Your ignition is too far advanced and making the engine fire after TDC and not before thus it runs backwards. Old Bultaco's were reknowned for this. Remove your magneto and loosen the points/plate or stator plate as some call it. IF you dont have a specific manual stating the correct timing procedure then move the plate anti clockwise a little at a time. Reset your points each time to the correct gap and things will only get better. Under hard accelleration if the engine rattles excessively then retard the ignition more as it's still too far advanced and suffering detonation which isn't good even in small doses. Hope this helps a little.
VMX Suzuki Central, January 2009
