Lean backfire on downshift, 440 4 speed

[69Runner]

A little background - we recently converted our 67 Coronet from auto to 4 speed. The engine behaves well for a 1976 440 when on the gas (no hesitation or backfires) up to 4000 rpm. Still breaking in the clutch but exploring the higher rpm ranges every week. Anyhow air/fuel is around 12 when on it hard(er) and mid 14's at partial throttle.

If the rpm is above 2000 and I let off the gas (such as when downshifting) it backfires. The A/F reading jumps to 16+ at this time, leading me to believe it is lean. If I press the pedal slightly then it does not backfire.

Internet searches have led me to motorcycle websites - similar backfires are discussed in relation to Harley's where it is regarded as "normal" following exhaust upgrades. Nothing fancy on my 67, 2.25" exhaust all the way back. But it's definitely a backfire in the exhaust.

Carb is a 650 Edelbrock AVS (vacuum secondaries), with a Stage I richer rods and stiffer piston springs.

Any thoughts or suggestions?

 

[727]

Before you spend a lot of time trying to tune out the backfire. Put a bigger carb on that 440 then go from there!

 

[65Fury440]

I'd be looking more at the timing being advanced. Turn it back a couple degrees and try er out. The lean A/F may be a byproduct of the backfire. I ran that same carb on a stockish 440, good response, was a better runner than with a 750.

 

[bm02tj]

On the lean burn cars they added a little pot to slow the throttle from closing all the way quickly
and slow transition to idle circuit
Try this if you are unable to resolve with timing and tuning

 

[69Runner]

Turn it back a couple degrees and try er out.

Tried this just now, still backfiring. Distributor advance is connected to timed vacuum, so I switched to manifold vacuum with same result.

 

[69Runner]

a little pot to slow the throttle from closing all the way quickly
and slow transition to idle circuit

Cool, maybe I can rig something up.

 

[69Runner]

Thanks to bm02tj's hint I started looking for a dash pot.

The trick was finding one that would work for the Edelbrock. Turns out 63 64 65 Corvettes with 327s used Carters. Lucky for me, the DP-44 dashpot fits an Edelbrock perfectly.
Dashpots were quite common on OEM carbs but eventually get tossed for whatever reason, component failure, carb swap etc. We'll see how it behaves on the street...

 

[69Runner]

Turns out the driver side exhaust manifold gasket was disintegrated and the 2 bolts had loosened off. Replacing the gasket was on the winter TO DO list, but now with a new gasket and bolts the backfire has been eliminated. The A/F readings have also stabilized in addition to running a point lower.

 

[383man]

Yep exh leak will cause it to pull air in on decell and it will do like an air pump does and cause any exta unburned fuel mixture to burn and that will cause a popping (backfire sound) in the exh. The air pump emissions systems dump the air to atmosphere on decell by a divertor valve so they wont backfire like that on decell. Ron

 

[69Runner]

Yep exh leak will cause it to pull air in on decell and it will do like an air pump does and cause any exta unburned fuel mixture to burn and that will cause a popping (backfire sound) in the exh. The air pump emissions systems dump the air to atmosphere on decell by a divertor valve so they wont backfire like that on decell. Ron

Thanks, I just learned something! Makes sense because the leak was causing the A/F gauge to show lean. In fact it was rich so I just now put the stock rods back in.

 

[69Runner]

I ran that same carb on a stockish 440, good response,

Confirmed. Up to 4000 rpm on the street @WOT all is good with stock rods and jets. I will keep the 650 for now. The exhaust leak was the cause of the backfire.