[Vckline313]

Back info pontiac 400 punched 60 over, raiii cam
Smi quadrajet, Dougs headers with 3" to magnaflow mufflers dumped after mufflers.
I get about 10" of vaccum if I recall initial timing 12 total 36 or in around there

Car drives around fine some surging at part throttle cruise but will accelerate no problem no matter the speed.

On the freeway if I let off the gas it will be random pops and bangs from the exhaust and like a stumbling feel if I hold the pedal in place at same speed, but if I push the pedal down (go faster) it'll all go away then the backfiring will come back after I get off the gas

Is this a timing issue?
A/f screw issue?
Apt screw issue?

I've read it could be exhaust leaks but I do not hear any


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[chuckies76ta]

I use to have this in one of my engines sometime ago and I was told it was from unburnt fuel in the exhaust. Try a different carb see if it helps. Never did figure out the cause.

[steve25]

Your condition sounds , especially taking into account the surge is due to a lean condition at just off idle.

I would also double check with the motor out of gear that when you let the motor drop back from let’s say 3500 rpm, that the timing drops right back with the reduction back to idle rpm.

The advance section of the Distributor be it shaft or weights may be hanging up especially when the motor is fully warmed up!

You need to get this corrected soon because with a surge condition during part throttle the motor is likely lightly pinging also and that’s beating up your Rod bearings little by little even though your not hearing it,
Better especially this time of year to be on the rich side, then lean!

[scott70]

Years ago I had a quadrajet that would surge at a steady part throttle...ended up being the throttle shaft had some play around( worn) it allowing air to enter and leaning it out. Ended up having the shaft hole bushed.

[i82much]

problems like this are why it is nice to have a spare carburetor laying around - you can swap the carb and see if the issue goes away, and that will tell you whether the issue is carb related.

i seem to vaguely recall similar symptoms when i wiped a cam lobe years ago, so maybe pull the valve covers and make sure all the rockers have full movement?

[Kenth]

Popping exhaust usually indicates retarded timing or malfunctional ignition system.

[PunchT37]

10 inches of vac on ra3 cam seems low.

[indymanjoe]

exhaust leak at header can cause a backfire on decel just went through that on my car.