If you're getting some popping and backfiring through the carb as mentioned that indicates a vacuum leak. This is usually at carb to manifold gaskets rather than intake manifold gaskets. Even a broken or disconnected vacuum line can cause it. Sometimes a prior owner has plugged manifold or carb vacuum fittings with those little plastic or rubber caps that either get hard or erode or fall off completely. In combination with a dirty carb and lots of miles that can steal all the power.

It can also defeat the vacuum advance of the timing advance in distributor, causing power loss when stepping on the gas. Good to disassemble the disto and lube up the points plate and centrifugal advance weights too so it all moves easily. If there's been a vacuum problem for a while the points plate can get stuck.