riding the egt

Exhaust gas temperature (egt) is a good indicator of the fuel mixture in your two stroke engine. The higher the temperature, the leaner the mixture, and eventually the piston will seize. The first extensive use of egts as a jetting tool was in airplanes, where the pilot would use it to adjust the fuel mixture as altitude and temperature varied.

Egts are also widely used as instrumentation on dynamometers, and in the 70s we started using them on the snowmobile to check the jetting in the field. Today there is a big selection of egt products to choose from, both simple analog gauges and sophisticated digital recording gauges with programmable warning lights. By installing an egt gauge you can now jet accurately for all conditions and see when you are getting in dangerous territory. Combine the egt with some of the new adjustable fuel flow technology being offered and you can easily "ride the egt" and change fuel mixture even while you are driving.

Two stroke engines are sensitive to fuel calibration, and when run on the lean side can suffer piston damage. Snowmobiles often run in temperatures varying from 30 degrees F above to 40 degrees F below zero. In the mountains, altitudes can easily vary 5000 feet on a day's trip. Combine these two facts, and you may be stranded somewhere in the wilderness on a cold day with a seized engine. This is why snowmobilers are always conscious about keeping their carbs jetted correctly. Over the years solutions to this problem have been many, from the adjustable jet needles on the early pumper carbs, to the complicated and expensive electronic fuel injection systems.

Manufacturers felt the adjustable pumper carbs were a little too adjustable, as people leaned them out too much and stuck the engines. The fixed jet Mikuni float bowl carbs took the outside adjustability away, and ever since, the consumers have tried to put adjustability of the fuel mixture back in. Changing jets is not a favorite thing to do, as it is a messy operation with fuel everywhere. With the new rack-mounted carbs and complicated airboxes, it can easily take half an hour to change a set of jets, if you are a good mechanic.

A lot of different solutions to quick jet changes have been offered over the years. One of my early favorites was the "adjustable float bowl." This design had two main jet circuits in parallel. One circuit had a main jet that was chosen to be the leanest you would dare to go without seizing the engine, and the other circuit had an adjustable needle like the old pumper carbs. The carb could be adjusted richer with the needle, but could never be turned down leaner than the jet in the main circuit. This was a popular product for many years, but was eventually dropped from production and is now hard to find. The next step in the quest for easy jet changes was the power jet system.

This system also consists of two parallel passages. The regular main jet is still on the bottom of the carb, but the power jet is mounted on top of the intake bell and only goes into effect after the throttle is more than halfway open. The powerjet can be changed from above, which means you don't have to take the carbs or airbox off to change jets.

The next step in the evolution was to introduce adjustable power jets, either with a screw type needle or an airbleed system. The Dial-A-Jet, from Thunder Products (320/597-2700) is an airbleed system where a number of different size airbleed holes can be "dialed" in place by the twist of a screwdriver. The larger the hole, the less fuel is pulled through the power jet, and the leaner the mixture becomes. Belt dust and snow can easily get into and plug the holes, so a prefilter is also offered as an added accessory for this system.

The problem with all these systems is that you have to stop to adjust them or to change jets. Wouldn't it be easier if the carb adjusted itself according to temperature or pressure changes while you were driving? Perhaps you could adjust it from the handlebar without having to stop? This technology is available now and is getting very popular as the carbs and airboxes are becoming more complicated.

The electronic fuel injection does this automatically via its computer, aided by sensors measuring temperature and barometric pressure. The first attempt to accomplish the same thing on a carb was Yamaha's electronic Smart Carb. Ski-Doo introduced its automatic altitude compensator, but both these products lacked the "tuneability" motorheads wanted. Performance guys wanted to control the mixture themselves and watch the egt gauge as they rode, and the fully automatic systems did not provide this feature, neither were they all that accurate in their compensation.

There are a number of reasons to adjust the carb, aside from atmospheric conditions. If you run tight trails you may want to lean down a little to get better throttle response, and when you hit that long lake at full throttle, you probably want to richen up to prevent a burndown under extended load as the engine warms up. A product that can provide this adjustability on the fly is the new Vari-Flow system. This product was introduced several years ago by Barry Holtzman (715/479-8727), a physicist with a keen interest in snowmobiling. Dr. Holtzman reasoned that if he could control the pressure in the float bowl, he could also control the fuel flow.

The amount of fuel that flows through a carburetor is proportional to the difference between the vacuum in the venturi as the airspeed increases, and the pressure in the floatbowl. The floatbowl is normally vented to the atmosphere, which usually means the underhood area. The Vari-Flow combines all the vent-tubes and runs them into a valve which then controls the pressure in the floatbowl and thereby changes the calibration.

A small fitting is introduced into the venturi area of one of the carbs. A tube goes from the fitting to the valve and air is pulled through the valve. In the middle of the airstream sits a calibration tube with a small hole that is coupled to the vent tubes. By turning the calibration tube so that the hole is positioned at different angles to the airstream, different amounts of vacuum are produced in the floatbowl. The more vacuum you create, the leaner the mixture becomes.

The valve can be mounted on the handlebar or on the dash, and the adjustment range is 12 percent, or four jet sizes. This means you jet for the coldest or richest conditions, and click down from there as you head up the mountain or down the trail. Since temperatures can vary greatly from morning to mid-day, and down again in the evening, the Vari-Flow is just as useful on the flatland as it is climbing mountainsides and compensating for altitude. You only need one valve to control up to three carburetors, and Holtzman Engineering's kit also includes calibration sleeves for different size motors and high altitude locations.

"Riding the egt" and adjusting the fuel flow does require that you have a reliable and consistent egt gauge. There are a number of excellent gauges that do a good job, from the less expensive analog gauges to very sophisticated digital instruments. Personally, I like to watch an analog gauge, because the needle position quickly shows me what range I am in. The digital gauges have a number of extra options that are of interest to the tuner. A popular option is the maximum temperature reading. By pushing a button you can read what your maximum temperature was during your trip or test run. A warning light can be programmed to come on if a preset temperature is exceeded, and this can save you from a burndown. Both the Avenger series and the Digatron gauges from Exhaust Gas Technologies (800/348-4678) have these features.

There seems to be a lot of different preferences as to where to mount the probe itself. My personal preference is to mount it between eight and 10 inches from the exhaust port. This seems to give consistent readings without too much influence from any raw fuel that may escape the exhaust port. The closer you get to the exhaust port, the greater the chance of inconsistent readings. Consistency is the key element when you "ride the egt," and with the large assortment of tuning products and gauges available it is getting easier every year to get good performance without the fear of burning down.