and backfires.

I’m also going to do a compression check and look at the cap. Spark plugs will be changed as well. Fuel vent on the outside of the boat will be taken off and cleaned….. fingers crossed