My Gilera SKP 50 won't tick over?

I have got a 1999 2 stroke Gilera SKP 50 which won't tick over. I have cleaned the carb and air filter, replaced the plug and plug cap. I have also tried tuning the carb but no joy. Also the acceleration is very slow and can be non-existent at times. If I keep the throttle open I can reach about 35 but if I have to maintain a steady 30 then try to increase I can't, the only way is to drop my speed to below about 25 then try again this normally works.