These symptoms are typically caused by one or more blocked jets in the carburettor.
Small particles of grit can block any one of the jets in carburettors. The fuel still gets to the engine through the other jets but the efficiency is much degraded. The number and size of the jets varies by model.
I have no experience with Yamaha motorbike carbs but I have seen this problem on various old cars.
You have two approaches:
1) On some older carburettors there will be a connector where you can connect a tyre pump and give a blast of air to clear the grit.
2) The jets easily unscrew, so you can blow through them individually. You will have to take the top off the float chamber to access some of them, this is just a matter of half a dozen screws. The float is connected to the top and will lift out as you remove the top.
While you have the float chamber exposed, it is worth checking if there is grit sitting at the bottom. If so, wipe it out with a cloth.
The symptoms you describe - stalling at idle - suggest that the Idle Jet is the one that is blocked. This is visibly smaller than the main jet and is therefore more prone to blocking in this manner.