Fuel starve

I had the same issue with my 67 273 and ethanol fuel a long time ago. I could watch the horizontal filter fill and empty in cycles after engine warmed. I finally decided that more pressure was building in the bowl than could escape the bowl vent. When the float opened the needle gate, fuel vapor came back into the filter. A thicker base gasket under the stock 2 brl. and fuel line routing (vertical filter) and pump from 73 up model cured it. Then later on real ethanol free fuel/real gas came available all around me so thats what I buy for every carborated engine I own. Mowers, weed eater, all work better. I haven't had to replace those little plastic fuel lines on the weed eater again since.