My truck (400 4 bbl TQ) would stutter and pop at very light throttle but run like a scalded cat when you drop the hammer. Started this about the time the weather got colder, so I tried raising the rods to rich it up a bit, thinking it might be too lean for the weather. Helped a bit, but still fussy on easy acceleration.

Coming from work last week, the less than a year old exhaust popped it's third flange gasket, or so I thought. It's had a slight exhaust leak for a while.
The bolts in the flange had loosened to the point that one nut was gone, and the pipe dropped off the manifold. The remaining bolt was holding it up so it didn't hit the road.

I replaced the bolts over the weekend and tightened everything up solid again, and lo and behold, the hesitation has completely disappeared.
My ? is how would a loose/leaking exhaust flange cause a backfire condition.

Dual exhaust BTW, stock diam. pipes, stock mufflers. Restricted? or lack of flow thru the crossover perhaps? It was the Pass side with the heat riser that dopped down.

Curious coincidence, if nothing else maybe.

Part throttle excessively lean with no restriction or backpressure on one side?Reversion??

Sounds like the exhaust leak was causing a too lean condition. You said it improved when you richened it up. I know that you usually have to run bigger jets when you run open headers, because it leans it out. Sounds like you encountered the same condition.

I thought that might be the case, but it was just a bit curious that the exhaust leak was minor until the gasket that was holding the flange to the manifold peeled loose. Wasn't overly loud or obnoxious, just the Pfft-pfft-pfft that says you need to look underneath before too long. Didn't get loud (opened up) till the end of last week.

Have to file that with the rest of the X-File problems that have oddball causes and cures I guess. Sometimes it's the simplest things.

Actually, it's a good thing I had to get at the pipe cause I was about to de-carb the thing and go thru it for the second time in 6 months.