....
If the heat riser butterfly sticks in the shut position,it might get the intake manifold hot enough to make it run real lean(when its closed,all the exhaust from that side of the engine is forced to go thru a passage made into the intake and out to the other cylinder head..that heats the base of the carb for better fuel vaporization)..
I doubt it would cause hesitation,(might cause pinging,or vapor lock on a hot day)--an accellerator pump or timing being off,or a tune up is needed (new plugs and wires,cap,rotor)..also vacuum leaks or weak fuel pump,can cause hesitation....
I sold new heat risers at the parts stores I worked at.."EVERCO" was the brand we carried,not sure if they are still in bussiness now..a Dealer or a place like NAPA probably carries them still..yours might be the vacuum operated style if its rattling..a ported vacuum switch that runs the heat riser (or EFE valve,as its called if its vaccuum operated) could be defective,and the same switch might control the vacuum advance for the distributor too,which can affect acceleration if it doesn't work..
Many of those EFE valves rattled about 5 minutes after a cold start up--its just the vacuum switch deciding whether to stay open or not as the motor warms up..after its fully warm,the vacuum should be constant at the EFE valve,so it will stay open, and not rattle....oh yeah--gotta pull off the "Y" pipe, on that side at least to change it..pray the exhaust studs are in good shape!.
