[Dead Parrot]

Possible vacuum leak around intake gasket or carb base gasket. Do the spray with carb cleaner test.

Does it still have the exhaust riser valve? Those can stick shut and force half the exhaust through the intake crossover which kills higher rpm performance and can overheat the carb.

Does it still has the original Y pipe? Some of those had double walls and the inner wall would collapse, mostly blocking the exhaust, Also killing performance.

Check the rubber lines from the tank(s) to the frame. If those are original, they are probably crumbling away and letting air into the fuel system. Same for rubber lines from dual tank valve to frame if equipped. Even if hoses recently replaced, mice sometimes like to sample taste the hoses, also letting air in.

Nice looking service truck.