Hi jason, yes, they are rich for a couple reasons, you sorta hit one on the head, which I'll get to last. First, the carbs are a very simple design to provide a generic and adequate fuel mix for full power at any rpm. Fuel consumption/economy wasn't an issue to the military in those days.
Second, you are right, the richness is to cover up a problem... it's with the fuel delivery system. The culprit is on these carbs (I think). They have a normally-open vacuum-actuated butterfly throttle, and they lack an accelerator pump (which injects an additional shot of fuel into the mix whenever you press on the pedal). The carb on a 6602 needs to run rich all the time, or the engine will hesitate and (more often) cough out everytime the engine vacuum drops, as when loads are applied. It's a snowball effect. The engine starts to bog down, the vacuum drops, the butterfly opens and lets in more air, and if the engine is cold or mixture not rich enough, you get the typical cough and backfire up through the carb. The 5T gassers are famous for it. That's why they are supposed to be warmed up before moving out or else you will be fighting with the engine and playing with the choke constantly until you finally get up to speed. Once they are warm they run great (although rich). Next time your brother has his carb off, look at it and you will see the throttle butterfly wide open. That's why they cough and backfire when you start them cold and have to choke the crap out of them. They are trying to start wide open. It's a bad design because the last thing a cold engine wants if it hesitates or starts to die is to have the butterfly open up wide. But that's exactly what happens with this type of vacuum throttle. The only reason it's on there is to prevent overspeed, and it was also supposed to provide automatic throttle adjustment in direct response to loads on the engine.
If you ask me, I've never liked the design. Vacuum throttles were on some civvie trucks as well. One of these days I'm going to jet mine down and see what happens. It probably will help the fuel economy, but I'm sure I won't be able to even move the truck until the engine comes up to temp. But before I do that I should try to modify the throttle so that it's 100% foot pedal actuated, which means the butterfly will be closed at all times unless you push on the pedal, just like 95% of carburators. To heck with the vacuum governor, I have a tach on the dash and ears! Worse comes to worse, I'll put a "normal" carb on it, one that will respond to what my foot tells it to do. My 6602's work almost every day, off road and on road. They will do 60 on the flat if I let them. Economy and performance is something I need to address, and the stock carb is too basic and "idiot proof" to provide both.
