Odd spark plug issues

[Beebe4]

I recently re built my 1976 25HP Johnson. It was running well, but I decided to change the coils. The coils that were on there were not that old, but the plug wire kept coming off where it attaches to the coil. Anyway, I got two new Sierra brand coils, and tried it. It runs, but spits and acts funny. I took the cover off, and notice the boot on the top cylinder looks charred right where the boot ends and you can see the porcelain. I took the plugs out and found that the spark didn't always fire, I also noticed an arc from the charred spot. What gives? The boots were on by the manufacturer, and the plugs are new. I guess if it aint broke don't fix it. Looking back I should have glued the wires into the old coil.

 

[gm280]

Beebe4, sounds like that plug got so hot that the spark plug boot charred. And once the boot charred, it provided an easier jumping point for the spark voltage then the spark plug electrodes. So looks like you may have to either replace that coil, or use one of the older ones and try it out. What type and brand plugs are you running? Are they the type and specified heat range for your engine?

 

[Beebe4]

Thanks for the response. I'm using Champion J6C or is it JC6? It's a slightly hotter plug then the J4C (JC4) (which is stock). I ran both plug types on the old coils and had no issues. I went with the 6's because I don't WOT a lot, and read somewhere those help with fouling. The other cylinder seems to be just fine. Are Sierra coils not that good?