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Only 50 MPH?!?

T

the sMac

Member
Looking for a little advice or a "what to look for" comment.

I have a 1984 25' Concincor. Just dropped in a new 454 L29 MPI engine last fall, and also had the tranny and outdrive rebuilt.

After finally changing the oil this weekend, I went out it's first top-speed ride. Well, I was only getting about 50MPH (gps) at 4400 rpm.

It really didn't matter how I trimmed it, tabbed it, it wouldn't pull much more speed.

Any thoughts on this? My old 330hp 454 was getting the boat in the 60's... not gps.

Thanks in advance.
 
If I am correct, that new MPI motor you have is 315 h.p. I would not have expected a 10 mph loss, I would have thought it would be about the same. Is there a chance the new power is not running right?
 
well, that's my question. My old motor had a ton of hours, was old and tired, a rochester carb that was never touched since I owned the boat of 10 years, etc...

now, I have this newer power plant, MPI, fresh (20 hours) rebuilt outdrive and tranny, freshly waxed hull bottom and I'm only get 50 (at best)

I don't know what to look for. It sounds fine, starts fine, feels like it runs great... but the top end is missing.

I don't know if I should be getting more RPMs, if it's missing, or what is wrong.

The only thing I can think of, and this could affect it a little, is I have 4 inch exhaust going thru a 3 inch exit at the transom. But I had a 3 inch exhaust all the way out before.
 
I'd sure look at fuel pressure... I'm not sure if that engine has a vst tank on it... (vapor separating tank) if it does would be (looking front to rear on the engine) on the right side aft.. adjacent to the distributor.. these have lil micron filters in em that plug up.. it may need to be cleaned out... be careful with that screen... i think mercury charges a lil over 200 bucks for a piece that looks like it came out of a cherios box...

your tranny should be a 72c borg warner.. its unlikly that it is robbing your engine of that much speed... but check the fluid level in her.. the correct way to check the level is engine at operating temp... clutch in to forward and reverse... then check to see if your level is correct.. adjust as neccesary..
 
Thank you. I'll check that filter tonight. Seems like that wouldn't be the problem though since I am turning 4400 rpm. As long as that is the correct RPM for that engine. I'll need to check that also.
Thanks again,
 
the l29 should run up and rev limit out around 5200.. (least the 10 i take care of down here).. rock solid lil engines..
 
I can outrun my friends 22 ft rinker by a few mph, i clocked 53 on GPS last weekend and his speedometer read "60" so it could just be the difference in the speedo vs GPS. Also downloading a prop calculator program can be a great way to see if anything in wrong. If you know the weight of your boat, prop pitch, outrdive gear ratio you can calculate max speed and rpms and HP.
 
Thanks for the responses.

Happy, are you pretty sure the RPM is around 5200??? That could explain a lot. I'm jut pushing 4400.

I wonder what the difference is then from this motor to the old one... same prop, same outdrive.
Not enough power to turn the prop?

Maybe I finally have the excuse get rid of that old 3 blade cleaver prop for a shiny new one.
 
I would try a different prop. w/ less pitch. That motor pobobly is not producing peak Hp at 4400 rpm.

What is the timing set at? I have been able to get more rpms out of BBC by advancing the timing. BBC will run with 8-15 BTDC initial timing 30-38 BTDC full advance.
 
the timing on the l 29 is 8 degrees btdc.. if you try to fool the computer your only fooling yourself.. it will not run right.. you need to put the engine in service mode.. idling around 1k to 1200 rpm.. set it at 8 and be done.. you can check full advance.. albiet the line on the balancer will move around a bit.. but not much.. if your off wacky say bout 15 degrees @ 3000 rpm .. you may have issues w/your ignition system.. if you don't have a ddt.. then your guessing at more stuff.. you could have an ect, map.. throwing you off too..
 
You won't get top performance from a fresh rebuild... they need to be broken in... and
follow the procedure to a tee for max durability
and longetivity. Change oil after a week and
again in a few (eng. wears the quickest here)
Find a manual for this break in period to use.
Good luck!
 
5000RPM, nice. Thanks.

For some strange reason, I was able to get 53-54MPH (on gps) last weekend. Maybe I finally burned through the old gas from winter storage or something?

But I am only getting 4600-4800 RPM. Not bad, no complaints really. I usually run slow most of the time, around 3000 to 3300 rpm.

Thanks again,
 
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