Help - Merc 454 can only get to 3900 rpm wot
 

I have a 1992 26' Sleekcraft with a 454 Bravo1 set. The boat is new to me. It runs smooth and sounds good, but I can only get it to 45 pmh at 3900 rpm. I know these engines like 4600-4800 rpm. The previous owner said it would cruise at 65 mph, so I doubt the prop is the issue. Where should I start to figure this out? I already did an oil change and new plugs. BTW - I bought the boat in Pheonix but like in Calgary - I know altitude can affect the carb and jets, but I don't think that could make this big of a difference.

Any ideas?

Thanks

Did you play with the trim at all?

Yes. I played with the trim to get it to 3900 rpm. It just doesn't want to go higher. Could the altitude difference be it. Pheonix is 1124ft above sea level while Calgary is 3800ft. I believe every 1000 ft higher is 3% loss. So this would say I have 7.5% loss. I suppose rejetting would get that back? Thanks

You'll never get the hp loss from altitude back, but you can get back the hp loss you get for not being tuned right for the altitude.

BTW: We see many times on here and real life a boat owner selling their boat with some other prop than the one the boat liked best.

What rated hp is your boat ?

What prop is on it now ?

What does factory say it runs with that hp and prop that comes on it from factory ?

this is a stock 330 hp bravo 1? there is also the possibility that prev owner was being a bit optimistic with his perf numbers. i don't see that boat hitting 65 with less than 500 hp...

Yes, it is stock 330hp.

I did a few things now. First plugs were already all replaced before I posted this topic. But since I checked plug wire resistence and all were perfect. But I did notice on one that the connector that connects to the plug was below the boot area - so it really was not making good contact. So I fixed that. I also put on a new water separating fuel filter.

Took it out today with just my son and I on smooth water. Got 4200rpm doing 45mph. Better but I want to get to 4600-4800rpm.

What would you guys suggest I do first before I get mechanic help? I'm gonna test compression this weekend. Oh, when I test compression, is it necessary to warm up the boat first? If so, can I just take it for a 2 minute run and let cool down until it can be touched without burning, or does it need to be hot?

Thanks

If you are going to do a test think about a leak down test. It will tell you a lot more about your motor.
Not much more work than compression test especially if you do both.

I highly doubt that boat has ever even seen 60mph with a 330hp.

Are you using a GPS for speeds??? They still seem a little low.

What prop is on it???

DNA, a TIRED 454/330 on a heavy boat maxed out at 4200 RPM is about right. I know for a fact.
Merc spec'ed max rpm 4200-4600 RPM
Back to prop. Pitch/Material/# of Blades.
You keep it in the water, bottom paint? bottom fowled?
What is the timing set at?
Call Jim at Allstate Carburetor for proper Jets in carb. (altitude diff).
Good luck with the 21 prop and just ask for Bravo insert, no need to count splines.
Check speed with GPS on phone for more accurate speedo