Check that the drive chain is properly adjusted and not to slack, cause if this is to slack it will cause the bike to jump(make sure the Chain and sprockets r in good condition not worn and keep it well lubed).If the bike lying 4 a long time i would change the chain and sprocket as a matter of course cause. Their could be a seized link and that can put undue strain on gearbox bearings.
Busa Boy
March 2008