HHO, or two parts hydrogen and one part oxygen, is the gas created during electrolysis; a simple electrolysis device can be installed to produce this gas as a supplement to gasoline. Older vehicles can make the best candidates for an HHO conversion due to their lack of sensors and computerized equipment. The average backyard mechanic can tune the carburetor of an HHO-equipped vehicle in about an hour, but each car is different and times could vary.
M/F Inspect, measure and analyze the HHO system that you have installed, and how much gas is produced, by connecting the HHO generator output to a pressure gauge and determining the liters per minute. How the HHO system is connected to the carburetor, and how much HHO is produced, will determine how to best tune the equipment. One or two small HHO generators on a large motor, with the gas connected to the air intake manifold, will need slightly different tuning than a system in which the HHO goes directly into the service port vacuum of a small engine. Shorter HHO fuel lines will mean quicker warm-ups. A bubbler and flash arrestor can be added in-line to prevent carburetor backfire from igniting the HHO gas, and are highly recommended. There is no need for an MAF, or mass-airflow, sensor or O2 sensor adjustment equipment if the carbureted engine lacks emissions equipment (pre-1975).
M/F Adjust the carburetor's idle mixture screws, if applicable. These are the two screws normally found on the front of non-computerized carburetors, which can be turned clockwise for less fuel, and counterclockwise for more fuel. This also has an effect on the timing, but with minor adjustments (less than a full turn) the timing of the motor will be largely unaffected. Turn the screws clockwise slightly, until the motor begins to stumble, then back it out about a 1/4 of a turn. This will set the carb to a lean condition during idle.
M/F Change the metering rods inside the carburetor to set the loaded, or "highway" speed mixture to a lean setting by turning the small screws holding the rods in place on each side of the main barrels. They are usually small covers, and once the screw is loose they pop out slightly due to their tiny springs. They could appear black or worn and are easily replaced. Replacing the metering rods with longer risers (known as "fatter" rods at a parts store) causes the carburetor to not inject as much gasoline during sustained throttle positions, such as highway travel. This gasoline will be supplemented by the HHO, which is taken in at a lower rate during cruising speeds.
M/F Do not use HHO gas without a flash arrestor on a naturally aspirated motor.
M/F Use extreme caution when working with flammable fuels.