The little accelerator pump is a small brass piston with a tiny O-ring. Both wears quickly because they slide every time you play with the throttle. At first, the O-ring is designed for static or rotational motion. It doesn't work well and doesn't last long in a sliding motion but it's a very cheap component and apparently Stihl wanted to put the fewest money as possible in this carb "upgrade". Bad game for the customers.
Practically, that gives an air leak directly inside the carb. Barely perceptible at first, it becomes worse and worse, till the chainsaw can't work at all.
Some times ago, I found a solution on AS to correct the issue and I made a tuto with plenty of pics. It works well and it's a nice and easy fix, very far under the 100$. But the tuto is useless now because Imageshack sabotaged so many threads, trashing an enormous quantity of pics. I still have the pics on my pc and must to put them again on the net.
But you can get it on Magnus's site now, because he saved the whole post on his own pc. Smart move.
) It was same design as MS200 but with screw caps. When I bought my first MS200 in 2001 it and the one I bought several years later had the major lean off idle problem. 
