If you find yourself in a crappy situation when the bowl overflows just throw down some towels, grab your tools and get to work.
You might want to turn on a fan, too. It's going to smell.
A properly maintained 2-stroke includes a correctly adjusted float level and caring for the carburetor. A broken float, a clogged needle and seat, or a clogged tank vent all contribute to the float bowl overflowing with fuel which leaks gas and prevents your bike from running well.
Float Bowl: The bowl shaped section of the carburetor that holds fuel reserves.
Parts wear out, of course, but if the float level is not adjusted to manufacturers specifications you run the risk of the float running too high or too low which results in the float sticking if too low which closes off access to the reservoir of fuel thereby overflowing. If it floats too high, too much gas remains in the carburetor which can leak or flood the engine. If you've ever leaned your bike too far on one side that's essentially creating a high float situation which is why it's hard to start the bike immediately after.
The float is much like a bobber that rises as fuel fills the carburetor which acts as a reservoir for fuel. Once the float reaches the correct height it closes a valve consisting of the needle and seat. The needle and seat controls the amount of fuel from the carb into the engine. A correct float height determines the optimum amount of fuel allowed into the engine.
Carburetor Float: A bobber of sorts in the carburetor that determines the amount of fuel into the engine.
As noted, any number of reasons impact the amount of fuel in the carb but the most common is improperly adjusting the float height. Think of it like the filler float in the reservoir tank of a toilet. When water fills the tank after flushing, the filler valve rises with the water and at a determined height shuts off the filler valve and allows the flush valve to close. If anyone of these components breaks or the filler float is not adjusted accordingly you get a leak or not enough water for the next go-around.
Adjusting the float level on your dirt bike takes a bit more time than adjusting the filler float on a toilet but just like the porcelain throne you want productive seat time on your dirt bike.
Adjusting the Float Level
First take the carb apart so you can access the float. Remember to drain it first. Remove the float bowl and use a set of Vernier calipers or what's even easier is a more precise digital caliper to make the measure based on the specs outlined in the service manual.
The floats inside the carb vary in shape and appearance based on manufacturer but generally made from plastic and it's what you measure to determine float height. Measure the floats by tilting the top of the carb about 10 or 2 o'clock so the floats fall back enough to close the float valve without squeezing the float valve spring. Measure the float from the base of the exposed lip (or carb body) of the carb to the top of the float without compressing the floats further. Found this great diagram below:
You'd actually angle the carb rather than straight up and down for accurate float measurement - Photo by Andrew Hufford
A low float height makes the bike run rich and a high height makes it run lean. If the float does not measure to spec, adjust it by bending the tang up to allow a higher float or down to lower the height.
Tang: Metal adjustable tab that determines the height of the floats.
Once you've adjusted the float put the carb back together. Need help with jetting? Check out A Simple Guide To Jetting Your Carb.