Have you checked the plug to see if you have a nice light brown electrode? I know after fitting my Pinasco kit it can often take a while to get the slow running sorted properly with the mixture screw. Keep turning it out or in until you achieve the fastest idle for a particular tickover screw setting and then try the bike for a short distance and fine adjust 1/8-1/4 of a turn at a time until it feels right and keep checking the plug. Turning out is becoming richer, in is becoming leaner. Unsure of correct jetting for your kit but if you’re running in you don’t want to be lean. There are probably quite a few other posts from people with your setup for jetting info. Often pays to have a collection of slow running jets, air correctors, mixer tubes and main jets.
If it’s a newish machine with a soft seat these can restrict the breather hole underneath. If your standard exhaust is quite old this probably won’t help and may be choking the engine. If you change anything, just change one thing at a time otherwise you won’t be able to isolate the cause of the problem.
Good luck. AK
PS Don’t know anything about Cosa clutches and gearing set ups, do you? Just posted something last night.