slow down the idle.. check the timing for to see if its too far advanced,, dieseling will occur also with to far advanced timing just like knocking, it dont have to have an ignition source..will fire from compression, doubt it tho on an old inline.. if your worried about carbon deposits..run a little stream of water through the carb while running at fast idle..... (steam em out)....dont pour to much tho might hydro lock it.....99% of the time tho...slowin down the idle will fix it....
