Try opening the low speed needle just a tad. If you still have issues you may want to mount on a test stand inverted and try running it that way. Word of caution though, gassers do not get enough cooling while on a stand so keep the runs short. In fact even in the airplane I do not like to ground run my engine, I start and stop the engine at the end of our taxi way so at most they are only run 1 or 2 mins on the ground.