I just rebuilt my 600 cfm 4160, before it was rebuilt if i revved the motor it would cough then rev an idle quality slowly decreased. now it still does the same thing but will also barely idle, run poorly in general , and backfire thru the carb occasionally when revved.
the timing is set at 8 deg. initial

 

Whats the pump cam setting at?

 

i dont know how to check that, but it is a red cam and the pump lever moves as soon as the throttle is opened.
a friend said i might have a tight valve, would that cause a backfire. i just installed a new cam and the motor ran great during break-in but had that cough.
i'm gonna change the plugs and see if that helps any.

 

the plugs solved most of the problems along with uping the timing to 16 deg. i still have that little cough. i think it is mechanical advance related though.

 

A backfire through the carb is a lean condition, you may need a larger pump cam. The Holley on my 396 came with the orange cam, and off idle problems untill I put on a green cam.

 

the plugs were the cause, they were pretty fouled. it doesn't backfire any more, im gonna get an advance kit and a couple of pump cams to see it i can get rid of that little cough.