So you seem to feel it is an engine power problem -correct? Does it seem like a lean or rich issue? Yours may have an issue with the choke- so you should verify that the choke plate is open fully. Some of these had a choke wizzard that is spring loaded and do strange things. Have you ever dropped the carb bowl to check for water/ debris? Same thing when you do line service of any kind. A sample should be taken of the fuel and observed. Fuel cap venting?