Hi need help .have a 68 Chevy p10 step van with little later motor i believe to be a 292 w electronic ignition 3 speed trans - early to late 70s , washed motor that morning took to shop were they rebuilt my carb idling fast and zip ties on choke for some unknown reason had it rebuilt idles good seems responsive until i;m driving it no power but had power before so went back and they said tune up needed so i replaced all parts for tune up but not spark plug still no power feels like to me a vacuum leak maybe or out of adjustment they said carb checks out i'm not wanting to take back to these guys any ideas what i can do to fix my self