Air flow

I did get my truck to pass emissions last year, but after it passed I have to remove the belt that drives the air pump to stop exhaust backfire and I would like to know why. The only thing that is not stock is the exhaust from the y pipe back, it is 3” single with a flowmaster muffler. Could the additional scavenging of the 1/2” larger pipe be pulling unburned fuel from the combustion chamber and with the introduction of smog pump air in the manifolds cause that backfire to occur during deceleration? If so, would additional intake air flow help with this or just increase the problem? It’s easy enough to disable the system after emission checks, so this is more curiosity than anything else.

Usually there is some sort of anti-backfire provision in the thermactor (Ford's name for the air injection system) plumbing. On Darth there were two valves (one for each pump) that would dump to the air cleaner housing. If the valve is there and either slow to respond or possibly has a leaking diaphragm then it would probably close to quickly leading to a backfire. Darth's actually had two vacuum signals to each valve, I am surprised you only have one pump. Darth is a 1986 F350 crew cab dually with a 460 and originally a C6. When I converted to EFI there was a single air pump, but all it did was put air into the catalytic converter which is not required on a 1986 over 8500 GVW truck.

Since VA has an update/backdate law, going to a newer engine, I still only need what was required when the truck was built.

I do have two pumps, so I guess my bypass valve is not operating as designed. More tracing of vacuum lines and troubleshooting ahead. Thank you.

Do you have the emission diagram for your truck? If not I have a picture of mine.

I do have it, and it just like the one in the document pages of this forum.