Couple Possibilities
There's a number of things of course that can cause a stumble and that's why it's so much FUN!
Thinking "out loud" here -
At idle, you should be pulling full vacuum advance but no mechanical. Giving it some throttle would reduce the vac, so the vac can should back off some pretty much immediately. If the can isn't backing off quickly enuf because of a "gummed up" advance mechanism, I don't know that it would cause a stumble. Same thing from light throttle cruise - full vac, little mechanical depending on your rpms, but likely not much anyway. Again, stepping on it the vac advance should pull back some, the amount depending on how hard you're stepping it down. Delaying the pull back (losing some advance) I wouldn't think would cause it to stumble.
I woudl suspect something more in the line of accelerator pump settings on the carb not coming in to action quite quickly enuf to make up for the additional air being let in to the intake.
It's always good to check all the settings on a new carb, esp accelerator pump (or in the case of a Mopar with an AVS or AFB, the metering rods being pushed up out of the primary jets by the plunger spring to enrichen the mixture). When you step on the accelerator you open the throttle blades and the vacuum in the carb and intake goes down, reducing signal to the boosters in the carb, thereby reducing gas flow and leaning out the mixture temporarily until vacuum catches up. The metering rods in the primaries are controlled by a plunger that is pulled down by more vacuum and the spring under the plunger pushes up the plunger and the
metering rods when vacuum decreases when you step on the throttle. This pulls the rods farther up out of the jets, to a spot where the rods are thinner, allowing more gas thru. Same effect as an accelerator pump or power valve on a Holley.
I've forgotten a lot about the AFB/AVS type carbs, don't remember if there's any adjustment screw in the plunger to mechanically adjust the height, or if you have to change metering rods to ones that are thinner at the same relative position - in other words have a different amount and rate of taper, or change to a stiffer spring to push the plunger up quicker when vacuum goes down. Any one or combo of those would act like putting quicker or greater stroke on a Holley accelerator pump, or putting in a power valve with a smaller opening vacuum value.
I believe there's likely some specs on metering rod #'s, or something related, in the manual also. You'd have to get your carb # and then look the specs up for your exact carb.
Back to the vac can - if it opens late, you're not getting as much advance as you should at light cruise which will primarily reduce your gas mileage somewhat, and depending on if it
springs back more quickly because of greater vacuum required to open it, or if it's slower in springing back because of a gummed up advance mechanism in the distributor, would alter your actual rate of advance or retard some from the specs, but I wouldn't think it would be sufficient to cause the stumble. Think of it like this, you can change your timing a few degrees either way from specs and you usually won't cause a stumble like that, although it will have an effect on power and economy at any given rpm.
Make any sense?
Hope it helps somehow.
Good luck!
There's a number of things of course that can cause a stumble and that's why it's so much FUN!
Thinking "out loud" here -
At idle, you should be pulling full vacuum advance but no mechanical. Giving it some throttle would reduce the vac, so the vac can should back off some pretty much immediately. If the can isn't backing off quickly enuf because of a "gummed up" advance mechanism, I don't know that it would cause a stumble. Same thing from light throttle cruise - full vac, little mechanical depending on your rpms, but likely not much anyway. Again, stepping on it the vac advance should pull back some, the amount depending on how hard you're stepping it down. Delaying the pull back (losing some advance) I wouldn't think would cause it to stumble.
I woudl suspect something more in the line of accelerator pump settings on the carb not coming in to action quite quickly enuf to make up for the additional air being let in to the intake.
It's always good to check all the settings on a new carb, esp accelerator pump (or in the case of a Mopar with an AVS or AFB, the metering rods being pushed up out of the primary jets by the plunger spring to enrichen the mixture). When you step on the accelerator you open the throttle blades and the vacuum in the carb and intake goes down, reducing signal to the boosters in the carb, thereby reducing gas flow and leaning out the mixture temporarily until vacuum catches up. The metering rods in the primaries are controlled by a plunger that is pulled down by more vacuum and the spring under the plunger pushes up the plunger and the
metering rods when vacuum decreases when you step on the throttle. This pulls the rods farther up out of the jets, to a spot where the rods are thinner, allowing more gas thru. Same effect as an accelerator pump or power valve on a Holley.
I've forgotten a lot about the AFB/AVS type carbs, don't remember if there's any adjustment screw in the plunger to mechanically adjust the height, or if you have to change metering rods to ones that are thinner at the same relative position - in other words have a different amount and rate of taper, or change to a stiffer spring to push the plunger up quicker when vacuum goes down. Any one or combo of those would act like putting quicker or greater stroke on a Holley accelerator pump, or putting in a power valve with a smaller opening vacuum value.
I believe there's likely some specs on metering rod #'s, or something related, in the manual also. You'd have to get your carb # and then look the specs up for your exact carb.
Back to the vac can - if it opens late, you're not getting as much advance as you should at light cruise which will primarily reduce your gas mileage somewhat, and depending on if it
springs back more quickly because of greater vacuum required to open it, or if it's slower in springing back because of a gummed up advance mechanism in the distributor, would alter your actual rate of advance or retard some from the specs, but I wouldn't think it would be sufficient to cause the stumble. Think of it like this, you can change your timing a few degrees either way from specs and you usually won't cause a stumble like that, although it will have an effect on power and economy at any given rpm.
Make any sense?
Hope it helps somehow.
Good luck!
Of course, if you've gone to a longer duration cam or bigger intake, you need to change the rate of getting more gas into the manifold when you step on it . Lower number power valve for Holley, and/or different accelerator pump cam giving it a bigger and quicker shot, or in the case of an AFB/AVS, probably different metering rods at least. In either case, maybe even somewhat larger jets.
If it's an AVS, make sure the secondary air valve spring isn't set too soft and letting the air valve controlling airflow into the secondaries open too quickly. Same effect as too little gas on the metering rod or power valve/accelerator pump activation.
Oh, I'm also now remembering there are maybe different accelerator pump arm settings on those AVS/AFB carbs, like 2 or 3 little holes in the accelerator pump arm for linkage adjustment, which would also quicken the accelerator pump stroke. Likely only need to change that if you've done some fairly serious hops to the engine. But check to make sure it's in the right hole per specs even if not.
