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1973 Mercury 7.5 Hp Thunderbolt. Bogs down under load

T

tsteckle

New member
I have a 73 Mercury 7.5hp Thunderbolt. I had bought this motor 1.5 years ago not running. I went through the Carb/Fuel pump, New plugs, wires, points, and condensor. The motor ran great for the first 3 months and then late last year started bogging down/sputtering on acceleration with 2 people in the boat. Once the boat is on plane the sputtering goes away and runs good at higher speeds on plane. This has been getting worse each time I use it. Recently used the boat and now it is to the point that I cannot get the boat to plane out with 1 person in the boat, just bogs down spits and sputters. Engine runs great at idle and lower power settings.

Any ideas where to start? I am thinking something with the ignition.... faulty plugs or points? Any advise is greatly appreciated.
 
I am not sure. I would think it is though, after the boat is planed out speed seemed to be normal. I don't have much experience with these small marine engine but I was thinking if a cylinder was not firing it would be running rough and have a distinct misfire.
 
Can you bring back some power by pumping the sqeeze bulb? Have you checked for spark with a gap jumping tool? Make sure you get a nice crackle at 1/4 or even 3/8 inch. What is your gas to oil mixture? Tried new spark plugs yet? Using non oxy fuel I hope?
 
timguy said:
Can you bring back some power by pumping the sqeeze bulb? Have you checked for spark with a gap jumping tool? Make sure you get a nice crackle at 1/4 or even 3/8 inch. What is your gas to oil mixture? Tried new spark plugs yet? Using non oxy fuel I hope?
Click to expand...


I have tried to pump the bulb and have also added a little choke, both don't make a change. That is what leads me to think it is not a fuel issue. I have not checked spark or replaced the spark plugs, that was going to be my next step. I am using 100:1 AmsOil. I am using regular gas from the pump. What issues is that going to cause?
 
Bottom crankshaft seal comes to mind.------And the labarynth seal between the upper and lower crankcase needs oil to work.----So 100:1 may be an issue here.
 
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