Merc 80-hp rough idle

[brooksk]

I have an '78 Mercury 80-hp which runs rough at idle and full speed. I replaced all four spark plugs and added new fuel on top of ~3gal of old from last year - Stabil added and engine fogged before storage. After replacing the plugs last week I realized I had overtightened by ~1/4 turn. Late last summer the engine ran great but discovered that plug 4 was so loose I could unscrew it with my fingers. Yikes! I never even touched the plugs last year - had the engine services by a 'reputable' dealer. I have new plastic tank, new fuel lines, clean and rebuilt the carbs (late last summer) so something since then has happened.<br />Any suggestions?

 

[flybyboat]

Re: Merc 80-hp rough idle

you may have accidentaly put some dust or some thing into the piston house. this can create major damage. this is only a idea dont think it is all true

 

[jim dozier]

Re: Merc 80-hp rough idle

Do a compression check. Report the numbers here.<br /><br />Check to see if you are running on all cylinders by unplugging spark plug wires 1 at a time (ground the unplugged wire to the engine) and see if any cylinders are not firing (the one that makes no difference when you unplug it is not firing).