Small Backfire issue in Muffler on Deceleration below 2000 rpm's

[JohnnyTak]

Hello,
On my 69 Roadrunner 440 6bbl, I'm noticing a small backfire sound coming from the mufflers when in gear and the motor is just at or below 2000 rpm's. When I come to a red light, and I down shift through the gears above 2000 rpm's, there's no back fire sound until I approach the 2000 rpm mark or lower (small sound, not too loud). If I take it out of gear and coast in to the red light, no issues at all. There are not issues either when above 2000 rpm's either when down shifting. This started about a month ago. I wasn't sure what to do so I changed out spark plugs, and checked my distributor wires also. Does not seem to be an ignition problem. I did check my wires on the coil to make sure they were tight also. I did not check the distributor cap though. I have factory electronic ignition. On acceleration, the motor seems fine with no apparent missing. Not sure what to check next.
Car's exhaust system consists of TTi headers 1-3/4" with flowmaster 40's. I straight daul exhaust straight back to tail pipes. Mufflers were put on in 2008.
Thx,
Johnnytak

 

[padam]

I used to have bad backfire on deceleration.
I changed a bunch of things, rebuilt motor, valve job etc.
Two things that may have fixed it:
Sealed the leaks at headers with Remflex gaskets, and hooked up the vacuum advance.

 

[toolmanmike]

We used to call it "back cackle". It was always worse with small diameter pipes and glass pack mufflers.

 

[LowBikeMike]

Exhaust leak letting air get sucked in under deceleration. Check the all the joints for telltale signs of small leak

 

[Don Frelier]

We used to call it "back cackle". It was always worse with small diameter pipes and glass pack mufflers.

Cheap beer and chicken wings tend to give me back cackle

 

[SlinktRR]

Whenever I've had this it was due to misfire. Check ignition. Also could be stuck valve or something, can check compression or leak down test.

 

[toolmanmike]

Is it a loud pop or just a rhythmic rumble?

Hello,
On my 69 Roadrunner 440 6bbl, I'm noticing a small backfire sound coming from the mufflers when in gear and the motor is just at or below 2000 rpm's. When I come to a red light, and I down shift through the gears above 2000 rpm's, there's no back fire sound until I approach the 2000 rpm mark or lower (small sound, not too loud). If I take it out of gear and coast in to the red light, no issues at all. There are not issues either when above 2000 rpm's either when down shifting. This started about a month ago. I wasn't sure what to do so I changed out spark plugs, and checked my distributor wires also. Does not seem to be an ignition problem. I did check my wires on the coil to make sure they were tight also. I did not check the distributor cap though. I have factory electronic ignition. On acceleration, the motor seems fine with no apparent missing. Not sure what to check next.
Car's exhaust system consists of TTi headers 1-3/4" with flowmaster 40's. I straight daul exhaust straight back to tail pipes. Mufflers were put on in 2008.
Thx,
Johnnytak

 

[JohnnyTak]

I used to have bad backfire on deceleration.
I changed a bunch of things, rebuilt motor, valve job etc.
Two things that may have fixed it:
Sealed the leaks at headers with Remflex gaskets, and hooked up the vacuum advance.

Hi, I do not have a vacuum advance on my distributor. My 509 lift cam make very little vacuum. I may try though. Thank you, JohnnyTak

 

[JohnnyTak]

Whenever I've had this it was due to misfire. Check ignition. Also could be stuck valve or something, can check compression or leak down test.

Okay, thank you for the advice. I will look at these issues mentioned.

 

[JohnnyTak]

Exhaust leak letting air get sucked in under deceleration. Check the all the joints for telltale signs of small leak

I will check that, thank you for your feedback.
JohnnyTak

 

[JohnnyTak]

Is it a loud pop or just a rhythmic rumble?

Very low poping sound inside the mufflers at low rpm's (2000- or lower only). Only happens when in gear also during deceleration.

 

[toolmanmike]

Very low poping sound inside the mufflers at low rpm's (2000- or lower only). Only happens when in gear also during deceleration.

I would have to hear it but from what you describe it sounds pretty normal.

 

[Carcinogen]

I get a pop/bang during low speed acceleration/deceleration

Fuel air meter shows very lean mixture that does not fire in cylinder then lights off when it gets enough oxygen in exhaust pipe.

Have tried changing tune up kit (Edelbrock 800cfm/Victor440) but carb/manifold just has problem at low RPMs. I just try to quickly pass thru trouble RPM.

If you did not have problems before, I would suspect a vacuum leak that is causing a over-lean condition.