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  1. #1

    Default 5.7 MCM runs perfect for 15 minutes at 35mph then looses power/bogs badly?

    Hi guys I am stumped at what is going on with my boat. 1994 regal with 5.7 MCM 4barrel carb alpha drive, boat was garage kept fresh water used entire life 250 original hours. Off the trailer the yesterday for another test run, my buddy and I held the boat at around 3600rpms for 16mins straight at 32 mph. It ran perfect then seems like it falls on its face like itís starving of fuel. You can drop it to 2500 or so and keep the boat going but it will soon do the same thing where you have to drop it to idle. Sit and idle in water for a few minutes and go on your way for another few minutes and then same thing will happen. It seems like the longer you wait and idle or with boat off the longer it will run without an issue.

    Things I have done are:
    1. New fuel filter/water separator
    2. New fuel pump
    3. Removed carb and Cleaned with carb clean, did not take it apart at all.
    4. Checked fuel tank vent lines, completely open.
    5. Changed the oil to higher viscosity 15w40 ( read thread on guy with same problem had wrong oil and the hydraulic lifters couldnít build enough pressure with hot thinner oil caused the engine to starve of fuel). Didnít seem to help with my issue.

    Im stumped, today Iím going to pull the pick up in the tank see if thereís some sort of blockage. Is there a float in these 4 barrel carbs that could be starving of fuel from internal carb blockage? Not sure if itís an electrical or fuel problem at this point. We are going to test that next time Iím on water with carb clean to see if engine revs back up. Any other suggestions? The boat overall runs perfect besides this stupid problem. How can it run so long so weíll for it to be electrical? Thanks for the help.

  2. #2
    Join Date
    Jun 2008
    Location
    Island Heights, NJ,
    Posts
    3,153

    Default Re: 5.7 MCM runs perfect for 15 minutes at 35mph then looses power/bogs badly?

    Check to see if coil is HOT when this happens
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280

  3. #3
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    16,389

    Default Re: 5.7 MCM runs perfect for 15 minutes at 35mph then looses power/bogs badly?

    It sounds like fuel starvation. I suggest you tee in a fuel pressure gage after the pump and see what pressure it's holding--should be around 5-6 psi. That will show you once and for all what's wrong.

    Jeff

    PS: If this is a Quadrajet, did you pull and clean the filter inside the carb (where the fuel pipe attaches)?

  4. #4
    Join Date
    May 2010
    Location
    DC Burbs/Maryland Eastern Shore
    Posts
    1,666

    Default Re: 5.7 MCM runs perfect for 15 minutes at 35mph then looses power/bogs badly?

    Use heavy duty SAE 30. Buy it in 5 gallon pails off amazon. Change oil frequently.
    This would seem to be bad fuel. When was the last time you cleaned out the tank? Can you try a remote fuel supply to eliminate tis possibility?
    Practicing the ancient art of ren-ching

  5. #5

    Default Re: 5.7 MCM runs perfect for 15 minutes at 35mph then looses power/bogs badly?

    Update: removed the pick up from the tank and found some debris inside the elbow at the top of the tank which definitely could be restricting fuel flow. Going to take for test run on Friday.

    I will check if coil is getting hot as well. It’s a Rochester carb, no screen at end of fuel line. I have never cleaned out the tank.

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