I have a ford 400 stock two baral carb and it backfires when I'm driving. A lot more when I give it a little more gas. Is this bad timing or bad fuel to air mixture.

 

It could be timing or bad fuel/air caused by carb problems or vacuum leaks. Its best to verify timing first to eliminate it if nothing else. Factory specs vary but most 400's seem to do well with 12 degrees timing advance at idle with vacuum advance disconnected and plugged.

 

Don't know if thi changes it but it backfires throough carb not exhaust. The under side of my brand new air cleaner is black now

 

It could be any number of things. Without more info it would be difficult to give any kind of accurate advice.
Backfiring can be caused by any number of problems from something as simple as a clogged fuel filter to burned valves.
How long has it been since the truck was tuned up? What kind of shape are the pluq wires in? How does the truck idle?
Check the timing but I suspect the problem is elsewhere.

 

No idea how long since last tune up. It runs fine at idle the spark plug wires are few months old and in good condition

 

This is a vw page but it doesn't matter as it's the same principle. It can give you some ideas
as to why you're getting a backfire through the carb.
Backfiring