Carb Tuning - Page 6, By Eric Gorr

Eric Gorr

Engine Builder
Jun 29, 1999
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Fuel and Oil Mixture Ratios

When we talk about the "fuel" in the air-fuel mixture for a two-stroke engine, we are really talking about a mixture of fuel and oil. If you richen the pre-mix ratio (20:1 as opposed to 30:1) there is more oil and less fuel in the same volume of liquid, which effectively leans the air-fuel ratio. And this fact gives the clever tuner one more tool to use when the correct jet is not available or when none of the standard jets are exactly right. You can richen the jetting by slightly reducing the pre-mix ratio (less oil). You can lean the jetting by increasing the pre-mix ratio (more oil). The best part is that changes in the pre-mix ratio affect the jetting over the entire throttle-opening range, but the changes in ratio must be small to prevent excess wear from lack of lubricating oil or fouled plugs from too much oil.

Pre-mix oils are formulated for a fairly narrow range of pre-mix ratios. You should examine the oil bottle for the oil manufacturer's suggestion on the pre-mix ratio. All production two-stroke dirt bikes have a sticker on the rear fender suggesting that you set the pre-mix ratio to 20:1 That sticker is put there for legal purposes. Always refer to the oil manufacturer's suggestion on pre-mix ratios. In general, small-displacement engines require a richer pre-mix ratio than do large-displacement engines because smaller engines have a higher peak rpm than larger engines. The higher the engine revs, the more lubrication it requires.

Tuning Gauges
There are three types of gauges that professional tuners use to aid carb jetting:
1. Relative-air-density (RAD) gauge
2. Air-fuel (AF) ratio meter
3. Exhaust-gas-temperature (EGT) gauge

The following is a description of how each gauge functions and their advantages.

RAD gauge:
This is the best gauge for dirt bikes because of the convenience. The gauge is no good unless you get the jetting perfect once. The RAD gauge provides you with an indication of how much the air density changes, helping you compensate for the affects of changes in the air temperature, altitude, and barometric pressure. The gauge is calibrated in percentage points. Once you set the jetting with the ride and feel method, you can set the calibration screw on the gauge so the needle is pointing to 100 percent. When the air density changes, the RAD gauge will show the relative percent of change. Using a calculator you can multiply the percentage change shown on the RAD gauge by the jet size and determine the corrected jet size for the air density. The pilot/slow and main jet have number sizes that correlate with the RAD gauge, but the needle clip position can only be estimated. Normally for every two main jet increments, the needle clip must be adjusted one notch.

AF ratio meter: The AF meter measures the percentage of oxygen in the exhaust gasses, and displays the approximate air-fuel ratio of the carb. The gauge displays AF ratios from 10-16:1 The optimum AF ratio for a two-stroke engine is 12:1. The AF gauge utilizes a lambda sensor that is inserted into the center of the exhaust stream, approximately six inches from the piston in the header pipe of a four-stroke and in the baffle cone of a two-stroke engine. A permanent female pipe fitting (1/4in.) must be welded to the side of the exhaust pipe in order to fasten the sensor. The weld-on fitting set-up is also used on the temperature gauges, and the fitting can be plugged with a 1/4in. male pipe fitting when the gauge is not in use. This gauge is ideal for four-stroke engines.

EGT gauge: The EGT gauge measures the temperature of the gasses in the exhaust pipe by means of a temperature probe fastened into the exhaust pipe, six inches from the piston. This type of gauge enables you to tune the carb jetting and the pipe together, taking advantage of the fact that exhaust pipes are designed with a precise temperature in mind.

An exhaust pipe is designed to return a compression wave to the combustion chamber just before the exhaust port closes. Most pipes are designed for a peak temperature of 1,200 degrees Fahrenheit. Most dirt bikes are jetted too rich, which prevents the exhaust gasses from reaching their design temperature, so power output suffers. Sometimes just leaning the main jet and the needle-clip position makes a dramatic difference.

Digitron is the most popular brand of EGT gauge. It measures both EGT and rpm. This gauge is designed for go-kart racing so its not suited for wet weather conditions. It is designed to mount on the handlebars. That way the rider can focus in on it. Once you have performed the baseline jetting, send the rider out on the bike with the EGT. The rider observes the EGT to give you feedback on the necessary jetting changes. Once the jetting is dialed, we use the tachometer to check the peak rpm of the engine on the longest straight of the racetrack. For example, if the peak rpm exceeds the point of the engine's power-peak rpm, then change the rear sprocket to a higher final-drive ratio (rear sprocket with fewer teeth) until the rpm drops into the target range. An EGT gauge is ideal for dirt track bikes and go-karts, where peak rpm temperature is critical.

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