400 Backfiring

[bordin34]

My 73 400 is backfiring. What would cause this and how can I fix this. It also sputters and stumbled from about 1500-1800rpm, I am guessing because I do not have a tach. Could it be running too lean? The carb is a 670cfm holley street avenger and it has a mild cam and headers. It isn't just pop backfiring, it is who shot a gun backfiring.
Dan

[Chryco Psycho]

usually it is a leak in the exhaust , but it could be a problem with the power valve leaking or not enough vacuum to keep it closed
 is the carb new ?

[bordin34]

I think the carb is a couple years old. I also think that the header to exhaust gasket is leaking. Would that cause a backfire.

[Chryco Psycho]

any leak in the exhuast can draw in air & cause combustion inside the exhaust

[MEK-Dangerfield]

Is this a new problem? Do your spark plugs look white, like they are way lean? I find it interesting that this just happens at a certain RPM range.  Does it happen under throttle, or while coasting?

  Mike

[bordin34]

It backfires from idle to high rpms. It sputters from 1500-1800.

[MEK-Dangerfield]

It backfires from idle to high rpms. It sputters from 1500-1800.

From this, I'll guess multiple demons at work here. Chryco was probably right about the exhaust thing. I would think the leak would need to be huge to cause that though.  Your sputtering is probably a carb problem. Have you messed with it at all? Meaning, do the plugs look good? Maybe the secondaries are opening too fast or too slow? Have you tried different springs in there(lighter, or heavier)?


  Mike

[bordin34]

I have done nothing with the car. All I have done is let it idle up to temp and drive it about 600ft. a couple times. I have not opened the secondaries or gone above 30mph. But when I give it more gas like enough to make it downshift if it were in 2nd gear, it wants to stall and sometimes does.

[vinb]

Hey bordin34, I'll be up in Fair Lawn on River Road Sunday the 21st for a car show if doesn't rain. If you bring it over I'll  be more than happy to look at it for you. I will be parked near Berdan Ave on River Road by the Dunkin Donuts. Just look for my Cuda. Trouble said he will stop by, but I think without his Cuda..

[bordin34]

Won't be around saturday, thanks for the offer anyway.

[bordin34]

Anyway, here is a picture of the car.
And a video of it revving, not backfiring in it.