f 150 backfires

Well, the truck runs pretty good now but it backfires when I am going down the highway and take my foot off of the accelerator completely. One person I talked to said it might be because the heat riser valve doesn't have a vacuum hose on it. What port is that suppose to be connected to? I have a Motorcraft 2150 carburetor. Is there something else I should check?

try retarding the timing, could be a blown powervalve.

do you have an air(smog) pump? a defective air diverter valve will do the same thing.