A little progress report on why my '88 Accord DX (150k miles) failed the NOx part of a smog test and a couple of questions to the experience carb/vacuum folks.

After replacing the usual maintence items (plugs, dist cap & rotor, wires, etc.) and testing the ^%$# out of the emission control components I had made NO progress! I did find three hoses with holes or had rotted in half, but my car still failed with high NOx.

Brilliant idea #1: watch the output from the O2 sensor. Hmm, it oscillated as it should but it barely got above stoich (.5v) and spent most of its time near .2v to .3v. Okay, replace the sensor since it's original. No change.

A slight misfire, from idle on up, has been bugging me for a while and I began to think it had something to do with the high NOx. After the previous component replacements there wasn't much that was cheap or easy to do. Brilliant idea #2: I tried a propane-enrichment test just to see if I was running lean. YUP. RPM increased by nearly 100 (should be 50 plus/minus a bit) and the mis-fire disappeared. Whoopee! For a bit. Then I began to wonder why it was lean. So a couple of questions:

- Does a carb usually go lean as it ages?

- How can I tell a vacuum leak from a lean A/F mixture?

Thanks,
Marc