So the carb/saw is running on all 3 transition ports in the carb, the L/M/H so as to burn all the fuel W/O flooding AND when the H port on the engine side of the carb loses little air the engine speeds up and is getting a better fuel air mix as the throttle plate starts closing.
Did you put the new PROPER SIZE check valve in your carb? (that you ordered)
I seem to remember that I used my mity/vac pressure/vac tester and I could test/confirm the forward and backward checking of the new check valve using a plastic needle tip adapter on a piece of tygon tubing gently held on the check valve and also confirmed some old carbs as checking ok. (but that does not totally confirm proper operation when checking and the engine running) A itty bitty piece of a knats bristle will hold the little valve open during operation..
If you have replaced the check valve and it's still odd
I would
TRY ANOTHER CARB.
Did you put the new PROPER SIZE check valve in your carb? (that you ordered)
I seem to remember that I used my mity/vac pressure/vac tester and I could test/confirm the forward and backward checking of the new check valve using a plastic needle tip adapter on a piece of tygon tubing gently held on the check valve and also confirmed some old carbs as checking ok. (but that does not totally confirm proper operation when checking and the engine running) A itty bitty piece of a knats bristle will hold the little valve open during operation..
If you have replaced the check valve and it's still odd
I would
TRY ANOTHER CARB.