stock 351 backfire

Backfire through carb or exhaust? start all of a sudden or gradually?

How is that distributor drive gear? Pin sheared?

Sticky valve(s)?

Check the distributor . . . .

Backfire is in the exhaust,I will check the distibutor drive gear. Thanks

When mine did that, it was a stuck exhaust valve plus an intermittent coil problem.

A backfire means the timing has shifted or the carb accelerator pump has become inop. Check the distributor drive pin by removing the distributor and physically sighting through the hole for straightness; sometimes, the pin only bends or shears on one side, retarding the timing severely but not quite releasing the rotor. Second, the cam chain wears severely on 351-Cs due to the heavy valves; if the pin is OK but the timing as chacked by a timing light has inexplicably shifted, it may be a stretched timing chain. Readjusting the timing with a stretched chain will radically change the cam timing, resulting in poor running at best. Finally, remove the air cleaner and physically check the action of the accelerator pump squirters by snapping the throttle open while looking down the carb throat (engine off). If no fuel squirts from the little acc. jets, the carb needs rebuilding.

I had that also at high tork & rpm, but my carb was too lean. Tweaked it and fine now.

Thanks Jack, carb. rebuild did take care of the backfire, Back on the road today!