1978 Mecury 90HP I-6 power prob

[stick31817]

Can put the Boat in the water when cold and has all the power until it warms up. Once warmed up motor has no power. All it wants to do is spit and sputter. I've checked the compression on all six cylinders and everthing is good. Found some rust on the bottom cylinder plug. Replaced fuel tank and filter. Check all the carbs seemed pretty clean. Any suggestions to the problem it might be.

 

[bullbat]

Re: 1978 Mecury 90HP I-6 power prob

id say check your fuel pump and make sure your not losing prime in the fuel system and also make sure that the engine is not overheating

 

[hkeiner]

Re: 1978 Mecury 90HP I-6 power prob

There are many possiblilities, so doing some simple troubleshooting steps may lead you to the problem area more quickly than fixing/replacing parts on a hunch or educated guess. I suggest that you check the spark on all cylinders to make sure you are not losing spark or getting a weak spark when the motor warms up. You can use an inline spark tester, inductive timing light, or DVA meter to do this. If theh spark checks out OK, then I would suggest squeezing the fuel bulb to see if the motor responds favorably or not. After doing these two simple tests, report back on what you found.

 

[stick31817]

Re: 1978 Mecury 90HP I-6 power prob

The fuel system is working fairly good from what I can tell. The bulb stays tight. Getting good response in the water when the boat is first dropped in, but after about 45 min. the nothing. Hardly any power. Doesn't do it at home though with the muffs on it. Just in the water. Expecially with more the one person or load on it. Changed the fuel system and checked the carbs. And the compression on all cylinders.

 

[bullbat]

Re: 1978 Mecury 90HP I-6 power prob

are you sure that you are not overheating the engine?

 

[hkeiner]

Re: 1978 Mecury 90HP I-6 power prob

Did you check the spark yet? If you are losing spark on a few cylinders when the motor warms up, the motor will seem to run fine on muffs (because there is no load) but run poorly under load. Ignition parts sometimes fail only when warmed up by the motor and start working OK when cooled off. By the way, a spark can weaken but not completely fail, so be carefull that you test the spark correctly.