RE: Ys 45 2-stroke
How fresh is your check valve? That'd be the 1st place I look. Beyond that, take the pump apart and make sure everything is clean / gaskets tight and clean / pay particular attention to the plunger / little silicone nipple. When all's right those YS's are some of the most consistent running engines you'll see. The throttle response should be flawless and instantenous every time.
Also, there could be a foreign matter in one of the small fuel / air passages in the carb / pump body. A piece of monofilament fishing line works well to clear those up.
Instructions Attached
-François