Olds 455 Stumbling
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Olds 455 Stumbling

  1. #1
    Idaho Jet Craig4937's Avatar
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    Default Olds 455 Stumbling

    I have a 19' Tahiti hull with a Olds 455 and a Holley 750 Double pumper. It has a issue when you are cruzing at 2500 to 3000 rpm and you stab it, it starts to stumble and flatten out. I thought it was a fuel psi issue but it's a rock solid 9psi at any rpm. Thats what i thought it was so I am stumbed. so any ideas? carb jetting?

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    Senior Member 1975sleekcraft's Avatar
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    it could be the accelerator pump...most likely on the secondary side...

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    Seriously off center slowride's Avatar
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    First off, the fuel pressure is a bit high. 7-7.5 is what a Holley likes. That aside, look at your distributor. What make and style? Automotive with vacuum advance or full mechanical? If vacuum advance, lock it out.... check that your mechanical advance is operating properly. How much total advance at 3000 rpm? As mentioned also look at the accelerator pumps and their adjustment. Double check your float levels, too.






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  6. #4
    Idaho Jet Craig4937's Avatar
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    Quote Originally Posted by slowride View Post
    First off, the fuel pressure is a bit high. 7-7.5 is what a Holley likes. That aside, look at your distributor. What make and style? Automotive with vacuum advance or full mechanical? If vacuum advance, lock it out.... check that your mechanical advance is operating properly. How much total advance at 3000 rpm? As mentioned also look at the accelerator pumps and their adjustment. Double check your float levels, too.

    It's full mechanical, and its a new distributor, ok i'll check it tonight. what should it be at 3000rpm. and i'm going to take the carb off and rebuild it. it needs it anyway...

  7. #5
    Junior Member 83Daytona19's Avatar
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    Is this a new problem or did you just get the boat? a couple of thoughts:
    what happens when you accelerate WOT from idle to above 4000rpm/top speed? No problem doing that points to issues in accelerator pump setup. If it chokes doing that start with fuel filter (where are you taking fuel pressure reading upstream or downstream of filter and are you doing it under load on the water or parked running on a hose with minimal fuel demand?) Look down into carb with engine off, see if dust and such might be blocking some of the bleed holes, spark arresters don't keep dust/dirt out.

    Check for cracked and old hard sloppy fitting hoses that no longer seal/vacuum leaks, base gasket/loose fasteners, Holley such a simple carb that too often the simple things that can't possibly be the problem are the problem.
    Rust inside the distributor might be hanging up the advance mechanism.

    Good call by others on fuel pressure... get adjustable regulator and reset pressure and float levels (floats will drop lower with less pressure pushing on float needle valve)

    If boat is new to you and you have eliminated the other possible causes, it might be the carb was set up for automotive use, at 3000 rpm unlike on the street, the pump cams might be too far up the ramp to give a good shot, you'd have to watch where on the cam the accel pump arm is at speed to catch that, or perhaps the spring loaded actuater rod riding off the pump cam lever is not adjusted properly and you are not getting a full shot. Again if set up for the street might need a different set of pump shooters to deliver the volume of the pump shot a bit faster/slower. Shooters are easy to swap, but mess with changing cams last, good way to mess up the operation if you didn't note which cam and which hole in cam and arm was used to start with. Great use for that cell phone cam and a sharpie marker.

    May also be the power valve (opens to let more fuel in when vac in carb drops below a certain point when punching throttle open) may be stuck shut/plugged or failed, though if diaphram blows out usual symptom is lots of gas going into the carb lots of black smoke and fouling plugs pretty quick, (too high a float level will result in same) but neither of these fits your operational symptoms.

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    Head Janitor FormulaZR's Avatar
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    I had this issue when I had an accelerator pump that leaked. I didn't realize it leaked at the time...but the boat would stumble when you stabbed the throttle. I replaced the accelerator pumps (one leaked, the other I replaced just so I didn't have to later) and the problem went away.



    Also, I had some fuel pump issues with a diaphram type pump...went to a Holley HP 125 Gerotor type (not marine certified) that has a preset PSI. Boat ran great until I took the carb off to switch to EFI.
    Last edited by FormulaZR; 06-22-2012 at 06:40 PM.


    "No, my boat doesn't need any more power" - No one ever

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