1968 GMC 250 Inline 6 4.1L Chugging

[JAV6]

I went through a similar problem with my 292, replaced many parts just as you during the process of elimination. All of the items helped, HEI, fuel pump, filter, carb rebuild, etc. but problem would return after awhile. Turned out the fuel tank had rust buildup that would come loose, plug a carb jet or eventually plug the fuel filter. Pulled the carb apart several times to just find more rust getting past filter. Cleaned it all out and ran great! Just my 2 cents.

Originally Posted by 68gmcdude

UPDATE..

I have done the following:

Replaced the fuel pump
Replaced the sock filter on the fuel sending unit
Replaced the fuel filter in the carb
Blew out fuel line from sending unit to fuel pump
Blew out fuel line from fuel pump to carb
Replaced rubber fuel line from sending unit steel line to fuel pump
Replaced vacuum line from distributor to carb
Replaced vacuum line from smog pump to carb
Confirmed vacuum advance canister is good
Confirmed vacuum advance arm is working
Checked vacuum at carb
Confirmed manual choke works properly
Confirmed points/dwell set properly
Confirmed timing
Confirmed RPMs
New exhaust (it was needed)

Going to have the carb checked again (was rebuilt last year). I don't know where to go from here...