mercury 120xr plugs 2 and 3 not firing -appreciate any help

[Stevireno]

Thanks for reading this and potentially helping me out...

I bought a used 1996 sugar sands mirage with a 120xr (I think, the flywheel cover is missing, so no serial number). I took it out and I couldn't get it up past 10 or so mph. Idled rough and would not hit high rpm under load. It was idling so rough, I suspected not all plugs were firing. I pulled the 4 plugs, top 2 plugs looked brand new, bottom 2 showed signs of normal use. I tested the plugs by cranking the engine with the plugs removed and found that the top 2 plugs are not firing at all, ever, and the bottom 2 fire everytime.

I know there are some known problems with the electrical box for these mercury models, and I am likely looking at replacing the switchbox, but I was hoping for some advice before doing so.

I did pull the top carb and it is fine, I am getting fuel to all cyls.

BTW, any other place a serial number is recorded besides the flywheel cover?

thanks...


-Steve

 

[calledinsick]

Re: mercury 120xr plugs 2 and 3 not firing -appreciate any help

Check the sticky at the top of this forum...there are some methods for initial diagnostics. Post up results and we can go from there.

Serial number should be on the block above the head/thermostat.

Doesn't quite sound right. If the top 2 plugs aren't firing, they should be wet/oily. You should also have the plugs well grounded to check for spark, and it needs to be a bright blue spark (or get an inline spark tester- cheap).

I made a simple grounding device from a household bare copper ground wire.

I took an old spark plug and wrapped the copper wire around the threaded end to make 4 loops. When you pull the plugs you can place the base of each in each loop (spark plug boot on) and the fourth hole can be grounded to a bolt on the engine. (BTW, mine is a SportJet 90, so only 3 plugs)

Crude drawing: