I have an '85 GT that I put a GT-40 crate engine in. It originally being an EGR car, I initially retained the EGR plate, etc. I put a Road Demon 625 on the car, but quickly realized this was too little of a carb (wouldn't idle down, ran rich, etc. due to cam overlap). It ran fine at speed, accelerated fine, even ran great at full throttle once I put a different (lower tension) vacuum secondary spring in it. But, it still wouldn't idle out, and I felt there was more in the engine. So, I replaced it with a new Speed Demon 650 with special jetting precisely for this engine and combo. It's a mechanical secondary carb, so I had to remove the EGR plate for clearance issues. I left the EGR-compatible Edelbrock intake on the car, but blocked off the port in the carburetor mating base with a plate. Now, the car backfires like a bean-fed donkey under light to moderate acceleration around 2800 rpm. If I floor it then, it accelerates like a scalded cat, but driving it around is plain embarrassing with the backfire problem. Is this related to capping off the EGR port? If so, I'll order a different intake and do away with it. I've adjusted everything else in the system; timing, vacuum reference, idle mixture screws, float levels, idle transfer slot settings, you name it. Any carb-savvy gurus out there with a suggestion, or better yet a fix?