If there is hesitaion on acceleration at about 2000 rpm you should increase the size perhaps by 25%. I did on my series II 1600 with great improvement.
This is mostly related to 1600's. I and a few other 1600 owners now run at 55 or 60 for improved transition with the solex 42's. Not heard much about standard 1300's having this problem with solex 35's when everything else is in order (valve timing, ignition timing, good compression, and valve tappet clearances).