Those carbs are finnicky and very sensitive to the adjustment screw. Play with the adjustment screw, try 1/4 turn out, then retract in 1/4 turn increments until it works. Make sure that you start with new sparkplugs.
To engage sled with maltuned carb, try giving it a little choke while trying to engage, add throttle SLOWLY (don't just punch it and hold it) until the engine speed has increased *significantly*, then start "blipping" the throttle to speed it up more.
Make sure that the engine is warmed up fully.