The wonders of the exhaust gas analyzer
Few outside of the powersports professional fraternity ever give thought to the exhaust gas analyzer in relation to motorcycles. Here I am not talking about the lambda tool, actually an O2 sensor also known as a wide band sensor (or AFR) many use for carburetor jetting. That's a worthwhile tool and I want to take nothing away from it. Nor am I ignoring the viability of inexpensive CO-only tools such as the brit-made Gunson G4125. But our subject is in fact the traditional infrared type multiple exhaust gas analyzing machine made famous by Sun instruments and once a mainstay of automotive shops all over and frequently found in powersports use as well.
This is the old workhorse, the once ubiquitous Sun-manufactured analog infrared sampling exhaust gas analyzer, or EGA.
Interestingly, of the Big Five manufacturers, only Yamaha ever formally embraced EGA use. The other powersports OEMs have ignored it. But that doesn't mean repair shops have. Whether franchised dealers or independents, the best shops are very familiar with the exhaust gas analyzer. They use it to tune engines, for troubleshooting, and of course for preparing vehicles to pass mandated emissions tests. It's a powerful tool.
The Two-Gas EGA System
The earliest EGA machines measured just two gases, CO (carbon monoxide) and HC (hydrocarbon). However, make no mistake. These two gases provide a wealth of diagnostic information, enough that many techs continue to find two-gas units more than sufficient for their tuning and troubleshooting needs. You can still buy such units today in fact. The two gases tested are CO and HC. CO, chemically equal parts carbon and oxygen, is very unlike similar-sounding CO2's (carbon dioxide) two oxygen components. Thus CO is oxygen-starved. It even displaces oxygen in the bloodstream when breathed, making it a seriuos death risk. Nasty stuff. Just as importantly, it signals the fact that combustion is awry, messed up. The burn is happening, it just isn't quite right chemically. The wrong mix. Which hints strongly at the carburetor's air/fuel mixture. HC on the other hand isn't just messy combustion like CO. HC actually signals that combustion is AWOL, missing in action. It's gone altogether, as in ignition misfires. A very different thing.
A high CO reading indicates incomplete combustion, that is, too little oxygen On the fuel side of the air/fuel mixture, this could mean too-rich idle mixture settings, high float levels, or leaking float valves. Also, the needle jets may be worn, or the choke plungers may be leaking or the carburetor's air bleeds restricted. If a fuel-injected engine, the fuel pressure may be too high. Carb or fuel injection, the air filter may be dirty.
High HC on the other hand indicates combustion breakdown. Some of the fuel is getting from the fuel tank to the exhaust pipe without being burned at all. Raw fuel, in other words. There are four things to check when HC is high. First, the most likely is an electrical misfire. This could be due to worn spark plugs, arcing plug wires, or other ignition system problems. Second, high HC may also be due to a fuel misfire, that is, an air/fuel mixture so rich or lean it results in misfire. It could be extreme richness due to a torn petcock vacuum diaphragm, or extreme leanness due to a vacuum leak. Third, a high HC reading may also be due to mechanical causes, including valves and piston rings, a leaking cylinder head gasket, and similar faults. Finally, high HC readings can result from the presence of fuel in the engine oil. This is common in powersports vehicles, because they often overflow fuel from their carburetors into the crankcase. The crankcase oil then has a high HC content, and this is transferred to the intake system through crankcase venting.
Even given all these examples, high HC readings are nonetheless trickier to diagnose than bad CO indications. The key to finding the cause of a high HC reading is in simultaneously viewing the CO. Here are some examples of that.
High HC due to electrical misfire: An electrical misfire is indicated when the HC is high and the CO is normal. This is especially true if the HC jumps upward periodically, with the CO dipping correspondingly. This points toward intermittent ignition.
High HC due to fuel misfire: A high HC reading accompanied by a low CO reading may be either air/fuel mixture related or ignition related. That is, the ignition could be performing far below capacity, rather than intermittently as before. This would reduce the CO. Determine whether the problem is fuel or ignition by very slightly applying the choke, or by taping up part of the air filter. If the CO increases, the ignition system is okay. It demonstrably can support combustion. The fuel system then is at fault. That is, there is a lean misfire.
High HC due to mechanical causes: A high HC reading accompanied by a high CO reading is caused by either mechanical or fuel system faults. The ignition system is okay because the high CO verifies that it can support combustion. Simply perform a compression check on the engine to verify a mechanical cause for the high HC.
In addition to these, it is also possible to have high HC for reasons that won't effect CO, reasons that aren't very intuitive. For example, high HC due to fuel in the oil. Verify fuel in the oil by measuring the HC in the exhaust the usual way, then simply unplug the crankcase breather and measure it again. There should be no difference. Another way is to shut off the engine and insert the EGA probe into the crankcase or oil tank. Do not start the engine and do not get oil on the EGA probe. There should be no reading. If there is a reading, there is fuel in the crankcase.
The EGA can help with carburetor jetting, too. One way is to perform the cruise check. Because an engine is designed to work best at high rpm, its combustion should be more efficient as rpm are increased, and conversely, less efficient at low rpm. Above idle, both HC and CO should drop somewhat. In fact the CO especially should drop to about half the idle reading. If the CO instead stays the same or increases at mid throttle, the cruise part of the fuel system is too rich. Check jetting, dirty air filter, or worn or incorrect carburetor parts or adjustments. Also verify correct fuel pressure on EFI vehicles. If the CO goes down like it should but the HC does not, treat this as a misfire. Do the enrichment test to determine whether the fuel or ignition system is the cause. This ability of the EGA to accurately gauge carburetor mixtures of an engine running under a load at various rpm is how Mark Dobeck's Dynojet Corporation got started. In fact, that's how people used the Dynojet dyno at first, with an EGA, for jetting carburetors.
The Four-Gas EGA System
The four-gas EGA measures CO2 (carbon dioxide) and O2 (oxygen) in addition to CO and HC. Comparing all four of these gases with each other speeds up diagnostics. It's also the best way to diagnose catalytic converter equipped vehicles, as the extra gases compensate for the masking effect the catalyst has on CO and HC.
The key to four-gas troubleshooting is found in two relationships. The first relationship is the one between CO and O2. These gases should be nearly equal to each other in modern vehicles, and they are inversely proportional -- when one goes up the other goes down, and visa-versa. If CO is high, O2 will be low, reinforcing the CO's indication of richness. Conversely, if O2 is high, CO will be excessively low, confirming leanness. Just as in two-gas diagnostics, low CO is leanness, high CO indicates richness.
The second critical relationship in four-gas testing is the one between CO and CO2. Excessive CO2 by itself indicates either richness or leanness, but doesn't tell us which. The CO level then indicates which it is, richness or leanness.
Five-gas exhaust analyzers are also available and actually the most common today. These machines add NoX, oxides of nitrogen, an important emissions test gas and one that has a direct link to combustion temperature.
An engine designed to be efficient at mid-range rpm is least efficient at idle and near-idle. Idle's very low air speed and the influence of altitude and other variables causes the carburetor's idle circuit, unlike the other circuits, to be frequently in need of fine-tuning. The pilot or idle mixture screw is therefore a necessity on a carburetor, though authorities forced it to become tamper-proofed in later years.
Warming up the engine first is extremely important. Adjustments made to a cold engine will be too rich when the engine is warm. Conversely, an adjustment made on an overheated engine will be too lean when the engine is at normal temperature. Warm up and calibrate the EGA. Insert the EGA probe as far as possible into the exhaust system to prevent the readings from being diluted by outside air which enters the exhaust between exhaust pulses.
Adjust the carburetor's idle mixture to the correct CO reading. Never mind the HC when doing simple idle mixture adjustments. It will take care of itself as the CO is adjusted. As long as the HC is below 400 ppm in late model vehicles, or 800 ppm in older ones, it's okay. As for the CO, late model, electronically-controlled and fuel injected engines are "happiest" at 1~2% CO, and some of the most recent bikes even as lean as 0.5%. However, vintage engines run best with their idle mixture screws adjusted to between 2.5~3.5% CO, and a few really old bikes even richer at 4.0~5.0%.
Another way to arrive at an emissions-compliant carburetor idle mixture setting, without using an EGA, is something called the idle drop procedure. It was exclusively promoted by one powersports manufacturer, Honda, and derived from their carbureted car oriented technique, the propane enrichment adjustment. The powersports-specific idle drop procedure relies on a super-sensitive electronic tachometer tool instead of an EGA. The tachometer must be able to pick up changes in rpm as small as 50 rpm as the idle mixture screw is turned. First the screw is adjusted until the rpm is maximized, as would be done either with an EGA or by a trained ear. Then this optimum setting is deliberately worsened a precise amount, gauged by the rpm dropping either 50 or 100 rpm from the first reading. This procedure approximates the idle mixture air/fuel ratio the manufacturer believes will satisfy emissions regulations. Note that, being an emissions adjustment, this in no way results in the optimum setting four your motorcycle's carburetors.
The exhaust gas analyzer. An incredibly powerful tool that despite its longevity on the automotive scene is under appreciated by many. The EGA's troubleshooting and tuning potential is huge.