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