Alright... I rebuilt my carburetor not to long ago and have pretty much the same result. When i put my foot into the secondary a little bit it keeps revving up and has a bit more power but if i completely floor it and open up the secondary all the way it just boggs down the engine. I think ive narrowed it to either throttle shaft bushings or a weak pump. Ive checked the float level and everything and its within oem spec. Im open to suggestions because I cant really even use the secondary on my carb so climbing hills sucks. I struggle climbing a small grade with no weight. I do have 33s but I shouldnt have to sit at 4k in 3rd gear just to maintain 50mph up a hill.

 

fuel filter?

 

...brand new

 

well the pump may be weak but I think you're in for a rebuild of the carb

 

Have you checked both diaphragms on the vacuum advance can? Usually the inner diaphragm craps out first.

 

^^ What he said, i had that problem on my truck, but both diaphragms on my distributor were bad. just to let you know, its cheaper to replace the distrubutor than the vacuum diaphragm, i found that out the hard way.