KZ 400-S2 suddenly stops running

Hello, I'm Smythe.
I purchased my 1976 KZ400-S2 new in 1977. It has 26,870 on it. I have maintained it myself and it is in original condition. Until eight years ago it has been trouble free. Then, a section of the left carburetor body that holds the main jet simply crumbled away. It sat in my shop while I rode my BMW or my Triumph.
My wife asked me several times to sell it, but I didn't want to do so. I kept the engine free knowing I would ride it again someday.
About five weeks ago I bought a set of old carburetors on-line. Carb job finished, I replaced the battery and oil and went riding. Then I began to notice sometimes I would need to kick it over 10 or more times to get the engine running. Normally one swing through with the kick starter is all that was needed.
Then after 110 miles of riding ( not all at once) since the carb repairs the engine suddenly stopped while I was on the highway.
I have installed a new battery (fully charged, leads tight), new ignition coil with new spark plug leads, new plugs (correct gaps), new condenser and points. Of course I have fresh non-ethanol gas, new fuel hoses and filters and have cleaned the pet-cock.
High and low beam head light works and the brake light works from foot pedal and lever on the handlebar. The tail light is functional. Front and rear turn signals work on both sides. The horn is clear and loud. Instrument lights work except for the high beam indicator which has just burned out.
I cannot figure out why the engine will start and run nicely with one swing on the kick starter. Then, simply stop after about 10 seconds. When it stops, I have to wait a while to restart. It seems to be an issue of intermittent firing.
What are your thoughts?
Thank you.

You may have done so already, but if not make sure the fuel line is not kinked. Ed

I'd check the fuel cap venting and make sure the gas tank is clean and free of rust. Also check the petcock function. And you didn't mention checking the valve clearance? That wouldn't typically cause the engine to stop dead but it could explain the hard starting.

In my experience, the Kz400's are among the easiest and most reliable starting bikes out there. If it ran recently, and it's taking more than two kicks, there is probably something wrong electrically.

Intermittent issues are a real pain to find. There are devices you can buy that go on the spark plug that lets you see if there is spark. You would only need one, and it would stay on all the time. So whenever kicking doesn't start it, you can see whether you have spark or not.

The most likely areas to check are the fuse box, ignition switch where the wires attach, the kill switch (very often the problem), and the wires that run from the frame to the steering area. It could even be a worn out contact in the ignition switch itself.

Another test you can try is to run a temporary wire from the battery positive to the coil's positive connection. Put an easy to reach toggle switch in the temporary wire. Then when the bike is difficult to start, see if turning on the switch makes it run. If so, then you have a faulty connection somewhere as mentioned above. This temporary wire would be in addition to the existing wires so you will have to use a piggyback connector on the coil.

smythe wrote: fresh non-ethanol gas, new fuel hoses and filters

What sort of filters (plural?) did you use? Some of them do not allow enough flow when operating under the small amount of vacuum that a motorcycle engine will provide.

You may want to remove the filters from the equation: run a new length of fuel line from the tank to the carbs, set the petcock to prime, and see if the problem persists.