Bill and Robert,

Thanks for the help, I think I figured it out. My car had cruise control when it was new, but the P.O. took it out for some reason. When the engine was rebuilt last winter, the mechanic just put a plug in the vacuum line tee that comes off the decel valve. I guess the plug fell off and caused a major vacuum leak, which in turn caused the rough idle and backfires when you lifted off the gas. I recapped it and WOW what a difference. No rough idle, no backfires, better power! So the reason I couldn't find the problem, was the cruise control box was completly removed from the engine bay. I only figured it out buy asking a guy what the thing on the left back wall was in his car (cruise box) and I followed a vacuum line back to the spot where the tee joint is.

Thanks again,

Paul
81 911SC/RS

P.S. Sorry I hi-jacked this post.

Those of you who are reporting backfiring with a lean mixture, like with a vacuum leak for example, are hearing an INTAKE backfire, I believe. Lean mixtures do cause backfiring in the intake. According to some trusted local mechanics, backfiring in the exhaust is ONLY caused by an exhaust leak, like a gasket that is sucking air. That's what they say and I cannot dispute it.

My car runs much better with the Decel Valve unhooked, as described above.

Finally, Mike reports a high idle that comes down and stays down if you use the clutch to force the engine to the lower idle. This is evidence of mechanical advance weights sticking. Mike, your distributor needs to be disassembled and cleaned, and this will stop that symptom. The high idle is caused by the centrifugal (mechanical) advance mechanism is sticking in an 'advance' position.

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'83 SC