What causes backfire?

[gator508]

As others mention, the exhaust flange attached to the heads may be loose.
Loosen then then re-torque top nut 10-20 "lbs, bottom nut 100-120 "lbs, top nut 100-12"lbs. See if that helps.

 

[cdn-bigfoot]

Is it better to torque the exhaust flange nuts on a cold or hot engine?
And is it necessary to replace the exhaust flange gaskets if having this exhaust leak problem on re-torque?

As others mention, the exhaust flange attached to the heads may be loose.
Loosen then then re-torque top nut 10-20 "lbs, bottom nut 100-120 "lbs, top nut 100-12"lbs. See if that helps.

I just noticed this.
Make sure you torque to only 100-120 INCH lbs.
NOT 100 FOOT POUNDS!

Is it better to torque the exhaust flange nuts on a cold or hot engine?
And is it necessary to replace the exhaust flange gaskets if having this exhaust leak problem on re-torque?

Also, to answer my own question above....
Torque cold engine with new flange gaskets. If "crush type" gaskets are used and over-torqued once they will be flat and not work correctly. With new gaskets you only need to torque to 80 IN-LBS. The gaskets will leak slightly until they fill with carbon. More torque destroys how these gaskets work.