Post by CJ
Does anyone know what this sensor does?
That model isn't sold in the USA. You might try googling for online
mail order catalogs in the UK that have parts fiches which customers
can look at and perhaps figure out what the sensor does by its
description. Your engine seems to have a Teikei YP24 carburetor,
according to the only English website that I found with specifications
for the TW125.
Maybe your sensor has to do with shutting off fuel to the idle
circuit. Your high RPM at idle indicates that the throttle butterfly
is too far open for idling at the specified speed. The transition
ports are uncovered by the edge of the throttle butterfly, so the
engine is getting excess fuel at idle and it takes several seconds to
An engine is supposed to idle smoothly with the smallest amount of
throttle opening and with the idle mixture screws turned out the
fewest number of turns possible.
I have read about carbs with throttle position sensors that will shut
off the fuel supply if the position of the throttle twist grip
disagrees with the position of the throttle butterfly or slide.
This is supposed to protect newbies from having a motorcycle run away
with them clinging on for dear life.
Other electrical devices on carburetors operate a little valve in the
side of the carburetor that shuts off an air passage. This is
necessary when coasting on closed throttle to keep the engine from
But that is usually done with a vacuum sensing rubber diaphragm on the
left side of the carb.