Hello,
I'm trying to diagnose this problem:
The NC29 will start with the choke. After a while and at full choke the RPM will go up and down between 2K and 3K
-If I turn the choke off , the bike will not maintain RPM and it will turn off.
-It will start without a choke , but it will not stay on unless I press the gas and pass the 3K.
-When I press the gas, I will turn it half way with no acceleration at all and then it will accelerate quickly to 5K.
-The bike will run ok pass the 4K

I'm looking for your expertise before taking the bike apart.

-The bike had the carb clean 9 months ago, but two weeks ago I found that the small foam filter was dry and in pieces.
Could parts of the foam gone trough the airbox and clog the carburetor?
Could this be a problem that could be solve with a Carb synch?

Thanks in advance for your help.

Best bet would be to take carbs off and clean them out,as then you know it's not the carbs at fault..although it sounds like a %100 carb related problem...was it running ok before you found bits of filter in there??

Definitely sounds like a carb problem to me - I suspect there will be bits of filter clogging the slow running jet (the long thin one)

.....and check the air jets at the mouth of the carb, that's the first place the foam is going hit.

Thank you for the advice.
I will clean the carbs and report back.