I own a derbi drd pro sm 50 (2 stroke), it's completely standered apart from the back sprocket has been changed from a 53 to a 48 so it's hits 50mph, running the standered 12 t on the front... I brought a brand new full arrow pipe and I want to know what main jet size I will need to make it run how it should... The standered jet is a 80... I have tried the exhaust before and it doesn't go past 4k, so clearly it needs upjetting which Is obvious, any help on what jet size would be great, Thankyou, the bike is a 60 plate.
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