fuel feed problem

Friend has a rebuilt chevy six,dual webber carbs ,dual exhaust from Clifford . Ran like a top for 600 miles then quit. He can run the engine up to max RPM's if he does it slowly. If he dumps on it it sounds like backfiring thru the exhaust. He has replaced all ignition parts , carbs taken apart and appear ok and in specs,all pressures are normal,fuel vacuum etc.Filter is new and clean. Timing is in specs, cam and dist timing chain controlled and are set properly. A note here I am not a mechanic and am relating what he has told me. He has had a 1/2 doz or so of real experts look at and all shake their heads . To say the least he is quite frustrated . Any and all suggestions will be considered . Thanks

"He can run the engine up to max RPM's if he does it slowly. If he dumps on it it sounds like backfiring thru the exhaust."

Not a fuel problem. Ignition problem. Get new plugs and gap properly. Check wires for resistance. Segregate wires from each other or cross them at 90 degrees to prevent induction voltage from one wire to the other. Check inside distributor cap for carbon tracking. If points distributor, replace points and condenser.

it does sound like ignition go buy one of those cheap spark gap testers and see what a cyl does when its running I spent a complete test and tune day dreaming that my car was lean same problem as you describle and it was ign, I ended up putting a crame points eliminator kit in it and ran fine after that

Thanks for the info. I do know he has replaced everything except the wires and they were new when he started . I will relay your suggestions .

Check the engine foundation of your vehicle.

Also check for worn out distribuator. Had that happen once on a 66 pick up I had.