My friend has a 1987 scarab excel with what we believe is a 330hp 454, He recently had heads redone, and new exhaust manifolds, and all associated done.... On inspection, the mechanic claims the engine had been redone previously and was bored out .040 over. Previous to the repairs, we had gotten 43 mph out of her at about 4000 rpm, with 4 guys (2 that are pushing 250lbs) after the repairs with 3 on board we opened her up (about 4200) and only got about 38 mph out of her. The engine sounds great and accelerates smoothly.

What could this possibly be???
My first thought is a prop but the boat was never run aground, any other possibilities?

Are you using a gps for your speed? Losing one person out of the boat and gaining 200 rpm and you lost speed? Something has to be off. What type of prop and how many blades

Yes all the mph are by gps, The first time out we didn't open it up this time we did.... Its a three blade, clever style I guess, I think pitch was 19.5" but I'm not certain as it sits in the water...

Without knowing what other work that might be done to the engine it's all going to be guess work. How much fuel on board, direction of wind, tide/current, cooler of beer? IMO that prop is junk. Is that the max rpm you can reach? I think you should be up around 4600 at WOT. Prop pitch doesn't sound too tall actually I would think a 21P or 23P but if engine is topping out at 4200 there might be a fuel supply issue. Pump going bad or filter clogged. Either way I'd trash the clever beaver. Just my 2ct's.

no ideas guys?

bottom dirty !

You should be running 4600 RPM and about 55mph.

My scarab excel used to run 52mph on the speedo which i think was fairly accurate.That was with the 330hp and Alpha 1 outdrive.

First off, as said, retire the cleaver to paper weight, very old technology and way wrong prop for that boat. Clean the bottom and get a new prop. No need to do anything before you do that.

These are the things to look for if a new prop does not work. Verify with another gps, a real gps, not an Iphone app or the like. Verify tach working. Bottom problems that can lead to a very high slip number, ie hook, dirt or damage ect. Cone clutch slipping in the drive or other drive issues. Drive coupler problems. If nothing external has changed and you go up in rpms and down in speed, it's not a fuel or motor issue, it's very simple, if it were, you'd lose both speed and roms. It is most likely GPS, tach or driveline related. And as others said, you are not spinning your motor fast enough. Get it up to 4600-4700 light load. A 21" Mirage+ would be the prop for baseline testing.

Check the rev limiter on the engine, old stock 330 hp were set at 4350 . If tach is off a little there is your answer