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  1. #1
    Join Date
    Jul 2020
    Location
    Perry, GA
    Posts
    4

    Default 84 Mercury 75HP OB Bogging/dies under load

    I am new to forums, please forgive my ignorance to rules or places this should be posted...
    Please help!!!
    I have a 1984 Mercury OB 75HP 4cyl; Ser#6503669.
    History: bought used boat 2 months ago. I knew it needed a lot of work. I rebuilt carbs; new fuel lines all the way to tank (built-in); cleaned out tank vent; rebuilt triangle fuel pump; replaced wires and plugs; 1 & 2 cylinders were not firing and figured out it was the switchbox (replaced) all 4 cylinders firing strong. I set timing best I could based on information that I reasearched. All 4 cylinders have 140 PSI of compression +/- 5PSI
    My motor starts easy and runs great on the trailer, in fwd; neutral; and reverse.
    Here is my problem:
    As soon as I put it in the water and put it in gear it dies. I was able to mess with the screws and idle bolt enough to get it to sort of idle. I was able to finally throttle up but it is not running correctly...acts like it is starving for fuel. I had to run the two low speed mixture screws about 2 or more turns to get it to idle under load. It almost acts like something is binding and holding back from idle or powering up.
    I also replaced water pump and seals in the lower unit (oil looked like milk).
    Where do I go from here? A "local" mercury guy says my motor is just worn out. I just can't believe with that much compression, it is "worn out" and not worth fixing??
    Suggestions?
    Comments?
    Cures?
    Thanks
    Greg
    Last edited by McRider1962; 07-03-2020 at 04:33 PM.

  2. #2
    Join Date
    May 2013
    Location
    TX.
    Posts
    407

    Default Re: 84 Mercury 75HP OB Bogging/dies under load

    If he says it's just worn out a compression test will answer that question. Merc manual says that anything below 120 psi could be a problem.......not to say that you couldn't limp along at 100 but now you have a number. Come back and we'll go from there. If you don't have one most any auto parts store has them.

  3. #3
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    16,814

    Default Re: 84 Mercury 75HP OB Bogging/dies under load

    That's good compression. I suggest timing it with a light. Remove the top carb's cam lever so you can hold it wide open (3,000 revs) and set the timing accurately.

    Also, check to see if both carbs are opening the same. There's an adjustment between the two if they aren't.

    Jeff

  4. #4
    Join Date
    Jul 2020
    Location
    Perry, GA
    Posts
    4

    Default Re: 84 Mercury 75HP OB Bogging/dies under load

    Quote Originally Posted by Texasmark View Post
    If he says it's just worn out a compression test will answer that question. Merc manual says that anything below 120 psi could be a problem.......not to say that you couldn't limp along at 100 but now you have a number. Come back and we'll go from there. If you don't have one most any auto parts store has them.
    Thanks Texasmark,
    The compression is 140 give or take a couple psi
    Last edited by McRider1962; 07-03-2020 at 07:33 PM.

  5. #5
    Join Date
    Jul 2020
    Location
    Perry, GA
    Posts
    4

    Default Re: 84 Mercury 75HP OB Bogging/dies under load

    Quote Originally Posted by fastjeff View Post
    That's good compression. I suggest timing it with a light. Remove the top carb's cam lever so you can hold it wide open (3,000 revs) and set the timing accurately.

    Also, check to see if both carbs are opening the same. There's an adjustment between the two if they aren't.

    Jeff
    Thanks Jeff...
    I will try those things... I am at my wits end with this... I cannot believe this motor is a lost cause...

  6. #6
    Join Date
    Nov 2011
    Location
    tennessee
    Posts
    4,510

    Default Re: 84 Mercury 75HP OB Bogging/dies under load

    Both carbs opening?
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  7. #7
    Join Date
    Jul 2020
    Location
    Perry, GA
    Posts
    4

    Default Re: 84 Mercury 75HP OB Bogging/dies under load

    Quote Originally Posted by faztbullet View Post
    Both carbs opening?
    Yes. They both are cracked open at idle and wide open at wide open...

  8. #8
    Join Date
    May 2013
    Location
    TX.
    Posts
    407

    Default Re: 84 Mercury 75HP OB Bogging/dies under load

    Quote Originally Posted by McRider1962 View Post
    Thanks Texasmark,
    The compression is 140 give or take a couple psi
    Sorry I missed that the first time scanning through your initial post. That is great compression.

  9. #9
    Join Date
    Sep 2010
    Posts
    19,767

    Default Re: 84 Mercury 75HP OB Bogging/dies under load

    Find another mechanic !!

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