Mercury Marine outboard misfire

[alanrr]

We have a Mercury Marine 30 hp outboard that was run with low oil level. The buzzer apparently sounded but was ignored. The water pump impeller chose that moment to shed it's vanes and the engine overheated and cut out. On examination I found the wiring harnesses melted but not shorting. The overheat sensor(Wow thats expensive!) had gone short and had blown the 20 amp fuse.I replaced the water pump impeller, the overheat sensor and fuse and repaired the wiring. I checked the bores and did a compression check which was ok. The engine fired up fine and ticked over sweetly. The problem is that it revs up to about 1/3 rd throttle and then rhythmically 'hunts'. It occurs at the same rpm each time and I suspect it could be some sort of rev limiter cutting in rather like a f1 race car sounds in the pit lane. I have changed the plugs and no other adjustments have been done. Any ideas!

 

[orca]

Re: Mercury Marine outboard misfire

G'Day. Another pom in trouble. When the alarm sounds for low oil you usually have a couple of hours running before the oil runs out. I suspect that the alarm may have been for overheating. To the best of my Knowlege there is no Rev limiter on these motors. Double check all wiring for shorts.<br />When a motor has overheated the coils, stator and/or switchbox may have fritzed and are failing under load. Check the spark with a timing light while the motor is running under load. <br />When the motor overheated the gaskets for the water jackets may have shrunk allowing water into the cylinders.

 

[alanrr]

Re: Mercury Marine outboard misfire

I did check the wiring very carefully. Only the wiring around the top of the cylinders was affected. The engine will run all day without trouble, it just won't rev over 1/3 rd throttle. Could anyone e-mail me a wiring diagram maybe?