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Hi I am having the same problem on a 1981 kz650 but on cylinder 1 & 4.
bike runs great from time to time then intermittent spark.
seems worse when the bike warms up.

Adjust the valves?
Steve

Hi Steve
checked the valve clearance all ok compression check ok.
still runs terrible. thinking weak spark.
i guess next check is the wiring.
any suggestions would be appreciated.
thanks.

Pic of the plugs?
Steve

Ha! I'm not sure what pinball.ken is talking about he must have a similar problem. Anyways I have not checked the valves as it seems to be an electrical problem I am dealing with and the bike ran well last time I ran it. I have a parts bike that I gave to my father in law who is very OCD about his things. He has been going through that bike methodically nursing it back to health. I have been going over to his house and taking pieces off of it to help my troubleshooting but it would be much easier if I had access to both at the same time. I have the coils to try next after work today....

Hi i hope this helps, I found the problem on the bike i am working on.
check the voltage to the coils on the red wire to the coil to positive side of meter negative side of meter to ground.
mine was only 11.3 volts. it would not fire both coils.
found a broken wire in the harness running from the electrical box to the headlight.
fixed the wire and now have 12.6 volts to coils and bike runs great.
let me know if you want a picture.

ugh looking at when I started this post it's been three weeks already. Yes Please Pinball Ken I would love a picture. thank you for the help, I will check the voltage at the coil wires like you suggest.

here is the pic of the damaged wires. they were very hard to find inside the vinyl casing in front of the tank running into the headlight.

thanks pinball.ken here I was assuming there was a faulty part that was causing my problems. I should have known better and checked the wiring. My friend always jokes when I find something fucked up- I bought this bike off of an "airplane mechanic". anyways I was trying to troubleshoot the coils and I had the key switched on. I leaned my hand against the main wiring harness and the lights turned off. I noticed this and tapped it and they turned on then off. I realized the vinyl was gone and it was electrical tape covering the wiring harness. After some inspection I found a broken wire.

Airplane mechanic?
Steve

Yeah airplane mechanic. The PO literally told me he was an airplane mechanic when I bought the bike from him. I've found all kinds of shady stuff someone did to this bike while I've been going through it. Doesn't instill confidence in air travel.

are these crimps stock? I can't imagine that they are. wrapped with blue electrical tape to identify them as crimped wires. then wrapped with black electrical tape over the whole bundle to protect them....