Bike stumble when accelerating KZ400 B1 78 Help

Sharky wrote: Hello,
I am currently hitting a bump with my KZ400. No services around that will help with such old bikes.
Bought the bike 2 weeks ago.
Problems: Idles well, starts without choke, runs rich(puffy black sparks), when I accelerate it stumbles and the picks up speed and then stumbles again,it is like riding a horse.
One oh the vacuum pistons is 3mm shorter than the other( may cause a rich problem but not that bad, i think)
I did clean the carbs, they look ok, Jets are the normal size. I have a spark on both spark plugs, carb float level is 2mm below the carb line, fuel line clear, no inline filter mounted, petcock working. Diaphragm not broken.
Any ideas?

Sharky wrote: Hi,
I am quite new to the forum. Bought a K400B 1978. When I started to clean the carbs I noticed that one of the vaccum pistons sits higher that the other. Inspected both, no sigh of wear or bend. Is that normal?Does it have something to do with the fact that it is a singe exhaust motorcycle?!
Thank you

TexasKZ wrote: They should be at exactly the same height, and should be all the way down when the engine is not running or when the carbs are off the bike. Sounds like something is binding one. Gently push each one up, then let them fall. They should fall smoothly with a clunk when they bottom out. Dried fuel, corrosion, a bent needle, or maybe even a small bat colony could cause one to hang up,