KZ440 with KZ400 B2 Engine with hanging idle and backfire problem

Do a full throttle run

Then tape up half of your air filter intake to create a richer mixture.
Do another run. Did it pull harder?

Take off the tape then unscrew your air filter and leave a slight opening between your air filter housing and airbox housing.
Do not remove your air filter housing as that would be too much air.
You are creating a leaner mixture.
Do another full throttle run.

Which run was better? normal, lean, or rich?
Make sure your carbs are synced before testing.