Carb Help Needed

Sounds like once it`s heated up, expansion maybe allowing an air leak, thus leaning it out/higher rpms? or fuel pump not delivering enough fuel? Check all hose/hardline connections. Spray carb cleaner around carb and base, also intake manifold. If you have pretty paint substitute with wd40. A rise or drop in rpm will indicate a vac leak, in that area. Could be time for a good carb rebuild. I assume you have a mechanical secondary carb? If so, you`ll have some serious tuneing to do. to get economy. For economy and not too shabby performance, a spreadbore intake, thermoquad, can get you 20+ MPG or a holley Vac secondary carb.