Help on color of springs for race 50polini

Hello I have an Aprillia sr motard
It has front malossi variator with 3.5 rollers . Race 50cc polini cylinder .17.5stock carb and yasuni 21 exhaust. I have lowered the rollers but the acceleration is still low what else can I do to the back clutch to help with the acceleration (what color springs should I get?)
Thanks

 

The power band will be quite narrow, maybe between 8000 _ 8500 rpm so first check that the rollers are the right weight to maintain that rpm through the full speed range. To get faster initial pull away means getting the clutch to engage close the the power band. Stiffer clutch springs mean that the engine will reach a higher rpm before the clutch shoes open and engage therefore it will rev high and bite better, quickly entering the power band and shifting up.
Buy a pack of springs, they usually come in three colours. Open your clutch and compare the spring coils to the new ones. Less coils and thicker metal means stiffer springs so choose the colour that are stiffer then your current ones.