I have over 20 of these UAT's from Pilotrc - all 250ml - but I do make a small mod. I replace the brass tube with a 4MMID felt clunk from Tillotson:

https://www.lawnmowerpartstore.com/f...-ow802-ow-802/

I bought these a few years back for my prop planes and jets (it is the same one that comes stock with the CARF mephisto). I drill the hole to 4MM (some of the older clunks were 2.5MM but the housing has room for 4MM ID).

I had 2 flameout when using it on my TOPRC Voyager on a very high aerobatics flight - with the K260. Autorestart fixed the flameout - landed and saw that I had maybe 40ML or so of air in UAT - (which is not much.)
On the next flight - I added the felt clunk and ever since no issues. (still have as high as 60-70ml of air - but no flameouts)