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91 Mercury 200hp - revs up and can't kill it

2

200merc

New member
Have an issue with 91 Mercury 200hp where the revs take off and you can't stop it with key or killswitch it just screams until it runs out of fuel!

1st time it happened last season the impeller failed while out on the lake and the motor overheated, I found some wires(green and white ones from CDI) that had melted together, seemed like the obvious cause of the problem, repaired and replaced wires (and impeller!) and all has been well since over several outings until yesterday while running in test tank at home just idling it happened gain - the revs just shot up and killswitch, key, battery had no effect on it - had to pull fuel line and wait for the fuel to run out.

I have seen a bit on the net about outboards not stopping due to faulty key/swicth/ground wire etc but nothing on the engine revving right up as well.

Both times the motor has been in neutral but it scares the S*&T out of me to think this may happen while in gear with the kids on board or something so really need to get to the bottom of it,

any ideas would be greatly appreciated!
 
Is it carbed or EFI? Having a factory wiring schematic and digital voltmeter will make it easier to troubleshoot the problem. Start with checking for a "faulty key/swicth/ground wire etc. Recheck the repairs that you made. Open up the bindings on the engine wiring harnesses and carefully inspect the wiring for chafing/shorted wires. Report what you find.
 
When a 2 stroke runs away its called "4 stroking" and the ignition will not kill it as you have found out. This is usually a carb sync and link issue and it can cause engine damage. Also check to see if the advance module wires are melted and grounded especially the wht/blk as it will raise the ignition timing...
 
Thanks for the responses,

It is a carvbed, I have a manual and multimeter etc. It appears to have had the idle stabilizer removed already and oil injection removed also to just run premix.

I spent the day stripping back all the wiring, checking wires and connections, found a couple of wires were the insulation had come back a bit by the connector but can't see how they could short out anywhere, I guess it's possible though (it was the white and the violet wires on the outside power pack, previous repairs are sound and coils etc test ok as per manual.

Kill circuit also checked out fine and no sign of any issues, it performs as it should over and our again.

Ran motor for the last 2 hours turning it on and off, changing idle speed, and revving. Couldn't fault it but still don't trust it! Hmm what 2 do........
 
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