I don't think that's it either, I have a 3/8 vent in the tank. As for temp.....outside air was about 52 on the way in and 68 on the way home. The temp gauge hit 180 and stayed there. I keep thinking ignition but today I tried initial timing of everything between 5 and 15 degrees. I recurved the distributor for more and less total adv. and in sooner and later......it all makes no difference....