Spark question..

[darin99]

I've been having problems with this 1990 90 horse 3 cylinder, the motor just doesn't seem to be running right.

So tonight while the motor was running, I pulled the spark plug wires off one by one to see what it would do. The middle cylinder every time I have pulled the plug out lately has been really covered in black soot, so I pulled the plug wire on it, and the motor seemed to increase in rpm.

The bottom cylinder would die almost instantly, and the top cylinder would somewhat drop in rpm but would not kill the motor.

Just curious if any of this makes sense?

 

[hkeiner]

Re: Spark question..

It does seem likely that you have an ignition problem. Systematic troubleshooting is the best way to identify the exact cause (failed part). A DVA meter can be used to check some of the parts in the ignition circuit. You can also move the coils around to see if the problem follows the coil or stays with the cylinder. Mercury OEM shop manuals have very good ignition sytem testing and diagnostic procedures in them. A DVA meter and OEM shop manual are two very good investments if you plan to find and fix the problem yourself. Also, it is always a good idea to do a compression test before spending much time/money trying to heal a sick motor.

The following links may also be helpful.

http://www.cdielectronics.com/downloads/troubleshooting guide/Mercury 3 Cylinder.pdf

http://dolphinmarineservice.homestead.com/ign5.html