911 chronicly backfiring

[Zane'sFinancial]

My name is Zane, I just bought my 911 and it's my second porsche, my first was a 1984 944. I love the car, it's gorgious, however it's developed a problem with chronic backfiring. Also, it's fuel consumption has become horrible, like maybe 15 mpg if I'm lucky. It's a little hard to start when cold, and has an erratic idle. Also it backfires constantly on deceleration. While powering around is mostly unaffected although I seem to have lost a little power.

What would cause this sudden over-richening of the mixture to such extremes? The only thing that I've really done with the car is I removed the A/C pump and bracket assembly. I looked in the owner's manual and see that there is a back-fire valve somewhere in there but it's a technical diagram that is poorly labeled. Is this somewhat interconnected with the a/c system? Is there a mixture adjustment somewhere?

So, what are your guys' ideas? I'm mechanically knowledgeable and have turned me some wrenches. However, being so unfamiliar with these car's engines/fuel injection systems I'm rather lost. Any help is appreciated.

[Latka]

What year is the car? That might help people diagnose it.

[Zane'sFinancial]

My bad, it's a 1977 with a 2.7L

[azasadny]

Zane,
My car is a '74 with the 2.7L CIS and is doing exactly the same thing. I'll let you know what I find out as soon as I make some progress.

[afterburn 549]

Back fire--- secondary breakdown ( coil, wires plugs etc) or a real lean fuel condition