Mercury Mariner idle problem

[Runningman]

I have an older Mariner 48 h.p. 2 cyl.outboard which starts fine but runs at idle very rough. After several minutes the motor begins to over rev. The kill switch would not shut off the motor and pulling the leads off of the plugs had no effect. It continued to run on its own. The only way to shut it down is to give it full choke. It will immediately start again and repeat the process. A mystery as it ran fine when I put it in storage for the winter. Any help with a diagnosis would be appreciated.

 

[SpinnerBait_Nut]

Re: Mercury Mariner idle problem

Welcome to iboats.
Mercury/Mariner outboard forum.

 

[HondaPower]

Re: Mercury Mariner idle problem

I do not have any knowledge on your motor so there may well be an issue specific to it which causes this problem, but my thought is it may be a heavy carbon deposit. The carbon becomes hot enough to ignite your fuel mixture without the plugs firing, so the engine will run on even though the ignition is turned off. This condition on a car engine is called dieseling. If this is the cause it would run much rougher than normal. Decarbing the motor should make this go away if that is the cause. You may have to do it several times to get it clean, do a search here for decarbing and or seafoam.