What goes in must come out. Air and fuel are mixed via the carburetor and ingested into the engine on the intake stroke. Every molecule of that air and fuel exits via the tailpipe (Theoretically. Some goes past the piston rings and into the oil, but we can ignore that since it is such a small amount).
The reciprocating piston internal combustion engine is a heat engine. Air and fuel are converted to heat and pressure which act upon the piston to produce power. This heat and pressure is produced by the chemical reaction between the air and fuel. If the mixture were stoichiometrically correct (meaning every molecule of fuel reacts with every molecule of air) then all we would have coming from the tailpipe would be CO2 and water and some inert gases.
We can not run an engine on perfectly stoichiometric mixtures because a carburetor can not control fuel mixture that precisely and because the engine will not run well. Especially in carbureted engines, the fuel mixture must be run on the rich side anytime the engine is accelerating.
During part-throttle cruise conditions we can run near stoich. Modern fuel injected engines are very precise and can cruise on a mixture considerably leaner than stoich and this is the main reason modern engines get such good fuel economy.
So how do we know if our fuel mixture is correct using a gas analyzer? First, you need to know what the gases mean. Gas analyzers typically come as 2, 4, or 5 gas analyzer. The 2 gas analyzers will be cheapest and they provide us readings to the 2 most useful gases in the exhaust.
CO (carbon monoxide, measured in percentage) is produced from rich mixtures. The more CO in the exhaust, the richer the mixture. Lean mixtures do not produce CO. As a matter fact, anything leaner than stoich will indicate 0% CO.
HC (hydrocarbons, measured in PPM or parts per million) is raw gasoline that has passed through the engine unburned. Ideally, all the gasoline will have burned in the cylinder but this is never the case. There is always some molecules of fuel that did not find themselves close enough to an oxygen molecule (O2 is diatomic) to be oxidized.
HC is always indicative of misfire. By definition that is what misfire means. The engine did not fire and the gasoline passed through straight to the exhaust. You can have high HC with either a rich or a lean mixture. Near stoich, they are lowest.
The 4 and 5 gas analyzers can measure CO and HC as well as 2 or 3 other gases. It is perfectly acceptable to tune with a 2 gas analyzer and you can achieve great results. When you are trying to diagnose a running problem with the engine, the other gases can clue you in to what is going on so they are always advantageous to have.
O2 (atmospheric oxygen, measured in percentage) indicates the amount of oxygen that has passed through the engine without uniting with a fuel molecule. The air went in and made its way all the way down to the tailpipe without reacting chemically.
CO2 (carbon dioxide) indicates the amount of CO2 produced, which is a natural by-product of combustion. We didn’t deem it particularly harmful until we realized that it was displacing good stuff. We use it to determine combustion efficiency.
When you buy the super-deluxe 5 gas analyzer you also get NOx (oxides of nitrogen, a constituent of photo-chemical smog). NOx is produced as the result of high temperatures and pressures. It is a big player in the formation of the brown gunk that used to hang in our air. For performance tuning, this reading will only give us useful information under load.
Now that we know what the gases are we can discuss how to use them to tune a carburetor. In my next blog post.