I've got a Puch Polini 64cc running a Polini-CP17.5 carb, and Tecno Circuit pipe (for the name).
Pulls well, she does almost a half ton.
However... If I ride hard, in that sustained 3/4 to WOT range, then let off the throttle and coast or brake down to a stop or low speed, the revs drop below 1000 and it takes it's time slowly building up revs to the set idle.
I put in the largest idle jet that I have 52 and upped the idle to about 1800 (main jet is at 110 now, but still rich). I was thinking it was a lack of fuel coming down from the high RPM/temp.
But now I am thinking this is a side effect of the rich main jet. The manifold and case are soaked after those high rpm runs where I'm backing off from 4 stroking. When the rpms come down, it needs to burn off all that fuel and isn't getting enough air.
So... Continuing to lean out the main jet may solve this?