So i have a few questions about my rebuilt 350 rocket. I have broken it in properly and now it has about 2000 miles on it. Everything seems to run great except if i accelerate quickly to more then 40 mph. When i let off the gas it backfires for a few seconds then stops. Any ideas on what it is or how to fix it?? please email me directly with answers. keefe_neise@yahoo.com

 

You must have loud exhaust with short or no tailpipes!
The rich air/fuel mix in the intake manifold comes into play, along with reversion, cam overlap, etc.
Ever hear a car with open headers - same thing!
Put taipipes on it - gone!

 

Pre-ignition because of incorrect timing might be it.

 

^^ agreed

 

• Exhaust leak (Headers are good for this) - which is essentially what Rickman said
• Timing wrong
• Valves not adjusted correctly which will end up burned

You didn't say is it in the exhaust or back through the carb?

 

Just my .02 but it typically won't do that under no load high vacuum conditions.

I'll go with rich or even lean mixture from various causes, or if you have a bit of timing in it and you have your vacuum advance hooked to the manifold vacuum under high vacuum conditions which is what you're describing, it will pull full vac advance. Might check there first.

Or as stated, you could have an exhaust system issue.