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86 Mercury 35hp only runs at idle speed

J

Joe Forsee

New member
When I first got the boat it ran great the first time out, next time after driving about 5 min it went from full speed to only running at a idle speed. I can put it in neutral and it revs up but driving it only runs at a idle speed. it's a little harder to start but runs slowly it doesn't cut out or sputter. I've replaced the impeller, both coils, plugs and fuel tanks, fuel line and the block gasket, OK now the block gasket it blown again. I guessing it only running on one cylinder. The block cover doesn't seem to have any cracks. I cant see any cracks on the cylinders but I'm afraid that's my problem. What do you guys think? Also I took to a guy to work on it and it ran great for only one day and know its back to running at a idle speed. I really need help in figuring this out I can't afford to take it to bass pro at $80 hr labor.......
 
"Normal" 1986 Merc 35 hp motor don't have head gaskets. This might be one of those foreign designed Mercs, however.

Jeff
 
plugs.jpg The Top Plug was dirty.
 
I guessing it only running on one cylinder.
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The top plug looks shorted out w/carbon and gunk. Did you check the end caps of the plug wires for corrsion? I put a timing light on the plug wires as close to the plug as possible to see if the plug is getting spark. Decarbon the engine w/Seafoam or Mercury's engine cleaner.

OK now the block gasket it blown again.
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You did torque the block cover to specs? The cover may be warped. Lay the cover on a sheet of glass to see if the entire cover sealing area touches the glass. If not then replace the cover.
 
I just checked the cover it's not warped, but I didn't torque the block cover when I bolted it up. Do you have any ideal what they should be torque at?
 
If the motor has bolts that go into the aluminum block , then you can only torque to about 100 inch lbs. Any more will start to stretch the aluminum threads.------------If it has bolts with nuts then you might use high strength bolts.-------------Good preparation / clean surfaces and a little smear of sealer is all you need !!!
 
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