Hey all...

I'm new here...and after being out of the boating scene for the last 20 years...I've purchased a 1990 Mach 1 Endeavor 2900 -- it has a 454 Magnum, 365hp, carburetor engine. The seller had the engine rebuilt about 7 years ago, added a performance edlebrock carb, and the lower unit gone through 3 years ago.

It didn't see water last year -- put it in for the 1st time today, and the engine seems to be running great, and when initially throttling up all seems good...but when running at WOT (roughly 4,500 rpm) -- GPS read I was going a whopping 22 mph?? Needless to say -- not very happy. From what I've read -- this boat should run somewhere in the 50's WOT. At about 1/2 throttle to WOT the RPM's increase -- but just not the speed?

When I took the boat out tonight -- I was in 3 to 3 1/2 foot chop -- and didn't have a lot of time...

I do have to note though -- that after some adjustment of the trim tabs (which I'm new to having/using) -- I was able to get the boat to run at 32 mph pulling 3,500 +/- rpm, but didn't have more time to play with adjusting them -- but not sure that I'll pull gain another 20 mph with more adjustment of the tabs??

The engine itself seems to run flawlessly -- no hesitation, no backfiring, doesn't seem to have a miss, etc.

I'm running a Quicksilver 3 blade stainless steel 14 1/2 by 23 prop.

Other than normal pre-season maintenance (oil change, filters, new impeller, pulled lower unit - checked gimble and bellows) -- I haven't done anything else.

So --I'm looking for some thoughts on what the cause may be?? I'm not a mechanic by any means - and know just enough to be dangerous.

Things that I thought may play a role -- and please correct me if I'm wrong:

1) Fuel -- the boat didn't see water last year -- and there was roughly 30 gallons of fuel in the boat when I bought it, and started right up without a
problem -- but could old gas be a factor? Before launching it I added another 75 gallons...

2) Carburetor -- is it possible that all 4 barrels aren't working in this case? Or that the carb itself needs cleaned?

Of course I'm hoping it's something I can address myself -- so any and all help/thoughts would be appreciated!!

Thanks...

What were you using for a GPS? If it was a phone app some of them don't work well. Your RPM's and seem to be consistent with the prop you're using to get 55 mph with the correct amount of prop slip which I would guess would be around 16% with your boat. However right now your at 55% prop slip. So obviously something is wrong.

Is your prop cavitating? This would cause low speed.
I don't know how to check this but your coupler could be going, and your getting a lot of slip.
If your prop is in a rubber hub that could be slipping as well.
Is your bottom painted?

Just some ideas to look into, let us know what you find out.

Like Endeavor said. Check the prop hub. If its one of those rubber hubbed units, sitting for a couple years coulda caused it to shrink/dry out, and now its just spinning in its hub.

Hey endeavour32, and Mild Thunder...

I appreciate your suggestions....

1) Yes -- it was a GPS phone app....good point!

Took the boat out again tonight, and ran it with trim tabs completely in the up position....was pulling 4,500 rpm -- and per GPS (iphone app), was running at 32 mph.

From people I've spoke to today -- I should be able to pull 5,500 rpm -- which would get me closer to the 50 mph mark. Problem is even after trimming up the outdrive, I can't seem to get anymore rpm.

2) Cavitation? I don't believe it was -- but to be 100% honest I'm not sure that I would know if it was??

3) Rubber Hub? Again not sure -- I will definitely be looking at it this weekend.

4) Bottom painted -- yes it is.


I've spoke with a couple people today -- and there's some thought that fuel could be an issue - that the older gas could have caused carb jet blockage?

And that possibly timing could be off a few degrees -- causing lack of power?

Again -- thanks for the suggestions, and I'll keep you all posted.

Power wash the bottom???

Bottom paint will scub off tons of speed. When I bought my Velocity (as a repo) it had deposits from botom growth on the bottom. After acid washing all of the deposits off, my boat picked up close to 8 MPH.

I don't think the 454 365hp motor is built to run over 5k. I think 4500-4800 is the correct rpm range. I would look at the hub in the prop. But with bottom paint, it will never reach it's expected top speed.

You also mention you added an additional 75 gallons of fuel.... How much fuel is onboard? That is a lot of weight for a single screw, 365hp motor in a fairly heavy boat.

You may also want to look at stepping down to a 21p prop.

454 Mag is rated for 4600 - 5000 rpm. You should be within that range. I would definitely do plugs, wires, cap, rotor, fuel water separator, clean the carb, and clean the fuel filter where the fuel line hooks up to the carb. Timing check too.........I just checked mine and it was 6 degrees off. I think you should be at 8 degrees before top dead center at idle.

ok. take the gps phone app and try it in your car. if it says you are going 32 when on the highway doing 55 you know that answer. try to borrow another prop, even same pitch or a 17 or 19. the only real way to check a prop hub is to replace it. my old searay used to do about 30 with a 15 prop. does it feel like you are only doing 30?