Middle Primitive Elvish
yul
root. smoulder
sok
root. drink
sug
root. drink
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
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”.