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Damn good question! It started with me asking a couple of very experienced engine builders why my car stinks so bad at idle. I thought running rich would make it stink, an they both said, "because it's running too lean."
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My EFI car smelled bad at idle as well. It has a pretty healthy roller cam in it. That was with an oxygen sensor feeding back to the ECM.

I welded a pair of metal matrix, racing catalytic converters on it and the problem was solved. It didn't affect the power output either.