I have an '86 600 with a weak spark condition. It is weak enough that it doesn't even try to start. I have measured the resistance on the primary and secondary side of the coils and everything looks normal. I checked/cleaned the pickup coil and replaced the spark plugs. The battery voltage is good and the the engine turns over freely.
Im looking at the IC Igniter as the culprit but I dont know a lot about it.
check the ends of your plug wires. if they're corroded, and on such an old bike this is possible, they'll give weak spark. you can clip about 1/4" off each wire end to clean up the connection real good, and it'll work great again.