96 4.3 backfire at WOT

Ran fine first few times out. Now runs fine at 3000-3500 and short burst, 30 seconds, at WOT. Then after being run 30 minutes if I go to WOT after 15 to 30 seconds it will backfire once or twice out carb. each time i backed off to 3500 and it did not do it again. Changed cap rotor wires and plugs. Next time out same thing, runs good first 30 minutes 3500 with a few short runs at WOT, 4800 alone. I'll stop and fish 45mins., then on the way in after 5 mins. running 3500, I open it up for 30 seconds and bang it does it again.... just once, I back off right away, so i don;t know if it would keep doing it.... Seems too run fine below 4000. Thanks for any suggestions!

cross firing, bad spark plug causing cross firing, bad intake valve seat, soft intake valve spring, broken intake valve spring

A loud backfire out of the intake is caused by a lean condition.

I would start with a good carb Cleaning and make sure the fuel pump is up to spec. A lean condition will melt your valves and damage your engine even if it is not backfiring.

Ayuh,....... It's goin' lean at Wot,.....

Pull the fuel filter, 'n examine it's Contents for anything but clean fresh gasoline,.....