The carbs in the photo below are late model stock carburators from an XS Yamaha. This same carburator flooding problem can be found in carbs from: Kawasaki, Suzuki, Yamaha, Honda and other motorcycles that use imported carbs.
Sometimes what appears as a flooding problem is actualy caused by vacuum fuel petcock diaphrams that are mounted on the tank itself, or mounted on the cycle frame. They get old and brittle and get a hole in the thin rubber vacuum diaghram. Intake manifold vacuum will then draw fuel directly into the intake ports and flood the motor, and possibly cause hydraulic lock, (non compressible fluid above the piston that stops the motor solidly from turning over). Too high of an oil level is a telltale sign of carb flooding too, and should be drained with the oil and filter replaced right away before engine damage occurs, if it has not happened already. Most common carb flooding problems are caused by float needles and seats not seating properly because of wear, and or dirt and trash that has become lodged in the float needle inlet orifice so that it cannot close properly.
Some early model carbs have a brass seat assembly that screws into the carb body, usually with a red or black gasket that is actually a seal washer. If it is loose, it will cause the same problem defined below.
These later carbs have a little half circle retainer clip with a small phillips screw that holds the needle assembly in place in the carb body, (it is removed from the needle assembly on the carb on the left in the image below and the brass needle housing can be removed by lifting/pulling it out. Most stock carbs have this type of seat assembly for about the last 20 years. It is called a "cartridge type" needle and seat assembly.
It has driven ordinary and extraordinary people insane looking for a carb flooding problem! What happens is the "O" ring that seals the brass needle assembly drys out, cracks and or shrinks up. This lets fuel enter the float bowls slowly around the outside of the brass assemblies and continue to flood the carbs even with float needles and seats that appear to be perfect and closing properly. As a last ditch effort "mechanics" replace the entire needle and seat assembly and then install it with new O rings. They inadvertantly cured the mystery carb flooding problem and still don't know thats what is was all along. Generally you paid for the needles and seats and you think that cured it too. It is called "S" tax. (stupidity tax)
Other things to inspect as carb flooding problems are: brass floats that are filled with fuel because of a hole, these can be drained and soldered. Some have a type of black or brown outer resin coating over the foam core inside of the floats, when these become saturated with fuel they will not shut the float needle. Both types can be checked simply by placing them in a container of fuel and see if they float or sink. Also check for choke plungers not seating properly, float levels set too high, and float needle valve dampening springs that have become stuck compressed or totally collapsed.
Paper element fuel filters generally have a smaller micron ratimg and are prefferable to catch fine rust and debris, over the sintered bronze 99 cent type that also restrict fuel delivery.
Always use caution when working around gasoline or other flamable hydrocarbon fuel, per pound, it has more BTU's of energy than dynamite.
Occasionally, a motor will have a mystery bog and run lean because of trash that has accumulated above the brass float needle, most have a fine metal mesh screen that snaps on the tip of the float needle seat that is hidden from view in the completely assembled carbs. (see image below)
XS 650 Yamaha Cycle Links