With the YS engine there are two places to go when the engine starts to run different then it was last week. Every YS pattern pilot has extra diaphragms in there flight box. The glow plug and diaphragms are about the only two parts that will give you a problem. I use the old 120 and the newer 110s and they have different diaphragms. I get mine from Central Hobbies and order the gasket and diaphragm for the 120s and the 110s just have a diaphragm. At the end of the day I run the engine dry before I leave the field. If I know the engine will be stored and not used for any length of time I open the regulator and clean out any fuel and oil then put the cover with the diaphragm back on but loose so there is no pressure on the diaphragm.
There are several different models of the YS engine too and the older ones are hard to ID. When they went to the FZ it became a lot easier but still, the best place to get info/help with the YS engine is in the YS factory support forum.
These are my all time favorite glow engines,due to the availability of the parts and the trust and help YS gives to there users I pretty much gave up using the OS four strokes, my number two favorite.
Forgot, if you have any crud in the regulator it will act just like a bad diaphragm. Flushing out the passages is also easy if needed.