That pilot sounds right for where you live. Its sounding right for what you say is happening at slow speed & low rpm as well.
When you take a modded race engine & try to slow ride it, the jetting will normally be to rich when going slow & will puddle the excess fuel in the crankcase. Once your in the clear & ready to hammer on it, that excess fuel will then be whooped up & sent on into the ports, so the engine will be in kind of a flooded state until the excess is used up.
The only problem with what is happening, if you jet so that it wont happen, once your in the clear & hammering on it, your probably going to seize the piston from being to lean.
Its one of those thing like, say you sold the quad to a guy never had one like yours before, so his riding levels not the same as yours & then during riding he realizes it running rich, so he jets the carb down to his state of riding level. Then one day you come over & ride it hammering on it normal just like when it was yours, next thing you know the back tires are sliding & engine locked up, all because it was jetted for his riding level.
A 250R engine, only if it is a stock build, can be rode slow like that. There may be times on a modded engine, some slow riding can be done, but the throttle needs to be burped quite a bit to keep the in coming fuel cleared out, kinda like you see race cars on a pace lap after a wreck gassing it.
You might could change the slide needle, get one a bit larger to make it leaner at that throttle opening, say maybe a CEL, but you dont want one to make it run leaner on top.
Have you tried a different clip setting on the needle.
Neil