Engine rpm low, help a rookie out please!
 

Hi all, Looking for a little guidance about my boats (lack of) performance.

I have an 84 Scarab 1, 21 ft with the 5.7 260hp and alpha 1 drive. Turning a 14 1/4 x 21 stainless turbo prop.

Heres the problem, the tachometer never goes over 3000 rpm. The boats speedometer will read around 48 mph, but only 3k on the tach. I have done some research on this and found most guys running a small block in this boat use a 19 or 21 pitch prop. If I were to switch to a 19 I might gain 2-3 hundred rpm but still nowhere near operating range of 4200-4600. This would seem to suggest an engine performance problem, but the boat will still run almost 50mph. Also there is no smoke out the exhaust at all, good fuel, new filters, clean plugs, new cap rotor wires and coil. Boat has been converted to electronic ignition by previous owner so no points to worry about. The secondaries in the carb are opening up (qjet) and the boat starts and runs great other than this seemingly lack of power issue. Does anybody have any experience with this problem and can you recommend me a good place to start looking before this becomes a wild goose chase? I should add I am an auto mechanic by trade so I have tools and ability to work on it, just no experience with diagnosing boat engines. Thanks for reading my long post any help is appreciated!

Will it rev over 3K in neutral?

I haven't tried that, never been a fan of revving an unloaded engine up, but I guess I can try it once just to see if it goes any higher....

check your tach,3k=48 mph sounds like bad tach reading

+1

It's your tach. It is theoretically impossible to run 48mph with that prop @ 3000RPM.

Perhaps a stupid question..but are you trimming out the outdrives or running them in the fully down position?

Even trimmed down it should wind higher than 3000. I'm guessing bad tach as well.

If you work on cars you should have/have access to a hand held tach or timing light with digital tach in it. Hook up one of those same as if it was in a Chevy truck of the same vintage and see what you get.

I figured the same but I thought I would ask just to see if he checked. Wouldn't that be the first thing to do? He doesn't like to rev un-loaded engines????????? How the hell can you check anything? We're not talking 7000rpms here, I've had small blocks that wouldn't idle under that almost!

Since the ignition has been messed with I would start there. Either the signal to the tach or the tach itself is off. No way you can run 48 mph at 3000 rpm with that prop as others have eluded to.

Get your actual outdrive ratio gearing, actual tach speed and a good GPS speed.

MSD ignition requires a transformer to get a good reading from some tachs. It's in their catalog.