Have recently bought my first xs2 -72 with 38mm mikuniförgasare... When I start up the bike, the idling will begin to "walk up and down and around" and when I have run the bike a bit and then stops at a crossroad, for example, it is very high idle. Had a leather strap in my pocket as I clenched the carburetors in the frame up against the petrol tank ... Then it went just fine! Is this a common problem that they pull air leak?