Using the Exhaust Gas Analyzer
I was tuning an old Honda. The bike had been seriously molested and hacked up with all sorts of odd modifications, so it was helpful to bring the science of exhaust gas monitoring to the process. I was pleased with the numbers--the 2.8 percent CO (carbon monoxide) and attendant 350 parts per million HC (hydrocarbons) were perfect, exactly what this model should show and thus very encouraging because I had rebuilt the carburetors and fettled the ignition and was well on the way to solving many of the bike's performance glitches. But then, all of a sudden, while I watched in consternation, both readings plummeted. The CO dropped to just 1.2 and the HC to a mere 150, and the engine began acting up, its rpm dropping until it finally stalled. The bike had run out of fuel! Oops! Nice warning on the EGA, though! On another occasion I was doing some tuning on an engine when I experienced the opposite. The bike's CO was 3.5 and the HC 600, then both rapidly skyrocketed to 7.0 and 1200 and worse. The engine was fuel-flooding--it was experiencing severe richness. I knew this because the CO rose along with the HC.

It's simple. CO represents richness while HC indicates combustion irregularity, and the fact that both climbed together told me the ignition system was still supporting the super-rich combustion. So ignition was not the problem. However, had the CO dropped while the HC went to the moon, this would have indicated an ignition misfire: ignition could not properly support combustion. It would have meant that the abnormal combustion revealed by the high HC was in fact due to an ignition problem.

This is how an EGA is used. No single gas reading gives the whole picture. One gas must be compared with the other. While CO is the relevant gas in simple carburetor adjustment scenarios, HC is important too. A little HC is normal even in the best engines. But those designed later are more efficient than earlier ones, and there is an abnormal level of both CO and HC for each era's engines. 1960s and 1970s bikes are "dirtier" than 1980s machines, and 1990s than 2000s, and so on. Modern EGA machines actually compare four and five different gases, necessary due to the complexity of later vehicles' catalytic converters, air injection and other systems.

Pilot screw adjustment
Accurately adjusting idle mixture has long been a challenge in powersports, and sadly, today seems to be characterized by debate and disagreement. However, experienced mechanics are known for their ability to do the task fairly accurately by ear. It's just a skill you developed back in the 60s and 70s, because you had to. But while this was going on, automotive shops were using the exhaust gas analyzer to do the same adjustment. Car mechanics were dialing in idle mixture scientifically and quantitatively. But a few knowledgeable motorcycle shops also knew the value of the EGA. It is both fast and accurate.

Not all exhaust gas reading machines are equal. The traditional EGA is a fairly sophisticated machine that actually scans exhaust gases with infrared light. New or old, this is how the pro-level units work. Conversely, the lower-cost single-gas tools that have emerged in recent years use a much lower-tech, simple automotive tailpipe oxygen sensor instead. While those machines mathematically interpret from that the CO level, they aren't as accurate and more importantly, are designed solely for carburetor idle mixture adjustment and really nothing else. Sourcing their mixture information from a less accurate source, and lacking comparison gases capability, they can't begin to be diagnostic tools. By contrast, true EGAs are superlative diagnostic machines. They are very powerful in that space.

Although comparing with HC is important, CO is the focus in carburetor idle mixture screw adjustment. Every carburetor has a perfect CO scenario. When an EGA is used for carburetor adjustment purposes, if the HC stays within limits, it can be more or less ignored while the CO is adjusted by the carburetor's pilot screw setting. CO that is the correct percentage for a given engine tells us the air and fuel are mixing and burning at the right proportions. CO that is too low or too high signals that the air/fuel mixture is the wrong mix, the wrong proportions--there is either too little fuel or too much. Thus CO is a good way to adjust a carburetor's idle mixture. But care must be taken to do the adjustment when the engine is at the right operating temperature. An engine that reads a typical-for-vintage-Hondas 2.7 to 3 percent CO when it is at running temperature is going to indicate just 1.5 to 2.0 percent when cold. If adjusted when cold therefore, the adjustment will be too rich at temperature. Among other things, this illustrates the increased fuel efficiency of a warmed-up engine (its fuel needs decrease, showing on the EGA as a surplus), and it emphasizes the caution that must be taken to not make adjustments before the engine is at the correct temperature.

EGAs are professional-level tools. They are not really meant for the DIY-er. Understandably, they are fairly expensive to purchase. And rehabilitating an old 1970s or 1980s Sun EGA is unfortunately a prohibitively expensive tactic. A number of small-shop oriented EGAs are available today, all of them with digital displays, some with printers. Bridge Analyzers in Ohio is a good source offering high quality, made-in-the-US units, from two-gas to five-gas, machines used by several major powersports manufacturers in fact, beginning at about $3,000. The Carlson motorcycle tool company is another good source, as is also K&L. An EGA is not a hobbyist-level purchase, of course. But for the career mechanic, the EGA can solve mysteries.