My '86 wood hauler looks rough and has some issues but I've been addressing them as I can. Just swapped in a new brake booster and was admiring all of the dead ended plugs and mystery hoses when I noticed something missing between the air filter housing and heat shroud. Haynes tells me this should be the heat riser tube. After several YouTubes and articles, best I can understand is that this directs hot air into the carb to keep the choke loose in cold climates.? Does this sound right? The truck is used mainly during the warm months and has been running okay - minus some random stalls which I'm addressing - so do I need to replace this now or will I cause further issues since it's been running so long without it? My inner-tinkerer is screaming to replace that hose!

 

The heat riser tube is used to release the choke as it warms up.
During cold starts the carb will need to be choked so the motor can start and run. The warm gasses from the manifold control the choke so the butterfly plates open based on engine temperature. That then means the carb is allowing more air inside the carb (no longer choking the the air out).

If you do not have it attached, I think it's okay if you do not need to chock it to start on cold days. I'm not sure where the heat goes if there isn't a tube. Back in the day I had to add a riser tube but I was young, knew what it was for but really do not know what happens if the heat is dumped into the engine bay. I assume nothing will be wrong as radiant heat is already escaping but it's not focused through a funnel.

 

Gasoline doesn't burn. It has to be 1) vaporized, and 2) mixed with Oxygen. The cheapest place to get Oxygen is from air. The easiest way to vaporize gas is to heat it. So when the gasoline & air are cold (as during winter), it doesn't burn very well, so you have to put a lot more of it through the engine than normal; that's what choking the venturi does. It moves the vacuum from below the throttle plate to up over the jets (where gasoline gets sucked up & out of the bowls) so the gas moves through faster, and more gets into the engine. But the jets only atomize the liquid gas so it has more surface area in-contact with the air. It still takes heat to vaporize the gas, and that heat has to come from the air. The colder the air, the less vaporization, and the more gas is wasted (as HC emissions, and past the rings into the crankcase where it dilutes the oil). So if you can get some hotter air, the engine will run better. That's what the heat riser system does - it gives the engine the option of scavenging otherwise-wasted heat from the exhaust manifold to preheat the intake air when necessary to improve vaporization sooner.

If you live where it doesn't get that cold, the heat riser isn't as important.