1992 Mercury 115 clean and dirty plug?

[justchecking]

I just rebuilt my 1992 merc 115. I have been running on 25:1. I looked at the plugs after an hour of idle time and found plug #1 with dry carbon. #2 is sparkle clean? I pulled both boots while the engine was running and both are firing. (#3 and #4 are clean but aren't getting fuel at idle per design.)

Why is #2 so clean, or why is #1 so dirty?

 

[CharlieB]

Re: 1992 Mercury 115 clean and dirty plug?

Possibly the idle mixture screw on #1 is slightly richer than necessary.

Some of the 2+2 used different main jets in the carbs. You might want to look and write down which main jet is in which carb and compare to the factory parts manual. If #3's larger jet was inadvertently switched with #1 during cleaning, then #1 would run rich which #3 may be running slightly leaner than design. This may not be a problem at moderate throttle but it could be serious at WOT for any length of time.

Float height can also make a bit of difference if they are not adjusted well the same.

 

[racerone]

Re: 1992 Mercury 115 clean and dirty plug?

Clean plug likely means one of 2 things.----No fuel there or no spark._Do not run the motor untill you determine the problem.----Did you determine and correct the reason for the failure that resulted in the need to rebuild ???----If not then there is a risk that it may fail again !

 

[justchecking]

Re: 1992 Mercury 115 clean and dirty plug?

Thanks for the feedback. I did find all the carbs seriously gunked up before rebuild, so I did a complete carb rebuild, setting float height, etc. I also found the fuel pump assembled incorrectly and rebuilt that. I also completed an electrical and timing check and confirmed that to be in order.

Next time I start the engine I will adjust each idle screw to determine if they are flowing the same. I did confirm by pulling off the spark plug boots that they are both (1 and 2) contributing to idle.

 

[Faztbullet]

Re: 1992 Mercury 115 clean and dirty plug?

Could be a bad plug...seen a few lately

 

[justchecking]

Re: 1992 Mercury 115 clean and dirty plug?

Thanks faztbullet, I'll switch #1 and #2 and see what happens to try to isolate. Too much rain past few days, hope to get back to it Friday.