‘71 360 Down On Power, Backfiring Through Intake

[87Itasca]

Hi all,

Working with a friends old RV this time, a 71 Dodge Tradesman RV with a 360. About 5 years ago we did the Holley Sniper EFI conversion and it's been absolutely trouble free until the other week. I don't suspect the issue lies with the EFI though.

Engine will start up fine, but when accelerating it will backfire out of the intake then pick up steam. It struggles to reach 55 mph now. Just started right out of the blue. Sensor data shows nothing abnormal, O2 sensors report the air fuel mixture to be OK with no signs of it running lean. Went around intake and vacuum connections and found no leaks. Timing advance diaphragm is not torn either. Possibly a jumped/stretched chain? Did these have a phenolic cam gear that liked to fall apart?

Not really sure where else to start honestly. Cap and rotor were replaced with no change. Firing order is correct, plugs/wires have ~1200 miles on them.

[Eyez Open]

Intake firing is due to a open intake valve during a combustion event. old timing chain would  be highly suspect. Simple check would to pull no 1 plug place a screw driver in the cylinder and find  tdc, from there your harmonic balancer should be a tell all...aka timing mark.

[87Itasca]

That's what I'm afraid of. I'll probably be able to check this weekend