Alt-Fuel Speed: CL450 Racer - Installing an Air/Fuel Ratio Gauge

The standard low-cost method for tuning a carburetor is by performing a "plug chop". This is a process where a fresh set of spark plugs is installed in the engine, the vehicle is driven at a specific throttle opening for a few miles, and then the motor is quickly shut down and the plugs are removed and inspected. By checking the relative condition of the spark plugs, such as the location of carbon deposits, the color of the ceramic insulator, and the color of the exposed metal parts, it is possible to determine the relative tune of the engine.

Carburetors are surprisingly complex components, and are equipped with multiple jets to provide even fueling to the engine at all throttle openings and load conditions. Every jet has to be independently tuned, so it is important to maintain a steady engine speed/load in order to perform the plug readings. Acceleration and speed changes will vary the fuel circuits and jets that are used. To determine the state of tune of a specific jet, the engine must be started, maintained at the desired speed, and then quickly shut down (ideally by pulling in the clutch, killing the engine, and coasting to a halt).

The phrase "plug chop" refers to the practice of cutting down the side of the plugs so the entire ceramic insulator can be inspected. If the plug ceramic is black, the engine is running rich. Likewise, if the ceramic is white, then it is running lean. Carburetor jets should be swapped until the plug is a brownish/tan color through the entire load range.

However, this process is tedious, wasteful (you go through ALOT of plugs), and not particularly exact. It is especially time-consuming if you are maintaining a race bike, and need to tune the carbs for a variety of air filter, muffler, temperature, and air pressure variations. Rather than do things the old fashioned way, I decided to purchase an on-board Air/Fuel Ratio sensor and gauge from Innovate Motorsports.

Special Delivery!

Laying out the equipment

Panel-mount gauge

This kit includes a high-resolution wideband O2 sensor which permanently mounts to an exhaust header. Based on the composition of the exhaust gases, the sensor calculates the ratio of fuel to air in the carburetor, and displays it on an numerical LED screen. The ideal stoichiometric ratio for a gasoline engine is 14.7 parts air to 1 part gasoline. Any more than this, and the engine is running lean. Lower, and it is running rich.

The biggest problem with this kit is it is meant for automotive use, so there are no mounting kits available for powersports applications. Normally this would mount to a hole in a dashboard, so the first step was to make a robust aluminum housing to mount to the motorcycle.