Motorcycle Repair: high idle on Suzuki GS550E, fuel level sensor, vent hoses

Question
I just finished rebuilding and restoring a 1983 Suzuki GS550E.  I had the carbs off, cleaned and rebuilt; put a new air filter, spark plugs and battery on it.  It ran great for the first several days.  Recently, when it gets to operating temp the idle speed increases from the correct 1100 rpms to between 3500 and 4500 rpms.  When I shift into gear it evens out and runs ok, but when shifted back into nuetral, it returns to that high rpm.  Any ideas on what might cause this?  Thanks

Answer
Jed,

generally a change that leans out the fuel mixture or changes timing
can cause this. I imagine you have checked
the throttle cables already?
-check the vacuum hoses around the carbs and make sure
carb vent hoses are not pinched or routed incorrectly.
-Check fuel tap for good fuel flow. They have filter
screens inside the gas tank.

-check for any air leaks around the intake clamps
and rubber, use wd40 or carb cleaner, see if engine
speed changes. See if carb slides are free or sticking.

-have been some reports of tank rust or dust plugging
small carb jets. Sometimes it is the fuel level sensor
in the gas tank rusting.
The particles are very small, most filters are not
able to help much.
-if the idle jets plug, the fuel gets lean and the
bike can speed up or run erratically.
-does it speed up even if you slow the idle screw down?
check if the tank or gas cap has an air vent, make sure it's
clear.
-check battery cable connections.
-check your plug colours, see if they are  black/rich, tan/good,
white/lean fuel.
-Check for any exhaust system leaks.