81 FI intake backfire

[spider2081]

I have a 1981 FI Spider and recently it started intermittently back firing in the intake just as the car starts. There is a single pop. The bang is hard enough to blow the hose off the air flow sensor. Not sure what would cause this. I am thinking maybe a leaking fuel injector.

Any thoughts appreciated

[So Cal Mark]

intake backfires are usually associated with a lean condition

[rlux4]

A couple of answers would help us figure this out.
How does it idle when it's running?
Is your ignition timing correct?
You could try unplugging the cold start valve for a starting point if these things are normal.
You take the chance of damaging your air flow meter with this backfire, the flap can get bent.
Ron

[124JOE]

can you hear a leak anywhere around the intake?

[vandor]

Check for vacuum leaks, as Mark said this is usually caused by a lean condition.
Also check the intake hose for cracks, especially at the bottom of the accordion section.

[spider2081]

The backfire only occurred on two occasions. Both times the temperature was under 40. I checked the hoses and the only one of suspect was the breather hose so I cut it back and refastened it. There were no other vacuum hoses or air intake connections that looked the least suspect. The flap in the air flow sensor does not appear bent and still moves freely.

I removed the distributor cap and found the rotor had hit the cap and was bent. Could that happen from an intake backfire kicking the engine rotation backwards? I replaced the rotor the cap looked ok. I don't have a timing light here I think it's at one of my kids houses so I couldn't check the timing. Maybe it is too advanced.

Thanks for the suggestions.

[So Cal Mark]

the damaged rotor could have been the issue. I've seen several of the aftermarket caps and rotors that don't fit well and have contact issues.

[spider2081]

thanks mark, I usually buy parts from well known Fiat vendors but this cap and rotor was one of those toooo good to be true specials from an on-line site. Lesson learned.

Thank you for all your help and support of this site.