Odd thing happens under full throttle=can heard almost like a backfire noise

Okay this is not your everyday Burban.It's a 91' with a 98/99 454 but its been converted to carb.Now the guy had spent some money on the engine and then slapped a leaky,shot Q-jet on it .I replaced that with a Barry Grant Street demon 750cfm Vac sec..even though the carb looks new i still went thru it and he had the jets all wrong so prior to installed i set it up.Adjusted the electric choke,one pump on s cold start and bang she fires right had ti fiddle with the fast idle speed a bit but now when warm you can reach in and turn the key and it starts.

Now all the above wasn't bragging it was to show im an old dude that has played with alot of carbs and never ran into this.When you are accelerating hard and the secondary's come open you can feel the truck pick up speed but once or twice when they opened i felt as much as heard a pop..now BG's from the factory are like 74's in the front and 83's in the back which is a big set of jets.

I checked for vacuum leaks and ended up replacing 90% of the hoses under the hood.

Any feedback,suggestions,thoughts to steer me in right direction would be great.