[webebob]

Changing the plugs is a good start. Unfortunatly, a Demon won't be much better in the "power valve" dept. it uses the same valve as Holley.
A blown power valve can souce your plugs and prevent starting. It is worth a look. Wipe the gas off of it and give it a good suck to see if the diaphram is torn. As the posts above mentioned, a "stuck" open or closed powervalve (whatever a stuck power valve is?) wouldn't cause the full flow necessary to prevent starting, but of course a torn diaphram would allow fuel to rush in unmetered.
Any idea what caused your inital backfire?
Bob