Why do I get big backfires under over-run?

[[email protected]]

Hi. I get big backfires under over-run? They sound like a rifle going off. I think it happens when I dont release the throttle all the way on over-run. Is there a way to reduce this?

In the google link below are five items:

1. Tune file

2. Log file

3. Short video of the big flame and bang

4. Picture of the muffler internals and a video of the construction.

Here is a google drive link: https://drive.google.com/drive/folders/1nagdD2oQ64N-Kn3tbiXZkg-2WONczqTY?usp=sharing

My setup is Link G4+ Blue Storm. 3sge beams with ITB throttles. The muffler is a multiple chambered wrx subaru unit. The car backfired with a long 2.5" straight through muffler also. I thought it would go away with the full subaru cambered muffler but it didnt.

Thanks for your help.

[0x33]

Do you still get it with over run fueling turned off?

[Adamw]

Most commonly backfiring on overrun is caused by a leak in the exhaust system somewhere near the front, it will suck in air when exhaust flow is low which gives enough oxygen for combustion.  

Put your hand over the tail pipe when it is idling to listen for leaks up the front.

[[email protected]]

Yes it has a four into one collector that needs sealing up. Thanks for the answer.