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84 Mercury 75HP OB Bogging/dies under load

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McRider1962

New member
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
 
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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.
 
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
 
Texasmark said:
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.
Click to expand...
Thanks Texasmark,
The compression is 140 give or take a couple psi
 
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fastjeff said:
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
Click to expand...
Thanks Jeff...
I will try those things... I am at my wits end with this... I cannot believe this motor is a lost cause...
 
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