1977 Mercury 500 Has intermitent power loss and starts hard

[Mercurylips]

Purchased an older Sylvan with a 50 hp Mercury and when it went out on a test run it ran smooth but would do no more than 5 mph. So far I have cleaned and adjusted the carbs and replaced the bulb and hose, and the spark plugs. Found out the throttle linkage was a mess so had that redone.Now the motor runs strong after it warms up but suffers a top cylinder power loss at times. It starts great when warm but not cold. I am thinking plug wire or the top one of four coils could be faulty. I don't want to keep throwing money at it but I think it is a keeper and would like to know what else it could be. It runs great at top end and after a few minutes you get a drop in power but no sputter. Like someone just took a plug wire off. It will then go back to full power for a while and then a repeat. Easy to feel and hear. Thanks

 

[Mercurylips]

Took the muffs out and set up for a test in my driveway. The motor started a bit hard but seemed to run okay I didn't run it long and didn't run on full throttle much. Shut it down and removed the top three plugs. The top plug was wet and lower two dry. Problem seems to be related to top cylinder not firing. The intermittent loss or gain in power on the water (Lake)seemed to be about 20 to 25 %. What to try next?

 

[coin operated]

Try switching coils and see if the miss follows the coil.

 

[Mercurylips]

Thanks, Riopa. My marine mechanic and myself are heading out Monday morning to give it another test. I will see if I can get him on board with that idea. My merc seems to have hard coils to find. It is a 1977 and has red coils. I wonder what else may fit and work. I am seeing some red ones on Ebay for other HP motors. Thanks, Mike

 

[Mercurylips]

Late last summer I got the motor to run well at full throttle after it warmed up for about 15 minutes and would only do about 65% power at full throttle cold. I am wondering if this might be a fuel pump problem. There is a reed type in each carb?