Below for you Mikuni Carb Owners is the Sub-EGR valve. This valve refeeds exhaust back into the carb for reburning any possible left over fuels after it's first trip into motor. These valves tend to get very carboned up with black soot to the point it will cause a gas peddle to stick. The valve will get stuck in the soot and lodge itself in it's mounting and cause the gas peddle to have a strange sticking feeling. These valves are very easy to clean and should be checked occasionally.
Simply remove the valve and lightly clean all the carbon from the valve shaft. If you have a bottle brush to fit the opening the valve cylinder fits in, great. Push it in and clean the opening as well before returning the cylinder.
This entire line can build up with carbon like a clogged blood vessel. It goes all the way through the head and can completely seal up after years of use. I clean mine with a coat hanger to remove the soot buildup thru the head. The carb and intake line can do the same exact thing too. Clean as much as you can out of the trail and reinstall you're valve. Service the throttle arm and make sure it moves the sub valve normally when completed.

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