I have a problem with my rt125 same motor
it gets very upset if i open the throttle hand feed any at all when starting...
make sure throttle cable freeplay does not have the slide in the carb lifted any.. then adjust the fuel air to 1 1/2 to 2 turns out
and them the idle to about 1800 rpm / udm
i also use a gell cell for a battery ... the bigest one i can stuff in the
battery box also on cold days i use a jump start battery to run the
starter and the bike battery only runs the cdi box
mine started last winter at -20C yes we ride them when its that cold but
only about 10 km and full winter faring and leg sheilds
ar dave
ps i tried a different air filter but it just seemed to have less power than stock filter