78 400X carb issues?

[Sivrag]

I have a 78 400X which I received mostly assembled and not running. I spent some time verifying fuel air spark and got it back to running condition... For 10 minutes at a time.

I start with choke on and the bike idles around 1100. The right cylinder seems to be warmer than the left. It will occasionally give me little backfires when warming up, but at about the 10 minute mark the bike will either cut out and refuse to restart or rev high on its own until I shut it off, at this point I still have spark but can't start until the bike cools off. My fuel filter and fuel line are transparent so I can see fuel still entering the bowl.

I have cleaned the carbs, but have not done a full teardown and sync. I have replaced intake boots and o rings, gaskets for the entire top end, valve seals, verified valve clearance, timing, and set the air screw according to my manual (1.25 turns out).

I would like to think I have ruled out vacuum leaks since I have replaced top end gaskets and torqued everything down properly.
When I did my initial teardown there was a lot of carbon buildup in the right cylinder.

[KiwiAlfa156]

Hi there, you really can't clean carbs without tearing them down and visually inspecting for corrosion, sediment and damage. Compressed air and/or aerosol carb cleaner with a tube to squirt through all the jets, orifices, by-passes, and galleries to confirm that they are clear. It doesn't take much to stop a carb from metering properly. Never use wire to clean out anything in a carb. If you must poke something through, I wouldn't use anything harder than nylon fishing line.

[Sivrag]

Disassembled, cleaned, reassembled. Waiting on o rings for the needle jets.

[Gorminrider]

...while you're waiting, Check Ignition particularly timing advance mechanism, replace condenser now (if you have points) or investigate the "signal coils" if you have a black box ignition..also spark plugs, spark plug caps-dissemble to clean replace if doubtful..check high tension leads for cracks and low tension connections including kill switch etc...You still have spark when it doesn't start so assume coils (or black box) is ok until everything else checks out .

You have to do all this anyways on a bike new-to-you but more intensely on a badly running bike that doesn't improve with fuel and compression ok