This model has ECU, actually there are two of them. One is mounted on the top of the
air-flow meter and has aluminum casing (with Jetronic label): this is responsible for controlling
the fuel injectors.
The second one is mounted on the bulkhead, near the heating box. It has a black plastic
case, and it's responsible for the RPM dependent ignition advance correction.
I don't know the history (e.g: the car was unused for a long period,
or the problem arised after some repair, or arised suddenly), so
here are some general thoughts about the problem.
Backfire into the manifold may caused by the following:
- too lean mixture
- incorrect igniton advance
- incorrect firing order
Too lean mixture:
- some of the electroinjector valves are stuck: you can measure resistance across
the injector valve terminals,but the injector valve(s) won't open then voltage is applied
- inadequate fuel pressure: worn fuel pump and/or clogged fuel filter, defective
fuel pressure regulator
- the injection control unit not getting RPM signal from the igniton
coil terminal "1" (no injection at all), or the injection controller is defective
- old fuel in the tank
Incorrect ignition advance:
- the sparks are generated too late (after the TDC): incorrect basic advance setting.
e.g.: this might caused by the loosened, or wrongly aligned distributor
- failure of the RPM dependent ignition advance controller unit
Incorrect firing order:
- the ignition leads are mixed up
- if the oil pump was removed recently, it might not inserted back correctly: the pump
shaft is not aligned well during reassembling, causing the rotor to point to the wrong
segment in the distributor cap
I hope this might help.