Mercruiser running high rpm

[Blueoval0150]

Hello all,
New to the forums and needing some advice. I have a '92 marada 18' with a 4.3lx and alpha one drive. If I run it to wot it will hit 5500 rpm. Really high for a motor rated for 4600-4800 at wot. My first thought was wrong prop but it has a 21" pitch 3 blade.. Not sure on the diameter. The one time I ran it to wot it hit 48mph/gps verified. Assuming the tach is accurate what could cause the high rpm? Thanks in advance for any help.

 

[fossill]

Re: Mercruiser running high rpm

Was it always like that or did it just happen?

 

[Blueoval0150]

Re: Mercruiser running high rpm

I've had it out twice since I bought it and it's been that way both times. Don't know if the previous owner had the same issue or even knew enough to realize that it was over revving.

 

[fossill]

Re: Mercruiser running high rpm

Check the tach for accuracy against a shop tach. Check to see if prop is slipping on prop shaft and or outdrive shaft not slipping in coupler if the tach is ok. Also ensure that the prop is the right one for the boat..

 

[Blueoval0150]

Re: Mercruiser running high rpm

Thanks for the info! I'll get the tach checked and go from there. I'm hoping it's just a spun prop. The boat is mostly used for wakeboarding and tubing so I'd like to drop to a lower pitch for better holeshot. It's having a hard time planing without shifting weight around now. But got to get this rpm problem figured out first. Thanks again! I've learned a ton of info from a few days of reading through these forums.