What is the deal with backfire?

[67chevemall]

I'm trying to trouble shoot my back fire issues, and I can't fix it.
Ping Bing pop happens at idle and some levels of RPM's
But at a steady high RPM like 2500 it runs fine?

I always thought that backfires were from to much fuel, but I was
just reading my hanyes manual and it says "just taking the
air cleaner off sometimes will lean a carb enough to create
a backfire".
So I not sure if I'm to rich or lean.
The plugs are ok from all the "pops" but I HATE hearing that
noise. Next thing you know your trucks on fire and your running
down the road with grenades.

The book reads for trouble shooting a back fire:

1 Emissions system not functioning ( emissions?)

2 Ign. Timing Could be!

3 Sec. ign system. Plugs wire cap roter all new.

4 Carb neds adj or worn. Its a Qjet from a 78 C20 might need adj.

5 Vaccum leak Maybe the carb gaskets are still to loose

6 Valve issues Can't see it

7 crossed plug wires ? runs to good at 2500rpm

What do you guys think?? Thanks a TONN