Check your needle and jet. Clean them all out. If you block some of the intake will it rev. up. normally.
My 78 Puch stopped running right as soon as I put a new air cleaner on. Am running a 56 Jet. It runs great with the old clogged up air cleaner. Sputters out with the new free breathing filter. So I'm pretty sure I need to upjet to get things back in balance. These are both stock air cleaners, so I was quite surprised.