65 hp merc cutting out and bogging down

[rockytop93]

'75 merc 650 starts fine idles ok. when pushed to wot surges as it is running up to full rpm. At wot runs fine for a few minutes, then slows down suddenly to 2500 to 3000. moving throttle back and forth will not get it to speed up. Will be ok after sitting for a while, then same thing all over again. Choking causes it to die down, fuel prime bulb stays hard. New plugs.<br /> have taken carbs apart and cleaned. One thing, when testing coil first two tests in manual ok. Third test, tester between coil tower and ground showed nothing. Book says 57-74 ohms. If coil was bad wouldn't it cause problems at all speeds? How about stater in dist? Compression good all three clys. Has good spark all cyls. Hope someone can help

 

[sharkcat]

Re: 65 hp merc cutting out and bogging down

You are dropping a cylinder at WOT . Coils get hot & fail change the one that gave you the bad reading...Also check for corrosion at the ends of the plug leads.Hopefully this will solve your problems.

 

[rockytop93]

Re: 65 hp merc cutting out and bogging down

I have suspected the coil since the checks didn't all pass. However this model engine has only one coil. Perhaps a new one would be worth trying. Thanks for your reply. Allen

 

[sharkcat]

Re: 65 hp merc cutting out and bogging down

I used to mechanic on offroad racing buggies.I have been caught out a couple of times trying to dianose a high speed miss after replacing plugs,leads,dist.caps,rotors,condensors the only thing left was the coil & you guessed it that was the culprit.