This question is for a buddy of mine. He just got an 88 272 Formula with twin 350 MAG SBC's and he is having trouble with the starbord motor missing and derating under load. It runs smooth at idle and when reved up in neutral but when he puts it down and gets on plane (up to around 4500 rmp) the engine will start to miss consistantly and and drop in R's gradually untill it starts to backfire. By then he is back off plane he can put it in neutral and rev it up and it will do it all over again. Wonder what might cause this, where to look???
He doesnt have any history of the boat, he check the timing it it was 10 deg advanced, adjusted it with no change. Also changed the coil. Thanks in advance, MIKE

Sounds like it's going lean. I would make sure it's getting plenty of fuel and the carbs are clean, and in good working order. Todays fuels are horrible for fuel systems that sit.


Darrell.

anybody ck the dist cap,rotor,spark plugs and spark plug wires,it could just need a good tune-up ?

Water, debris or old gas turned to varnish in bottom of tank that is kicked up to fuel line pickup when on plane. Check for clogged filter...

Valve spings? Worn Valves. When you start popping a High RPM I would start looking for weak springs.

Well it could be any of the things discussed above. If the engine is starving for fuel it will go lean,lose power and backfire, also if you have lots of hours on the valve springs they could be going away causing a backfire.

I will pass the info to my buddy and see what he finds out. Thanks for the suggestions, hopefully it is some minor things like cap & rotor and mybe a fuel filter. Thanks, MIKE