Mercury outboard

[stevezandstra]

~~Mfg: Mercury Model: 850 SX Serial: 7091110 Canadian
HP: 85 Year: 1977
description_of_problem: What has been done. I took motor to the local boat dealer repair shop. $800 later and 6 weeks, worked okay briefly. Not happy!!!
New spark plugs, ohmed spark plug wires, new fuel/water filter, purged gas line and vent line, fresh non-ethanol gas.
Starts easily, not missing, idles okay, runs okay very low speeds and okay at first cold start up. Runs good 3 to 4 minutes at high speed, then loses 20 to 50 percent of power. After the motor sets for a few days, it will run great at high power for about 3 to 4 minutes again.
Tried pumping primer bulb at high throttle setting, no help. Tried choking high speed, no difference. Electric choke and at idle when choked will kill engine.
I suspect some type of carburetor problem. Possible too much fuel at high power after 3 to 4 minutes of initial high speed running when the motor really warms up.
I usually can fix a car engine but know very little about boat motors. An detailed email would be helpful.
It has been suggested to me that it might be a stator over heat problem at high speeds.

 

[SeaKaye12]

It could be fuel starvation but it could also be loss of spark in a couple of your cylinders; caused by stator, trigger or black box issues. You could try testing spark with those neon spark testers.....

CDI electronics has a tutorial for diagnosing Mercury ignition systems on their website.

Good luck; try and isolate the exact part(s) at fault....because ignition components on those are very expensive. As me how I know

 

[stevezandstra]

Thanks. Loss of spark at high power from a bad stator is the leading possibility and replacement is mostly my next repair. $$$$

 

[Nathan Bond]

So definitely follow the CDI troubleshooting guide and rule than in/out before doing it.