Bike stumble when accelerating KZ400 B1 78 Help

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: 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?

Would consider heeding TexasKZ's earlier advice:

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,

Would also check and assure correct valve clearances.
And proper functioning of the timing advancer (where so equipped).

Good Fortune!

A 3mm difference is huge. If the pistons are actually different sizes, you will likely never get it to run properly. If they are exactly the same size and one is higher than the other, that must be corrected to get the engine to run properly. If the engine starts from cold with no choke, the carbs are way too rich on the low speed circuit. Typically this is caused by too high fuel level (you say that has been eliminated), problems with the low speed mixture screws (broken tips on fuel screws, clogged passages on air screws), leaking enrichner system if so equipped, and remotely possible is that someone has badly gouged out the low speed fuel passages.

.....and as Patton wrote, correct valve clearance is crucial to proper carburetor function.

Thanks for the help.
Changed the vaccum piston, and all the jets. Just like a beast!!!
The only thing that I noticed : timing set correctly with strobe. When i advance the timing a bit I get an increase in revs and also has more power, runs nicer than with the owner manual ignition timing setup. Read many things about the ignition advancement...dunno weather I should leave it like this or as the manual quotes.
Kind regards friends

Hello.Iknow this reply is late,just kinda hoping you check in sometimes.What was the end result ,bigger or smaller jets?