i just finished my rebuild on my 400 pontiac. it makes a nasty popping sound after revving when you let the rpm come down. i firsst thought it was an incorrectly adjusted valve. i reset them and still no cure. i then figured maybe th ecarb was too lean but when i pulled a plug they weren't overly gray, maybe a even a little rich. im running a carter, crane springs and cam and headers. any suggestions. cars is perfectly drivable, its just annoying