NO spark

1980 650 xj is intermittently losing power and sputtering while cruising and under load. Seems to come and go for no apparent reason. Once when it would not go away I pulled the outside two plug wires off the plugs (cylinder 1 and 4) and the bike ran the same. So I am assuming I am losing spark to those two cylinders. It seems to be more of a problem in 40 to 50 degrees and not as apparent 60 degrees plus. Where should I start?

 

check the resistance on the coils - should have, i think, 11 kOhms*, between the spark plug caps on #1 and #4 (same reading for #2 and #3) and i think 3 or 4 Ohms across the "lead from the ignition" - i can't remember exacly what it's called - Hayne's manual explains more thoroughly.

my guess is that it's more problematic when it's cold because cold makes things shrink (insert shrinkage joke here).

i have 1 good coil off an '81 seca 750 if it turns out you need a replacement. pm me if you are interested.

*note: you might get something closer to 21 kOhms (instead of 11 kOhms) i think it has to do with whether you have the commonly used 5 kOhm resistor caps. not a big deal - the main part is to not have 0 Ohms or "INFINITY" Ohms

edit: oh yeah - might be the spark plugs too - those are CHEAP and easy to replace (and if they're not the problem - you're not out too much money, and you got a spare set of plugs).