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  1. #1

    Default High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    I have a Mercruiser 4.3 with an Alpha 1 drive. Bought it last year it ran fine and it was winterized and stored indoors. I am a rookie at this.

    This year it has run for an hour or so and worked OK but then it simply lost power. Won't run faster than 10 or 12 mph. I blamed this on ethanol fuel and and I replaced it. Ran Heet and sea foam through the system.

    That didn't work so I have replaced the fuel filter, and had a reputable boat shop rebuild the carburetor. The spark plugs are in excellent condition. I even removed the anti-syphon device temporarily until I can buy a new one. The electric fuel pump easily sprayed a stream of fuel when I tested it(I don't have a pressure gauge).

    So then someone suggested I have a spun hub. So I put it in gear, turned off the motor and tried to spin the prop. It won't turn at all in the counter clockwise direction. It makes a clicking noise when I turn it clockwise. Is that normal?

    I am at my wits end because it starts right up and idles fine. You can take it up to 3000 RPM fine. But no power. Anyone have an idea? Will take it to a mechanic but it seems like everyone is guessing.

  2. #2
    Join Date
    Jun 2006
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    Spotswood, NJ, USA
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    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    if you have a rubber prop hub it could be slipping

  3. #3

    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    Quote Originally Posted by Bt Doctur View Post
    if you have a rubber prop hub it could be slipping
    Thanks for replying. Checked that it is ok. Any other ideas?

  4. #4
    Join Date
    Apr 2016
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    barnegat NJ
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    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    Has compression been checked, Run an open air gap spark test

  5. #5

    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    Hi, the clicking you hear is normal. It will click in the other direction if you put it in reverse. Nothing to worry about there. Can you clarify if the engine is reving and it is not moving through the water like it should or the engine is boggin out?

  6. #6
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    Apr 2016
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    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    Is it possible your boat is carrying extra weight, water in bilge? Is bottom of hull clean?

  7. #7
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    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    A rubber hub is best checked by marking the inner hub and prop with a punch mark, test running, removing the prop to see if the marks still line up.

  8. #8

    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    Quote Originally Posted by Bt Doctur View Post
    A rubber hub is best checked by marking the inner hub and prop with a punch mark, test running, removing the prop to see if the marks still line up.
    Yes. Even a slipping rubber hub will still hold more torque than you can deliver with your hands. The clicking/locking in gear indicates the lower unit's innards are good. If your prop is holding per above, my next guess (unfortunately for you) is the engine coupler is slipping, or the engine yoke splines are damaged on the shaft, or more likely in the hub's alum. insert. You'll need to pull the lower unit, and get a flash light to inspect this.

  9. #9

    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    Gave it to the mechanic to check out. Will let you all know.
    BOAT = break out another thousand.
    Argg....

  10. #10
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    Jun 2008
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    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    If you're limited to 3000rpm and it went faster last season, then your prop is OK... Spun prop problems present as HIGH rpm and low boat speed. If you have a Thunderbolt ignition system and have not replaced the distributor cap recently, I'd start with a new cap rotor set. In 12 seasons I've never gotten a whole 3rd season out of a cap. I now change them every other year. If you want really trouble free boating, replace cap and rotor annually...plugs, if platinum tipped, last several seasons. regular tipped plugs are a seasonal change out IMHO. Good OEM wires or premium aftermarkets wires are good for about 5 seasons. Fuel filters are an annual or every other year ( if drained and refilled with fresh fuel annually) ... Oil and oil filter annually as is the raw water pump impeller in the drive ( per MERC Service bulletin). Check the ignition timing while you are at it...checking this on a Thunderbolt equipped engine is not the same as a mechanical points system.
    Last edited by sandkicker; 07-16-2018 at 04:30 PM.
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280
    1972 22ft Bristol
    Honda 7.5HP 4 stroke Long shaft

  11. #11

    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    Thanks. It is in the shop right now. Hope they aren't guessing and can see the problem.

  12. #12

    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    Out of the water with no load I can take it easy to 3 or 4000 RPM. It seems to surge a between 3 and 4000 RPM but sounds strong. When I put it in the water it revs to around 2000 RPM but only goes around 10 mph. Feels rougher then.

  13. #13
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    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    Yeh as noted you have something busted in the driveline. Coupler or hub or stripped out splines or something. All these possibilities will be taken care of by Big Bill.
    Practicing the ancient art of ren-ching

  14. #14
    Join Date
    Nov 2005
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    Mt. Zion, Illinois,
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    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    Quote Originally Posted by larsolr View Post
    Out of the water with no load I can take it easy to 3 or 4000 RPM. It seems to surge a between 3 and 4000 RPM but sounds strong. When I put it in the water it revs to around 2000 RPM but only goes around 10 mph. Feels rougher then.
    A couple years ago my Mercruiser 5.0L Thunderbolt V ignition developed an odd surging. When I would go WOT to get on plane it didn't have that throaty sound it had previously and when cruising RPM would drop, then pick up, drop......but on any regular cycle.

    I bought the boat used and had never done a tune-up on it so I decided to do a full swap: plugs, wires, cap, rotor, and sensor assembly. I can't say exactly which component (or combination) was the culprit, but doing that wholesale parts change fixed the problem. I had neglected to do any of that maintenance and was probably lucky that I hadn't experienced prior issues.

  15. #15
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    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    Quote Originally Posted by sandkicker View Post
    If you want really trouble free boating,
    1) replace cap and rotor annually
    2) plugs, if platinum tipped, last several seasons. regular tipped plugs are a seasonal change out IMHO.
    3) Good OEM wires or premium aftermarkets wires are good for about 5 seasons.
    4) Fuel filters are an annual or every other year ( if drained and refilled with fresh fuel annually)
    5) Oil and oil filter annually as is the raw water pump impeller in the drive ( per MERC Service bulletin).
    6) Check the ignition timing while you are at it...checking this on a Thunderbolt equipped engine is not the same as a mechanical points system.
    Good advice. Live and learn, but good maintenance is the key to reducing problems.

  16. #16
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    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    Quote Originally Posted by kevinj View Post
    Good advice. Live and learn, but good maintenance is the key to reducing problems.

    Further raw water pump impellers.... IMHO....If raw water inlet is thru HULL, you might get 2 or 3 seasons out of an impeller. If raw water inlet is thru the drive...annual replacement. Recent observed (not my boat!) failure mode of a 5.7L FWC inboard w/engine mounted raw water pump... Running OK previous season(s)... Started and ran without issues this season .... One day: Hard to crank, then engine started...funny high pitch squeal noise... then a BANG. Diagnosis... impeller seized in housing, crank pulley ate thru the belt... belt remains left engine at high velocity.
    Last edited by sandkicker; 07-18-2018 at 11:01 AM.
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280
    1972 22ft Bristol
    Honda 7.5HP 4 stroke Long shaft

  17. #17

    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    Looks like I'm running on borrowed time. My '96 4.3 LX Thunderbolt IV has original cap, rotor and wires. Low hrs. has probably saved me. The engine is running great, but I'm going to change these next week. I don't need any more performance, but these ignition parts are the only old ones after this winter's deep maintenance.

  18. #18
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    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    P/E process of elimination,replace one part at a time until you find the culprit, otherwise you will never know what component was bad

  19. #19
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    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    Quote Originally Posted by johnnygjr View Post
    P/E process of elimination,replace one part at a time until you find the culprit, otherwise you will never know what component was bad
    .... he's not saying you shouldn't replace all the parts you mentioned...
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280
    1972 22ft Bristol
    Honda 7.5HP 4 stroke Long shaft

  20. #20
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    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    That's correct capt Bob

  21. #21

    Default Re: High RPMs but no power Mercruiser 4.3 Alpha 1 Drive

    Ok thought I would top off this thread by telling you what the mechanic did. None of the obvious typical things were a problem. The fuel pump had the right pressure and the cylinders had per spec compression.

    First he installed new plugs, cap and rotor and this picked up 600 RPM instantly. It helped but the boat still dogged out. He also found one spark plug wire with high resistance so he changed it out. That helped some as well but not enough. He would have replaced them all but it seems the whole town is out of marine plug wires. He said all the shops in town are backlogged. So I will get a set and put them in soon.

    Anyway, still not enough power. He connected a test module to the shoe on the distributor cap and went reading by reading.

    So under load the test module read timing wasn't advancing to 22 deg BTC (I think that was the setting he mentioned). That was the problem but the mystery was why. Nothing looked amiss. He checked the alternator and it wasn't putting out enough power. I think he said it was only putting out 10 or 11 volts. Intuitively this didn't jump out at him because the boat cranks up very strong. But while he was in the water he was toying with the trim and he noticed that as he did the RPMs lowered. This clued him again.

    So he tested the alternator again at its base and the voltage was fine. It was the connections. So he cleaned them up and clamped them down tight and now it works great. What is weird is that the connections had no visible signs of corrosion at the alternator and battery. They looked perfectly clean.

    In his words, now the boat is fun again. I will bring it out this weekend to see.

    So thanks to all who made suggestions. I am a newbie at this and certainly learning. I hope the discussion thread helps someone in the same predicament. Happy boating!

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