Holley question

[cpetersen]

I have a 3310-1 which is a 780, metering blocks and 2 powervalves. I'm still having a occasionial backfire through the carb. And only when you are just giving it gas. Once every 2 weeks. Well it pops one or both powervalves everytime. I'm running a 6.5 in the front and a 8.5 in the rear. I'm running the factory jets. I'm planning on putting powervalve protection in both the front and the rear, but that still doesn't explain why it backfires from time to time. Am i running to light of powervalves? I have always ran carters and found this old carb, rebuilt it. My BBC loves it. I'm just on a learning curve here. Any help would be appreciated. Thanks