I would say since you just changed the intake and carb the problem lies there not the thermo stat. I bet you need to check your accelerator pump linkage and make sure that gas starts squirting the instant the throttle blades open. sounds like its getting air from the blades opening and then following an instant late with fuel. make sure both promary and secondary blades are set equally and keep them that way. only ajusting both equaly if need be. the transistion slots should only just barely show under the carb below the throttle blades. your pump linkage should have a small amount left in it at full throttle so you dont prematurely wear out the plunger or pump. but it should also be set like .020 from actually squirting gas out. lol like the linkage should be just barely touching the pump arm and you can look down the carb when you apply throttle by hand and make sure that gas and air comes on together. also start with the fuel air screws 1-11/2 turns out and adjust from there with a vacuum guage getting the highest vacuum reading you can and the fuell mixture screws equally turned. what power valve do you have compared to vacuum reading? hang in there bud you got this. also are you running a fuel pressure regulator? and your floats are set right?
Ive read double pumpers prefer single planes but ive seen all kinds of combos work and work good so that shouldnt be the problem. Have you tried a spacer? I just swapped from a open spacer to a 4 hole and I can definitely feel the better low end response. Although I have a single plane so maybe an open would give you more plenum area? idk oh you can also buy a set of colored pump cams and that will change the amount of pump shot sent to the squirters. maybe larger squirters would get fuel there faster too. you have plenty of room to grow before needing a bigger 50 cc pump shot too. Even new out of the box tho the carb should be checked thoroughly and set to spec.