A backfire through the carburetor occurs when there is a passage open from a cylinder to the carburetor when the cylinder fires.

This is generally caused by:

(A) ignition timing out of correct adjustment
(B) valve timing out of correct adjustment
(C) burned or sticking intake valve.

Before throwing rocks at your carburetor, I would suggest at least a compression test, and checking the ignition timing.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop