From

http://www.artsautomotive.com/thesmogstory.htm

And I quote:

"What causes high CO (carbon monoxide)? High CO is caused by a rich air/fuel
mixture. The air fuel ratio should be 14.7 parts air to to 1 part fuel.
Problems that can cause a rich mixture include:

Problems with the feedback system (the system that regulates the fuel being
added to the engine based on the amount of oxygen in the exhaust).

Problems with a computer sensor such as the coolant temperature sensor, air
flow meter, MAP sensor, etc..

Problems with the fuel pressure. Fuel pressure that is too high can cause a
rich mixture."
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Tom Young
Lafayette, CA 94549
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----- Original Message -----
From: "jimt" <wetwesty@tactical-bus.info>
To: "Tom Young" <tomyoung1@COMCAST.NET>; <vanagon@GERRY.VANAGON.COM>
Sent: Monday, October 25, 2004 8:46 PM
Subject: Re: Calif Smog Issues

Running to lean can give high CO.  from your readings it looks like you are
actually running lean.