Guys - the issue that lead me to retrofit the stock airbox (and rejet plus fit a different muffler) - is still there.
Bike idles fine, but on slow to medium take-off throttle is stutters a little and loses power, like there is no ignition or too little fuel now and then. At full throttle, the problem is almost gone. The plug is brushed and burnt clean of any residue, battery is full. Fresh gas.
Now...I did NOT take off the top of the carb when rejetting. I blew the floatbowl things and channels clean when apart, but I must have missed something.
Could this be the needle jet clogging up? The symptoms are identical to before I started tearing things apart. Setting the petcock to Prime or RES changes nothing.
Why oh why didn´t I split the whole thing while at it?
Or could this be ignition-related, like in oxidized connectors/intermittent earth loss and/or failing CDI box?