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  #1  
Old 11-09-2009, 09:45 PM
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Rickski Rickski is offline
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Default Motor issue

Started having a problem with my 1988, 80 HP, 3 cylinder Mercury. After a long day of trolling, she ran great right up to the ramp. The next time in, she ran so bad, I never even pulled away from the ramp.

Tried clean plugs, I have spark to all 3 cylinders, and I cleaned the carbs. I found a 30 PSI problem with my compression tester, and originally thought low compression was my problem. Turns out I'm running 95 to 105 across all 3 cylinders.

I get spark to all 3, But only one cylinder is firing. And the cylinder that is firing changes. At first only number one was firing. Shut it down, started it, and then only number 3 was firing. My check was pulling the plug wires off until I found the one that was firing. Still spark to all three. With all plugs out and turning over, I see a mist of gas coming out of cylinder 2, but none from 1 or 3. Wondering if I have some type of valve sticking problem, but if that was the case, then I would have seen it in the compression readings. Either way, the head has to come off so I can fix a spark plug hole that is on the edge of being striped out.

Any thoughts on what I'm chasing here? I've always had a problem getting the engine up to temperature on the temp gauge, so I'm wondering if the hours of trolling, on a cool motor, could have caused some carbon buildup. I've tried a few thermostats to address the engine running cool issue in the past.
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  #2  
Old 11-10-2009, 08:25 AM
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Default Re: Motor issue

Rick, when was the last time you changed the fuel pump???? it's a common problem as they get older, especialy with the alcohol based fuels, a tiny pin hole forms in the diaphram and flooding issues appear.. Coalman just recently went through the same thing... fuel pump solved his problem..

all this is asuming there is no water in the fuel
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Old 11-10-2009, 08:32 AM
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Default Re: Motor issue

also, I dont believe that motor has a removable head, it's only a cover plate over the water jacket, the jugs are a permanent part of the block... I have a spark plug thread chaser you can borrow to try and restore the thread... if that fails, you have to install a helicoil, it can be done with the engine intact...the other option is to split the block and remove the crank and pistons..

Always use never seize on the spark plugs... and it's a smart move to run a thread chaser into each hole to clean out any carbon...
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Old 11-10-2009, 09:14 AM
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Default Re: Motor issue

The fuel pumps were never replaced since I've had the motor. I've put 5 seasons on her, so I think I'm due. I think I need to go the helicoil route. I'm missing the first couple threads, and I can "feel" the others getting ready to let go. I've always been careful not to crank down on the plugs, but I've seen and felt the plug thread problem slowly getting worse, so it's time for the fix.

Thanks
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Old 11-10-2009, 11:40 AM
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Default Re: Motor issue

Rickski, All three of my cylinders were firing and I kept fouling #3. It would get soaked with gas. I also had mist coming out of my carb. After consulting with Roc and getting the minor fixes diagnosed it was down to fuel pump or carburetor. I chose fuel pump next.

If Riverside Marine in Salisbury sells Merc parts I recommend the nice lady at the parts department. I ordered the fuel pump parts over the phone. They arrived 2 days later with a detailed diagram of how to install the parts.

$40 later the fuel pump fixed my problem.

Thanks Roccus!
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Old 11-13-2009, 10:46 AM
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Default Re: Motor issue

I have a 90hp 3 cyl. 1986 merc. Had similar issues,finally rebuilt the fuel pump.i think the manual said 250hr.i was way over
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Old 01-23-2010, 01:04 AM
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Default Re: Motor issue

Using High test gas can help many problems like this. My 83' 7.5 Merc. 2 cycle wont run well on regular gas, even after rebuilding the carb.

"Chemtrol B-12" (WalMart) or "SeaFoam" (parts store) fuel conditioners are the 2 best on the market and also should be added along with Fuel Stabilizer, since gas today generates water after 60 days. There was a Merc. TSB on this 1-2 yrs. ago.
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