Idle Speed Racing

[nat]

Got a 1968 vespa sprint delivered 2 weeks ago. I stupidly rode it on reserve for a bit, since I've filled it up with petrol (and 2 stroke of course) I find that after I've been out for about 20 minutes or so the idle speed starts racing in neutral to the point where the bike starts vibrating rather worryingly. I assume its not the idle speed screw on the carb that needs adjusting as it didn't do this when I first got it.
Any advice gratefully received!!!

[Smallframe]

"thats an air leak that is" on the assumption you've not tweaked the carb settings , check how tight your carb and manifold box are and condition of gaskets. torque down the cylinder head. when idle is high, spray wd40 round suspect areas and see if revs come down. Have you any oil deposits on or close to flywheel and cover ( flywheel oil seal could be split ) ITS A PAIN, but its only a hobby.