A fingers-crossed update on the old Mantis. (Perhaps a bit early, but I'm so relieved right now!)
Started and ran pretty good on inspection & started just as well next day, BUT them bogged-down under load so tweaked high needle. All seemed OK until I turned it off and tried to restart later. Oh dear! Two very frustrating days of inconsistant running and tweaking/re-tweaking followed by every component test known to man (except a compression test because it seems to have good compression and, besides which, I don't have the means to test that). Eventually, after a final session with my father's assistance, it seems to want to run continuously and under load AND also restart when hot.
Eventual state of (interim) happiness was arrived at by tweaking low needle (with the odd tweak of Idle screw as well, but no tweak to High needle beyond the basic 1.25 turns) with the engine actually throttled-up slightly (via the throttle lever) over many minutes. We eventually let off the throttle &, lo and behold, the engine was in happy mode! Then, without that tad of throttle, a few v gentle, gradual tweaks to Low & Idle screws had her idling rather smoothly and also performing under load and restarting when hot. Sweet-spot revs for idling seem to be bit higher than specified 2,000 revs and there is the slightest rotation of the implement spindles presently, but enough is enough (for now).