A root in The Etymologies of the 1930s glossed “smoulder” with derivatives like ᴹQ. yúla/N. iûl “ember(s)” and ᴹQ. yulma/N. iolf “brand” (Ety/YUL). In all likelihood it was supplanted by the later root √YUL “drink”.
Neo-Eldarin: Despite it’s probable replacement by √YUL “drink”, I’d still recommend using some derivatives of ᴹ√YUL “smoulder” for purposes of Neo-Eldarin. Noldorin forms like iolf and iûl are not viable as derivatives in Sindarin, since initial yu- became the (vowel) y, for example: ýneg < ✶yūneke (VT47/41). But the Noldorin forms could be adapted into (Neo) Sindarin if they were derived from a Neo-Root ᴺ√YOL. Quenya forms could likewise be transferred to this root, or retained unmodified if we assume an occasional √YOL/YUL variation, similar to vowel variations like √SOK/SUK (PE18/45, 94). Assuming the existence of such a ᴺ√YOL/YUL Neo-Root would allow many of these 1930s forms to be salvaged without directly conflicting with √YUL “drink”.
In The Etymologies of the 1930s, the verb ᴹQ. yul- “smoulder” was mentioned in passing in a deleted entry for the root ᴹ√GUL (Ety/GUL). Since the root ᴹ√YUL “smoulder” remained undeleted in The Etymologies, perhaps this verb also remained valid in the 1930s (Ety/YUL). However, the later meaning of the verb Q. yul- was “drink” from the root √YUL of the same meaning (PE17/63).
Neo-Quenya: For purposes of Neo-Eldarin, I would adapt the root ᴹ√YUL “smoulder” as the Neo-Root ᴺ√YOL, and hence would adapt this 1930s verb as ᴺQ. yol- “smoulder” to sidestep the conflict with Q. yul- “drink”; see the entry on ᴺ√YOL/YUL for further discussion.