New to forum, recently acquired a 1970 521 w/74k mi. While accelerating, seems normal in 1st and 2nd, but when I hit 3rd it bogs down then sputters, may backfire, if I let up on throttle it’s ok, but if I add throttle it bogs. Doesn’t seem to do it in 4th either just 3rd. Can’t figure out why it would only do it in 3rd....? Accelerator pump? Vacuum advance? Any carburetor guru assistance would be greatly appreciated!

mac, accelerator pump's ACTION usually can be viewed looking straight down ea. barrel ( 1bbl or 2bbl here?) gotta have a "healthy" , steady ,UN-INTERRUPTED,stream -lasting most of the carb's throttle rotation - like 60-70%. Looking like coming out of a water pistol - for ea. "primary barrel".Now 3rd may actually "demand" more fuel than 4th , power /torque band , and most "upper end" acceleration may only be available in that car in 3rd( 4th just SLOWLY accel's to Top speed? )-SO a restriction like "inboard" carb filter , or inline gas filter. Inside the carb is float needle seat needs proper setting; don't forget the vacuum "power- piston" which controls "metering rods" - if those two components are in there. A carb " rebuild kit" really would virtually eliminate the carbureter (any filters ,gas,air replace - no brainer )Such an older car - most carbs over long time get waaay out of adj. ,besides DIRTY (many,many PASSAGES in them) and you never re-adj. & go and use the old gaskets , check-balls,accel-pump,springs etc. which ARE in a kit.

Thx so much for the info. Will get rebuild kit for 2 barrel oredered up and go from there.