this carb has been gone through (last week) by a good carb guy. It is on a 78' 2f, vac. advance moves inside the dizzy like it should, hooked the 78' ignition and ignitor up to one hot ingnition wire, truck idles and runs very strong/smooth but there is an area just off idle where it drops on its face and then comes back to life very abruptly. Carb guy thinks it just the transfer from idle circut to power circut, I have owned a lot of 40's and never had this issue. there are no vac leaks and the timing is set about 5 dgrees adv., where it seems to idle the smoothest. Also it only seems to do this under load, it revs up fine in neutral. Ideas?????