The upward motion of the piston in the combustion chamber draws air through the throat of the carb which essentially creates a vacuum. A small, yet precisely sized hole leads from the throat to the bowl of the carb. The bowl is essentially a reservoir of gasoline at the bottom of the carburetor.
This miniscule hole is located inside a jet, a brass part which can be removed for cleaning or resizing. The opening in the jet happens to be the means of evening out the pressure inside the carb. It does this by allowing a fine amount of gas to be drawn upward from the carburetor bowl. Basically it acts as a brass straw.
The fuel which is sucked into the throat of the carburetor is mixed with incoming air from the airbox side of the carburetor, and cast into the combustion chamber where it will be ignited by the spark plug at just the right time.
It sounds quite simple but it is really a marvel of engineering. All it takes is a mal-adjusted screw, a little bit of dirt, or a leaky seal and this little machine called a carburetor will not do as it was designed. I have listed a few of the basic carburetor troubleshooting tips but by no means is this an all inclusive listing. It is meant to be a guide to help with the most common 250R carburetor problems.
Let's look at some of the more common issues. Flooding, or too much fuel when not needed. The float valve may be set too high or stuck in a lower position thanks to dirt or varnish from old fuel. Fuel starvation is the exact opposite situation.
This will cause lack of power and a dimished running capacity for yiour engine. Again can be as simple as a stuck float in the bowl of the carb or possible a clogged jet. A faulty or leaky bowl gasket can also cuse this to occur. Idling problems are usually caused by a clogged pilot jet.
The pilot jet leads into the bowl of the carburetor. All jets in the carb can be cleaned with carburetor cleaner and compressed air. There are also commercially available jet cleaners that can be fed through the jets for stubborn debris. Lack of power can also be a symptom of an air leak.
It is important that there is a tight seal between the carb and the intake side of the combustion chamber. High idling is another common occurrence. This can be caused by the slide being stuck or improper setting of the slide in the first place. Fuel air mixture is set using the air screw.
Past experience has led me to turn the screw all the way in until it bottoms out, then back off approximately 3/4 to 1 full turn. This setting is not set in stone, but a guideline, you may need to adjust.
If you attempt to disassemble your carb in any way. Be sure to have a clean work area before you start. Cleanliness is key and organization is vital. Old fuel is bad fuel since it breaks down over time. And carburetor cleaner is cheap, just make sure you use it in a well ventilated area.
I hope this 250R carb guide has been of some help and shed some light on the aluminum mystery can.