TOPIC:

My 1980 KZ750 LTD won't start but it ran fine a about week ago. The starter turns, the spark plugs produce a blue spark, I've just cleaned the carbs to the best of my ability, and but the battery on a tender and it reads as full. What should I do or check next?

If you're getting spark at the plugs then you should look at your fuel supply. Does fuel flow out of the petcock? Could the tank be plugged?

I would also do a clear tube method to test the fuel level in the carbs.

If you want a quick confirmation that it is a fuel problem, Spray three quick squirts in each carb, then open the throttle and try starting it. Should fire up for a few seconds if all the timing and electrics are good. If so, this will confirm you have a fuel issue.

I checked the fuel line to the carbs and I could see the fuel flowing so the petcock/fuel tank don't seem to be the issue. Then I sprayed starter fluid in each carb before trying to start it and it still didn't fire even for a few seconds.

I'm not sure what the clear tube method for testing the fuel level in the carbs is, could you elaborate?

Did you test the plug for spark by substituting another sparkplug external to the bike?
Or did you remove the current sparkplug from the head and turn the engine over to verify spark.

I got caught out by this very scenario just a few days ago.
I pulled the leads and put in a substitute spark plug and turned the engine over. There was a nice fat blue spark.
But the motor would still not fire when I reconnected the spark lead.
Turns out that all my current spark plugs were fouled up.
I just replaced all the plugs and the bike fired immediately.

PS I prefer DENSO plugs over NGK. The NGK's cannot handle being fouled up whereas the DENSO's are much more resilient

Here is a link for clear tube method

kzrider.com/forum/3-carburetor/524078-78...0-help-please#524102