OT: My tow rig failed emissions! Options

[Eddie914]

My tow rig failed emissions today in Seattle!

1983 Chevy 454 with a Quadra jet pushing a 27 ft motorhome chassis.

It failed the Carbon Monoxide at idle test.

HYDROCARBON (ppm)

Cruise Limit: 220
Cruise Emissions: 33
Idle Limit: 220
Idle Emissions: 128


CARBON MONOXIDE (%)

Cruise Limit: 1.2%
Cruise Emissions: 0.75%
Idle Limit: 1.2%
Idle Emissions: 4.78%

Unfortunately the idle mixture screws are sealed on this carburetor (for the time being). The air pump has been removed.

What is interesting is that since I replaced the heads to rectify the badly worn valve seats the HC has gone substantially down and the CO has gone substantially down.

Are the HC and CO inversely proportional?

Should I lean it out? Should I richen it up? … or should I just shoot it?


Thanks

Eddie

[lapuwali]

Strictly as a function of AFR, HC and CO both fall as the mixture gets leaner. However, if you go so lean that you start to misfire at idle, HC will soar.

So, high HC, low CO means you're way too lean. High HC, high CO means you're way too rich.

NOx peaks at 14.7:1, and falls dramatically on the rich side, so for a car with no cat that has to pass a test with NOx, running just slightly rich will give best overall emissions. For a car with a cat, any failure generally means a new cat, as a functioning cat will lower emissions so much that you'll pass even fairly recent standards so long as they mixture isn't way off. If you have a car with a cat AND an O2 sensor, replace them both.

In this specific situation, it could be you've got a leaky jet in that carb somewhere adding too much fuel, so you may need to dismantle that carb and have a look. I'd pay particular attention to the idle mixture screw and it's seat, since CO is high at idle, but not at cruise.