I have a couple of the YS engines on my bench getting ready for some work. I just went out and used a .91 to show you the regulator and diaphragm. This one needs a new diaphragm!!I bought the engine as a parts engine but when I got it home I discovered all it needed was new bearings. Someone started to do the work but quit?? I don't have the hart to just use it for parts so 20 bucks for the engine and the cost of a set of bearings. I have the Bowman rings in stock.
First photo is the regulator, note the little block I have the pointer on, this goes to the front. The pointer in number two is on the diaphragm. This model engine has the low in air adjuster so the usual adjustment for the brass screw is flush with the regulator cover. Most all of the how to articles will tell you in detail what this screw does. Most new people to YS just have to twist it as part of there tuning like they are tuning a two stroke or something then wonder why they can't get there engine to run correctly?