454 Magnum, 365hp, carb -- getting the RPM but the not the speed???
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454 Magnum, 365hp, carb -- getting the RPM but the not the speed???

  1. #1
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    Default 454 Magnum, 365hp, carb -- getting the RPM but the not the speed???

    Hey all...

    I posted the following in the "On the Dyno" forum -- and some suggested I also post here....so here goes



    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...

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  3. #2
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    Aug 2010
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    Question

    While running the boat, did you by chance trim up (raise) the stern drive at all?

  4. #3
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    May 2011
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    Quote Originally Posted by jakkal68 View Post
    Hey all...

    I posted the following in the "On the Dyno" forum -- and some suggested I also post here....so here goes



    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...
    do you know that the tach is right 4500 rpms is 4500 rpms whether the engine is running correct or not the 32 mph at 3500 seems borderline ok depending where the trim was set I have seen prop hubs slip you could pull your prop and see if the hub is spun

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  6. #4
    It's what we do BDMarine's Avatar
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    Default

    You picked up 10 mph at 1000 rpm's less just by moving the tabs a little. I don't think you should be looking at the the engine. I bet the 4500 will increase as well as the speed when you figure out the trim and tab use.
    If you have a friend with lots of experience, maybe have him drive the boat and see how it works with more trim and less tab.
    http://www.performanceboats.com//signaturepics/sigpic837_1.gif
    B & D Marine Performance
    El Monte, Ca.

  7. #5
    Senior Member CampbellCarl's Avatar
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    Whittier, CA & Katherine Landing
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    Default

    You sure that the trailer wasn't still attached to the boat?..............................
    Originally Posted by Rattle Can Lou
    Again, if I came across as a v-drive snob..I apologize,
    Mike

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