1980 750K (not firing on #2 cylinder. Only sometimes)

[Joshuaeye]

Hey,

I have a 1980 750K.  If you go from left to right counting the cylinders it is the number two cylinder that won't fire.  It will sometimes.  Mostly it's just ruining the plug all the time so it's hard to know if it's working or not.

I've changed out the ignitors under the seat for another set i had.  Switched the coils out for another set, changed the wire, changed the boot, changed the plug...   sometimes it'll run fine, than usually when you come on to it it'll break down... Than after that usually the plug is so fouled that you're starting from scratch again.

The compression is a bit low and i believe it's on that cylinder.  But it's not extremely.  the others are 120 and it's like 110 ... something around there...    maybe 100 i can't remember.  i've had the carbs balanced and the valves shimmed.  it doesn't burn any oil.  

Does anyone have any idea what this problem could be?  It would be greatly appreciated.  I'm pretty frustrated with the bike at the moment.  This is the only thing that's really holding it back from being a fun bike.  

[Cheryl]

You may have already searched...but the wear of the plug can give you clues.
 
http://www.gnttype.org/techarea/engine/plugs.html

hope that helps a bit... good luck!

[LukeM]

Assuming the spark is strong on the other plug in this pair (1 and 4 are on one coil, 2 and 3 on the other), that narrows it down to the #2 wire, end cap, or spark plug.  First swap the plugs from 2 and 3 and see if the problem moves to the #3 cylinder.  If so, it's a bad plug.  If not, then the next likely culprit is the end cap.  These are a press fit onto the coil wire (and yes, it's wire, not the fiber stuff in newer car wires).  If there's enough length, remove the end cap for #2 plug, trim back about 1/4 inch of spark plug wire, and reinstall the end cap on the wire.  Make sure the point on the end cap contacts the wire in the cable.  Then put the end cap back on the spark plug and see how she runs.
If that's not it, it's possible the SP wire is broken at or inside the coil.  Then you're talking about a new coil.  EBay or one of the vintage shops would be your best bed.  

I hope this helps.  Keep us updated.
Luke M