Is My Engine wanting To Play Dead?

[Canadian694x4]

Well I have recently started to experience a little engine trouble. I need help to figure out what is wrong so that maybe I can fix it or I can take it to the experts I suppose. Here's what I can discribe...

My 283 just the other day seemed to be slow on acceleratation (no Power when I punched it). That was fine, cause I figured it was just my timing (my dizzy doesn't want to stay put). Well before the day was out and I could reset my timing my truck started to backfire on deceleration under braking!! What causes this?

Well when I got it home I tried playing with my timing then took it for a spin. All seemed well as I did get some of that Power I was missing back and she even sounded healthier. But alas, after about a 20 minute run she started to backfire under braking once more

Is it that I need to play with my Carb? Running too rich or sumthin maybe? Any clues or help would be great.

Can anybody help me to understand whats up?

I'm not a master engine guru...so any help would be greatly appreciated.

Thanks

Brian