Here's an interesting article about the combustion of gasoline with oxygen:
http://www.madsci.org/posts/archives...3229.Ch.r.html
Here's part of the article:
Quote:
The combustion of gasoline is a very complex chemical process for there are over 500 different hydrocarbons in gasoline plus many other elements and additives.
Hydrocarbons ( HCs ) are any molecules that just contain hydrogen and carbon, both of which are fuel molecules that can be burned to form water ( H2O ) or carbon dioxide ( CO2 ). If the combustion is not complete, carbon monoxide ( CO ) may be formed. As CO is burned to produce CO2, it also becomes a fuel.
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So based on this article it looks like HC=unburnt gasoline, and CO=result of incomplete combustion.
It seems to me that the most likely way to get high HC is if the ignition system is not working well, and therefore not igniting the fuel consistently. Since your HC is very low, I'd say your ignition system is producing consistent sparks to the spark plugs.
If the high CO is a result of poor combustion, then it seems to me that there are two likely causes:
1) The ignition timing is not correct. If ignition occurs too late in the combustion cycle, then the air/fuel mixture will not have time to burn adequately. Five degrees advanced seems a little low to me. I'm thinking 8 degrees might be better.
2) The air/fuel mixture is not correct. If it is too rich, then there won't be enough oxygen to burn all of the fuel present, which in my mind would also result in high HC which you don't have. If it is too lean, then the flame will have a hard time spreading throughout the entire cylinder. You said you have your jets leaned out a bit.
I would suggest either advancing your timing a bit, or rejet your carburetor to be slightly richer. It would be a good idea only to change one of these things before having your truck tested again, so if it gets worse, you can tell which thing you changed caused the problem. If you change two things at once, there's no way to tell which of the two things caused the problem.