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  1. #1

    Question Engine won't rev past 2000rpm under load

    I have 1985 Wellcraft Gran Sport with twin 454 MARINE POWER motors(1996).
    In October of last year I blew the V-drive on my starboard motor. I hauled engine out and installed a used V-drive. While the engine was out and at my shop the only thing I did is replace spark plugs, wires, cap and rotor. Dropped the engine back in and now for some reason it won't rev up past 2000rpm under load, it bogs down and backfires. At neutral works perfect, idles fine and revs up. Since that has been happening, I have rebuild the carburator twice, replace cap and rotor again, checked all wires, had the timing checked three different times by three different people and changed coil. It is still doing it. For the life of me I can't figure it out. It was working perfectly before the V-drive blow up. All it did was sit in my shop on an engine stand for six weeks until I found the V-drive. Any ideas?

  2. #2
    Join Date
    Dec 2011
    Location
    Left Coast
    Posts
    186

    Default Re: Engine won't rev past 2000rpm under load

    Do you have the means to test the fuel pressure while underload?

    Sounds like a fuel restriction, or air leak into the fuel system, or a failing fuel pump.
    Cheers,
    Rev Bob

    "if it's gonna happen, it'll happen out there!"


    My Dad always said; "Everything on your boat is either broken or about to be.......and you just don't know about it yet".

  3. #3

    Default Re: Engine won't rev past 2000rpm under load

    I don't know how to test the pressure. The pump is 4 moths old and it seems to be bringing a lot fuel up.

  4. #4
    Join Date
    Mar 2009
    Location
    Portland, Oregon,
    Posts
    6,945

    Default Re: Engine won't rev past 2000rpm under load

    In the future, try your best to not change more than one item at a time, and then test.
    This is more in line with the P of E (process of elimination) that best serves us when we do one item only at a time between testing.
    This also better prepares us for future trouble-shooting.

    If we were to back up here..... and if the new spark plugs, wires, cap and rotor are NOT the issue, then the only change that could have made the difference would be the V-drive............. of which may lead us to the question of ratio!!!!?????
    A substantially taller ratio may be over-burdening the engine, preventing it from reaching the correct RPM.
    There should be a tag on the transmission displaying the ratio.... yes/no?

    Otherwise, something within the ignition's spark quality and/or advancing system has likely changed.
    I'd begin by putting your timing light on it, increase RPM and watch for the correct progressive spark advance.
    Since you have twins, it's very easy to make a comparison between the two.

    .
    Rick ... aka Ricardo
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP's

  5. #5

    Default Re: Engine won't rev past 2000rpm under load

    Quote Originally Posted by RicardoMarine View Post
    In the future, try your best to not change more than one item at a time, and then test.
    This is more in line with the P of E (process of elimination) that best serves us when we do one item only at a time between testing.
    This also better prepares us for future trouble-shooting.

    If we were to back up here..... and if the new spark plugs, wires, cap and rotor are NOT the issue, then the only change that could have made the difference would be the V-drive............. of which may lead us to the question of ratio!!!!?????
    A substantially taller ratio may be over-burdening the engine, preventing it from reaching the correct RPM.
    There should be a tag on the transmission displaying the ratio.... yes/no?

    Otherwise, something within the ignition's spark quality and/or advancing system has likely changed.
    I'd begin by putting your timing light on it, increase RPM and watch for the correct progressive spark advance.
    Since you have twins, it's very easy to make a comparison between the two.

    .
    I have done one thing at a time so far. I check one thing and then go out. As far as the V-drive is concerned unless it has the wrong plate on it since I bought it used, it matches the old one and the port side. I will be checking the timing again next. I am hearing conflicting opinions as to where it should be set. My mechanic set it at 8 degrees, but I found the MARINE POWER book and it says it should be 12 degress. SO we will test that too.
    Thanks

  6. #6
    Join Date
    Sep 2008
    Location
    Maryland - USA
    Posts
    5,531

    Default Re: Engine won't rev past 2000rpm under load

    Heed Rick's last paragraph, above....I'd suspect the timing advance isn't happening.

  7. #7
    Join Date
    Mar 2009
    Location
    Portland, Oregon,
    Posts
    6,945

    Default Re: Engine won't rev past 2000rpm under load

    Quote Originally Posted by theobot View Post
    1... I have done one thing at a time so far. I check one thing and then go out. As far as the V-drive is concerned unless it has the wrong plate on it since I bought it used, it matches the old one and the port side.


    2.... I will be checking the timing again next. I am hearing conflicting opinions as to where it should be set. My mechanic set it at 8 degrees, but I found the MARINE POWER book and it says it should be 12 degress. SO we will test that too.
    Thanks
    1... The engine needs to be running for full transmission engagment... correct?
    Is the boat on the hard?
    Can both engines be run?
    If yes..... you could fire each one up (one at a time), run at the same idle speed, shift into FWD, and then strobe the prop with a laser tachometer and compare the two readings.
    This will tell you if the two ratios are the same or not.

    2... IMO (and going out on a limb here), your mechanic isn't doing you any favors by discussing BASE advance ONLY.
    Any good Marine Mechanic that is worth his salt is going to NOT ONLY set/check BASE, but he's going to also check the progressive advance!

    If your progressive advance has somehow quit functioning correctly, it may explain the low power on this one engine.... if nothing else has changed!
    This is very easy to test for by strobing your ignition timing marks as RPM is increased to approx 3.2K RPM.
    This takes a minute or two to connect up, and only seconds to do the test!

    It's an advantage if the harmonic balancer is marked off up to approx 35 degrees or so, but this can also be viewed with a digitally advancing strobe light.

    When doing any ignition related work, this should be protocol and should be done by any marine mechanic, IMO!


    NOTE:
    BASE advance is BASE advance.
    We fire up on BASE.... we idle on BASE... and that's about it!
    It means nothing in terms of what's occuring within the power RPM range.
    No progressive advance.... no power.
    Too much progressive advance and too early..... equals detonation potential.


    .
    Last edited by RicardoMarine; 02-15-2012 at 07:46 AM.
    Rick ... aka Ricardo
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP's

  8. #8
    Join Date
    Sep 2008
    Location
    Maryland - USA
    Posts
    5,531

    Default Re: Engine won't rev past 2000rpm under load

    Quote Originally Posted by RicardoMarine View Post
    1...
    If yes..... you could fire each one up (one at a time), run at the same idle speed, shift into FWD, and then strobe the prop with a laser tachometer and compare the two readings.
    This will tell you if the two ratios are the same or not.

    .
    I have to disagree with Rick on this one - in the details. I'd never spin the props with the engines on the hard. Strobing the coupler with the shaft disconnected would be ok.

  9. #9

    Default Re: Engine won't rev past 2000rpm under load

    I have had this problem many times with my 350 marine power motors. Water in the fuel. Each time I removed my spin on filter, either drained the visible water or replaced the filter and I was fine. (83 SIlverton)

  10. #10
    Join Date
    Mar 2009
    Location
    Portland, Oregon,
    Posts
    6,945

    Default Re: Engine won't rev past 2000rpm under load

    Quote Originally Posted by makomark View Post
    I have to disagree with Rick on this one - in the details. I'd never spin the props with the engines on the hard. Strobing the coupler with the shaft disconnected would be ok.
    OK.... I'll agree, and good point. I was not suggesting to do this for a long duration. Perhaps a helper woud engine the drives for a very short period!
    However, you'll still get the same results in that this would determine if the ratios were identical.
    If not..... then there's part, if not all, of his issue. Yes/No?
    Rick ... aka Ricardo
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP's

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