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  1. #1
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    Default 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    Hi Guys

    I have a 2000 4.3 gl carbed motor that falls on its face between 2500 -3000 rpm and often stalls, however is does not backfire thru the carb and restarts quickly.

    There is no fluctuation on the tach. I have run this motor directly off of a 5 gal gas can with new fuel line. Same issue.

    The secondary metering plate on the Holley 4160 was plugged so I rebuilt the carb. Same problem. I have 7 psi fuel pressure at idle but it drops to zero when it acts up.

    I have run the fuel pump with a jumper wire between 30 and 87 at the relay cavity and don't see a voltage drop on either the power or ground side at idle. The fuel pump is new. I would expect to see more than o psi at 2500+ at my guage which is teed in between the electric fuel pump and the carb.

    Even after returning to idle it takes some time to recover to 7 psi.

    I suppose I should monitor the ignition with a timing light flash but can't get over the fuel pressure drop. I also may run the power and ground leads to the fuel pump directly to the battery.

    Any help would be greatly appreciated.


    Thanks in advance
    Jon Allen

  2. #2

    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    Is there a screen going into the pump? Soumds like it has a fuel volume supply problem. You can get a inexpensive spark tester that plugs into the plug wires and lights up if there is spark.

  3. #3
    Join Date
    Jun 2008
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    Island Heights, NJ,
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    2,605

    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    Is this a new problem, i.e. did this boat ever run any better than this?
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280
    1972 22ft Bristol
    Honda 7.5HP 4 stroke Long shaft

  4. #4
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    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    Hi Guys

    Guppy, the fuel screen is part of the new pump and yes it is clean along with a new 20 Micron spin on fuel filter as well as a new screen in the carb inlet fitting.

    Sandkicker, I believe it just starting acting up this season. Performed several fuel volume tests for 15 sec. and received anywhere from 4oz to 11oz.

    As I mentioned the secondary metering plate was clogged with junk and would not allow any fuel to flow from the secondary boosters. It seems like the fuel pump isn't keeping up with the motor.

    Will run a direct power and ground to it to see if it improves. Was wondering if there may be a known problem such as a wire splice in the harness that is prone to failure. Will monitor ignition with a timing lite flash to see if I may be experiencing an ignition issue as well.

    Wondering if an amperage increase that may come with rpm may be causing an issue with a marginal connector leading to the fuel pump.

    Seeing o psi on a fuel pressure guage at 2500 rpm has me led me to the fuel pump. I believe that the fuel pump is losing power/ ground.

    Thanks again

    Jon Allen

  5. #5
    Join Date
    Jun 2008
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    Island Heights, NJ,
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    2,605

    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    re: "...Performed several fuel volume tests for 15 sec. and received anywhere from 4oz to 11oz." Merc fuel pumps are rated in tens of gallons per hour. 11 oz/15 seconds is consistent with 20 gph... 4 isn't. Measure volts @ pump when engine is running. It will likely be annoying to do.
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280
    1972 22ft Bristol
    Honda 7.5HP 4 stroke Long shaft

  6. #6
    Join Date
    Sep 2008
    Location
    Maryland - USA
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    6,824

    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    assuming the issue was present with the relay 'jumper', and the pump is brand name (they make lemons, too - just infrequently), I'd suggest focusing on the fuel supply to the pump.....a vacuum gauge connected to the 'extra' outlet on the filter makes for great troubleshooting....I'd be incline to think there's crud in the tank and its covering the screen on the bottom of the dip tube or has accumulated in the anti-syphon valve....

  7. #7
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    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    Hi Guys

    This problem will occur running off of a 5 gal can. I found a fuel pump connector w/ short leads from Autozone p/n 800-9213. I will connect it to the fuel pump and extend the leads which I will attach directly to the battery with alligator clips. This will eliminate a fair amount of potentially unfit wiring.

    I am curious as to what the volume spec is for an electric pump which is used in conjunction with a carb. I have never been able to find one. Does anyone have this info?. Thank you all for your help . I will post the solution. You guys are the best!

    Thanks
    Jon Allen

  8. #8
    Join Date
    Jun 2008
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    Island Heights, NJ,
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    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    Well... the only spec I saw said 20 to 30 gph free flow.... The observed fuel flow for that engine is about 18gph in published tests. Electric pumps free flow about the same at idle as at WOT. Usually pumps are pressure tested with a "t" fitting on a running engine.

    While the attached is a MERC doc....a 4.3 carbed is a 4.3 carb....at least at this level.


    electricfuelpumptesting.PDF
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280
    1972 22ft Bristol
    Honda 7.5HP 4 stroke Long shaft

  9. #9
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    Sep 2008
    Location
    Maryland - USA
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    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    The oil pressure safety switch is another suspect.....it should be wired in the supply side of the pump, between the pump and the relay - check the wiring diagram to make sure....it should be bypassed if you use the 'test connector' scheme you described....

  10. #10
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    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    Mark

    This particular V/P 4.3 does not utilize an oil pressure switch in it's fuel pump circuitry like a Mercruiser or possibly an older V/P.

    The F/pump relay is triggered by voltage recieved from the starter motor solonoid during cranking and alternator voltage when the alternator is turning and producing voltage.

    Running battery voltage temporarily to a fuel pump for testing purposes in not a scheme.

    Jon Allen

  11. #11
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    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    Jon:

    perhaps "approach" would have been a better choice than "scheme"; no ill intentions intended.

    I don't see how using the alternator's output satisfies the intent of the federal code for safety purposes....even if the field excitation was used.

  12. #12
    Join Date
    Jun 2008
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    Island Heights, NJ,
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    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    If there is a diode isolated output from the alternator, it would satisfy the USCG requirement.... No alternator round and round, no volts. Diode would prevent voltage feeding back from the battery and running fuel pump.
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280
    1972 22ft Bristol
    Honda 7.5HP 4 stroke Long shaft

  13. #13
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    Sep 2008
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    Maryland - USA
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    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    It would take more than an isolation diode.....any viable alternator mod would cost more than a pressure switch (in OEM quantities).....

  14. #14

    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    Just curios, Carter makes a marine certified electric fuel pump that we use on 500hp big blocks, wonder if he is having a fuel supply problem he could install this pump in place of the stock one, wire it up according to factory wiring and still pass USCG spec. This would definitely eliminate any supply problems.

  15. #15
    Join Date
    Jun 2008
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    Island Heights, NJ,
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    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    Quote Originally Posted by makomark View Post
    It would take more than an isolation diode.....any viable alternator mod would cost more than a pressure switch (in OEM quantities).....
    Yes, and no.... a simple isolation diode on the output won't do the job, however there is a point internal to the alternator, on the bridge rectifier and before the regulator circuit input, where a simple diode (about $0.06 in OEM quan) would do the job.
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280
    1972 22ft Bristol
    Honda 7.5HP 4 stroke Long shaft

  16. #16
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    Sep 2008
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    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    ok...now you got to mod the case for the new output.....not saying its impossible, just saying not likely due to economic drivers....

    Jon: is this a 'unique' alternator?

  17. #17
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    Default Re: 2000 4.3 GL carbed motor falls flat at 2500-3000 rpm

    Hi Guys

    I was able to fix the fuel delivery problem.

    I want to share it with you because I would never want to see any of you chasing your tail like I have for last couple of weeks.

    You guys have always helped me out over the years and I really do appreciate it.

    Mark, I really didn't take any offense to the word scheme. I apologize, you have been more than helpful over the years to me and everyone on this site. Shame on me.

    When I paid my first visit to the boat I took it out for a brief run and it fell flat at 3k. My diagnosis was lack of fuel. The owner of the boat was planning on using it before I could dig into it.

    He asked me if there was anything he may be able to try before I could look at it.

    He is a very talented plumber and I suggested that he look at the tank vent, the anti siphon valve, replace the spin on fuel filter and run a new 3/8 fuel hose from the tank to the motor.

    The anti siphon valve is aluminum and a tad larger than your ordinary 3/8 barb.

    When he attatcted the new hose to the anti siphon valve it was a very tight fit. While pushing on the new hose he snagged the inner liner and blocked off 80 percent of the hose opening. It was like the liner was rolled up in a ball.

    When I was still having issues after rebuilding the carb and replacing the fuel pump due to low pressure above 2500 rpm and low volume I decided to run the motor off of a 5 gal can. Still falling flat at 3k. I never suspected that the tank end of the new hose was severely damaged/restricted by being forced onto the anti siphon valve.

    I never thought to look. This situation led to a lot of wasted time and the unnecessary replacement of an expensive fuel pump which was not needed.

    The fact of the matter is that all this boat needed was a carb rebuild. It very well could have been me who forced the hose onto the fitting disregarding the fragility of the new style hose's inner liner. I hope this may be helpful to everyone.

    Thanks to all

    Jon Allen

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