Another thought - when you start the motor are you relying on the clutch alone spinning the engine to get it going or are you giving it a squirt of throttle as well - that might cause the rear tyre to spin...
A method I have seen used on cyclemotors using the various flavours of Villiers engines and others is as follows.
ride from a standstill and get up to speed with the relevant settings (ie decompressor/clutch to out, mixture rich and so on)
drop the decompressor or engage the clutch at which point the engine should fire without any use of the throttle.
disengage the clutch and continue peddling for a short while until the engine gets a little heat in it.
re-engage clutch and ride as usual.
This has the advantage of not spinning your tire and, in a worst case, dumping you face first into the gravel. It also allows the engine to warm so you arent running off on a totally cold engine.
Jemma xx