Old boat running rough

[87 Sunsport]

I have an 87 Supra Sunsport with the original 351 Ford and a New Holley Carb replaced five years ago. Every year I change the impeller, belts, fuel filter, oil and filter, and plugs. The boat has always run perfectly but this year when I put it in I could only get it to 28000 RPM, and it is sluggish out of the hole with a top speed of 26mph. It backfires when I accelerate. I'm trying to figure out what I did to it. Could it run this well if I mixed a plug wire? Temp, oil pressure all fine.
Bob

[DKJBama92Mariah]

Limited RPM and a backfire is one of two things: inadequate fuel flow causing a lean condition or improper timing. Sinc'e you've already changed the fuel filter, I'd pull the distributor cap and make sure the mechanichal advance weights and springs are operating smoothly. If that checks out, i'd suspect clogged jets somewhere in the carb. After 5 years, a carb rebuild is probably in order.

[wotan2525]

I was going to say the same thing.... sounds like your timing is off! Check the distributor and rotor...

[OUI]

Once you've had back firing through a Holley carb you'll have to replace the power valve.

[Salty87]

Could it run this well if I mixed a plug wire? Temp, oil pressure all fine.
Bob

yup, check the simple stuff first.