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.
Dan

[RD]

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.
Dan

check your spark... you could just have a bad spark plug wire (or a couple).  when was the last time you changed them and the spark plugs/cap/rotor?

backfiring is usually caused when the carb is loading up too much fuel, not leaning it out.  it may be a timing issue too.  bring #1 to TDC on compression stroke and see where the rotor bug is placed at.

there are a lot of things that can be causing it to run rough, can even be cam related.  start of with this and then we can go from there.

Lastly, did it just start doing this or has been doing this for a while?

oh, isnt troubleshooting fun?

[bordin34]

It was backfiring since I bought it, 2 weeks ago. But 2 weeks ago it was more of a light barely noticeable backfiring at idle and nothing past 2000rpm. Now its a hit the deck backfire at idle and backfires throughout the rpm range.

[supserdave]

Could be the timing chain? I'd check the basics first, check the timing. And then the carb.

[firefighter3931]

Usually a backfire condition means it's going lean. Make sure the carb is not leaking at the baseplate and all the vacuum ports are covered....check the vacuum hoses as well. 

I would start with a vacuum guage and see how much vac you have at idle. 



Ron