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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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Even near the end of the system a leak is a leak.

I haven't seen one in a while, but some carburetors used a "dashpot" as part of the metering system. It would prevent the throttle from closing as quickly to prevent a rich condition from increasing emissions or even stalling the engine. It sounds like it may be worthwhile to consider, if you have no luck otherwise.
 

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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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338 Posts
In the event that an exhaust leak or vacuum leak isn't the cause, I'm curious to see what the solution would be on a tri-power set up. On a 4 barrel a couple of ways around situations where more air is needed with the primaries in a good place on the transition slots is to slightly open the secondary or some folks drilled holes in the secondary butterfly. Then tune the idle circuits and so on from there. I have no experience with the tri-power to know how or if that would translate to that set up.

When you do meet with success on this, I hope you follow up. Welcome back!
 

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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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338 Posts
There are two things that cause exhaust backfires and that's retarded timing and air leaks.
I agree with what you said.

If there is not an air leak, I think the mechanism for the exhaust backfire is a situation that presents like retarded timing would.

The same way mechanical advance curves are "all in" by 2500 or 3000 and don't require the need for additional advance as a consequence of the velocity and better mixture in the combustion chamber allowing for a faster burn. Essentially the faster burn "creates" the scenario that additional timing would if the burn speed remained constant.

In the case of the abrupt (and maybe too complete) throttle closure, I think the column of atomized fuel (that was doing fine at higher velocity) loses it's ability to mix as well in the chamber. Leading to a low velocity over rich condition that can't burn fast enough. It presents the same way as retarded timing would with the incomplete burn in the chamber and the pop in the exhaust.

The open headers give that mixture quick access to fresh air. It may not happen or be noticeable in a full exhaust system.

For the same reason the dashpot helped with emissions, I figured smoothing out the transition from fast to slow moving mixture could help prevent or lessen the deceleration backfire. Maybe even buy some time for the now higher vacuum signal to compensate the timing a bit better?

The old adage that lean pops through the carburetor and rich pops in the exhaust might be a fitting explanation. I'm not arguing here. I guess I'm agreeing in a different way? :)
 

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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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338 Posts
Are these carburetors from a dedicated tri-power set up or did 3 of the same carburetors go on a tri-power intake?

From what I can tell, the 2 outer carburetors on the factory set up didn't have idle circuits and had special butterflies to permit better closure of the outer carburetors at idle. Do the outer carburetors have idle circuits?
 
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