If Im at a standstill and I nail my gas pedal I get a backfire out of my Holley 600.
If I'm at a 10mph roll and nail it
I get no backfire....I installed a K&N X-tream air filter and I think thats when I started to notice it. Does the carb need readjustment, or could it be timing??
sounds like you leaned it a little back out your fuel mixture screws a 1/8 turn each an try it until it improves. but keep track of how far from the original setting you backed them out. this should do it if this prob occured only after installing the new filter.
You also might want to make sure you have no play in your accelerator pump arm. You should have fuel coming out of the shooter nozzle as soon as there's any movement on the throttle. You also may have to change the pump cam or shooter nozzle to get rid of the stumble.
if its idling the same as before you changed the filter the valve should be ok.. if everything was great before the filter all you should have to do is adjust your fuel mixture if it is still hapening then check your timing. its a good idea to keep a record of all your settings ex. timing turns out on fuel mixture etc. that carb site will help if all else fails but proceed carefuly an keep a record.