While your results may vary depending on various factors (see below), we set our carb pop-off at 8.5 to 9.5 pounds (it isn’t too critical) and the metering lever height at 0.045 below the gasket. We use 8 ounces of Yamalube R to a gallon 100 octane or higher fuel. We set the low-speed needle at 1.125 turns out and the high speed to 0.75 to 0.50 out to start with. Piston-to-wall clearance is about 0.003 total. We assume you’ve judicially broken in the motor.
To read the color of the exhaust header:
Take some hot laps in your kart, certainly getting everything up to temp. Do your normal routine or whatever. Then cut the engine cleanly and pull quickly into the pits. Pull off the exhaust pipe and flex. Look into the header and note the color.
Black or dark brown … you’re too rich on the carburetor. Medium brown to light tan … about right but probably still a bit too rich. Light tan (about the color of a pair of khaki pants) … just right. Light tan to white … too lean.
If the color is dark, lean the high speed circuit of the carburetor by turning the high speed needle in an eighth of a turn. Then go out on the track again and repeat the procedure, each time checking the header for the proper color. Keep repeating until you get that light tan color. (If the color is white or very light tan, of course richen the high-speed and repeat until you get that right color).
When you have the right color at the end of the header, start leaning the carb in very small amounts. You’ll notice as you do this that the light khaki color will begin to get longer, moving from the end of the header back towards the piston. Once the khaki color covers about 2 inches of the header, you should be at the most efficient settings for that particular day/conditions.
We’ve found that this is a particularly effective method of tuning FOR NOVICES. At the same time as you’re doing this, also note maximum CHT (Cylinder Head Temperature) and EGT (Exhaust Gas Temperature) readings. As you approach the right color on the exhaust header, you’ll also start noting increases in both CHT and EGT, both approaching the most efficient readings for those current conditions. Be careful that you don’t in the future apply those same CHT/EGT readings under different air density or track conditions because as conditions change, the readings will change. EGT is by far the most effective gauge of what’s happening inside the combustion chamber.
Some factors that will affect the ultimate color:
– ambient weather conditions such as air density, temperature, etc.
– the type of fuel you use (AvGas for example will almost always give white color; avgas is not really recommended for a kart engine anyhow; you only need about 100 octane under most circumstances for a 11cc combustion chamber Yamaha).
– how much oil and what kind of oil you’re using. We recommend [and use] 8 ounces of Yamalube R to a gallon of 100 octane or better gasoline. Some folks think there is performance to be gained from running less oil. Perhaps. (Most tests I’m aware of show no appreciable performance advantage to using less oil; in fact, just the opposite.) The danger in breaking a motor is increased exponentially as the amount of lubrication is decreased and the amount of performance gain is negligible if at all.