well i finally desided on a carb a holley 670 street avenger and after i put it on my truck it will backfire through the exhaust. not all the time or even consistenly but every now and then at idle it will just 'pop'. its not loud like a backfire but more of a pop. i'm thinking vacumn leak but whats ya'll take on it? it never did this before untill i switched carbs truck has a mild 400 in it

 

Sounds like you need to adjust carb and check for vac leak. Also what carb was on there before? Did you touch the timing at all?

 

no i didn't mess with the timing. carb before it was a 600cfm holley vac secondary.

 

On a stock to mild build on a 400 a 600cfm is on the high end of the engines needs. You can try adjust the new carb or put the other back on if you didn't have issues with it.