Peugeot Speedfight Poor Acceleration & Revs Die Up Hills?

Hi,

My Speedfight was put away for the winter and when I got it back out it would only go 10mph at the most. I've since had it in the garage with new belt & rollers fitted even though the belt had only previously been on for a couple of months before. Since getting it back it will get to 50mph eventually on flat and downhill but the acceleration is not good at all and going up hills is terrible, the revs will die and the bike almost comes to a stop.

So I sent the bike to a different mechanic who has said the belt has frayed again already, a new one was only fitted a week ago! What could be causing this? He has had the carb stripped & cleaned, the exhaust has been de-coked and although better it's still accelerating poorly. Air filter has been checked and is clean.

Everything is standard apart from it has a Giannelli exhaust & Polini variator. Rollers are light (around 5g) & carb was jetted up from a 50 to 55. These were all fitted a year ago and it absolutely flew back then but not any more.

It's clearly a transmission problem but I've ran out of ideas and would appreciate any help / advice. Many Thanks.