question about tuning a Holley carb for Duster 340?

Hello,
are just about done and are having problems tuning the carburator so that it doesn't backfire. What is the best way to tune it?

Please help if you can ASAP

thanks,
eric  

First what makes you think it is the carb? When a engine back fires that is usually a sign of an ignition problem.
What model Holley are you running?
Quick checks you need to perform first.
Set timing to 10-12 degrees initial.
Make sure dist. vaccum advance unit is hooked up and working.
Make sure firing order is correct, 1,8,4,3,6,5,7,2.
Check plugs and make sure they are all firing.
Check compression.
Check coil and dist. cap for cracks.
Test ign. wires for resistance with an ohm meter.
Make sure your ign. box is grounded.
Carb, check float levels, front and back.
Check carb model and then find out stock jetting, a lean carb can cause back firing.  
Make sure accel. pump is working and the shooters are passing fuel when accelerated.
Set fuel mixture with idle screws.
If problem still exists after all this I would recommend taking the vehicle to a qualified shop were they can hook up a scope to assess the output of the coil and the ign. box.
Good Luck.
Doc.