Beware the governor on some Walbro carbs. I am posting this to hopefully save someone the grief I had trying to tune a Ryobi 31cc.
There are a few vague references to this, but none very detailed that I found. It seems Ryobi and maybe others need to limit the wide open rpm on their engine, due to most likely aweak con rod. Walbro solved this for them by inventing a little check ball spring combo that opens and enrichens (a lot) the high end mixture, causing the engine to 4 cycle and bog down to limit the rpm. It works by a combination of hich venturi vacuum and the fact the little ball resonates at the desired rpm acusing it to open. Several models of Walbro carbs have these, but are usually screwed in from the outside and more obvious. Mine, a Walbro WT-152 has it as an integral part of the main jet and I thought it was the high speed check valve which is used to prevent over rich on decel, boy was I wrong! I practally wore the screws out taking this thing apart ant putting back together. Tried different pop off pressure, fulcrum height, blah blah to no avail, it always went rich and 4 cycled at anything over about 6500 rpm.

You can plug it which I did, or if it is on a string trimmer just be aware you will NOT be able to tune for 2cycle operation at WOT and low/no load. This may be ok to save grenading it from someone running it wide open with no load.

Here is a picture of the governor, with the little ball visible in the middle.

And I thought I knew something about carbs and engines