I have recently experienced the same set of circumstances w/my Ryobi RY251PH strimmer. (Manufactured 10-22-15)
I must have dropped mine as I had a small piece coming off the carburetor with what looked like a ball behind it.
Order the $9 Amazon Chinese flavor of the day and attached.
What I discovered is that it runs at a totally different "richness/choke" setting which we cannot adjust.
Pull a few times with choke on 100%.
Whether it starts or not, then move choke to somewhere around mid setting.
Start it here.
Mine runs BUT if I totally shut choke off so more air goes thru it does exactly like yours.
I think the "richness" setting is off in the ChiComm specials and we can only adjust mixture with the choke setting.
If this works for you, be sure choke is tight so it doesn't vibrate totally open and then die again.
Please advise.
For proof that mine has lasted 10 years as a residential user a photo of manufacturing date sticker is attached.