Reply to "UPDATED - Backfire under Low RPM / Load (4th or 5th Gear)....."

power valve failure should result in a rich condition, it's a vacuum switch between the float bowl & the main metering system. there's some confusion as to whether a blown PV can affect the idle circuit, I'm not sure that it can but the main circuit / jets do affect just off idle in the transition circuit. if the stumble is in reality a rich condition it can be a bad Power Valve...

if the OP is using engine braking during the 'no throttle' period before the stumble, it's another point for it being a vacuum leak. engine braking can pull a lot more intake manifold vacuum than the engine can produce under throttle, a marginal gasket would fail under the most extreme conditions before it totally lets go

I wouldn't try to tune it out other than as a method of diagnosing the real problem. once the problem is found you'd have to undo any tuning done if it was right before