I have a 402 BB in a '72 Motorhome. The motor starts, idles and runs fine, but backfires through the carb when under a load or when given full throttle w/out a load. It had a points distributor and Q-jet carb; both with remanufactured stickers. I figured I'd put an HEI in it and see if that cured anything. But still does the same thing. I haven't swapped out carbs yet. I have advanced and retarded the timing, and it seems to have no effect. There are no vacuum leaks, and all plug wires are in proper order. At idle it's fine. But I can notice a slight ticking noise. It's more noticable standing along either side of the motorhome than from up on top of the motor. Initially I had considered a cam lobe or lifter as the problem. But I pulled both valve covers and watched all the rockers as it was running, and nothing seemed out of the ordinary. There was a very slight lesser movement on #2 intake, but barely noticable. Could have just been my eyes playing tricks on me. Wasn't enough to make it backfire like it does. If it's not a spark or fuel issue, and not a cam lobe or lifter, only other thing I can think of is a valve or bad seat. Am I missing any other possibilities? Other facts: The motor does not smoke, and has approximately 97,000 miles according to the odometer. All the date codes on the motor indicate it is the original motor. It backfires regardless of being cold or warmed up, in gear or not, but is more noticable when a load is on the motor. When I got it, the motor had a new cap, rotor and plugs. The Carb was rebuilt recently as well as the point distributor, and it all looks new and in good shape. This indicates to me that possibly someone was trying to fix the same problem in the past.