One bike I have, had a similar problem since late last summer up until today in fact. The bike has a 2011 batch Grubee Skyhawk 66 HT engine with a factory CNS YD EPA Carb with the three plastic air tube vents. The bike would start and idle fine. Turn the throttle a little bit and it was like the spark plug stopped firing. Down hill the engine would run at higher rpms. The engine would rev up at a stop like there was a manifold vacuum leak. I glued the carb on which did not help. Swapping out the complete ignition system did not help. Taking apart and checking the carb twice did not help. Taking a used bike inner tube, I cut off a section having the fill nozzle, glued and clamped one end shut and clamped the other end to the carb intake so a test could be done to detect manifold vacuum leaks. Using a compressor set to provide about 3 psi connected to this rubber device where the air cleaner was attached, a mist of soapy water was sprayed at the carb. The seal between the carb and the offset intake manifold is weak, but if the carb is shoved on as far at it will go, it will seal. The carb gas line brass fitting leaks around the edges but not bad. The drain plug and the idle adjustment screw leak, but this could be sealed with nylon tape. As part of all of this the problem was revealed and appears to be cured as demonstrated on a cold spring ride today. The rubber boots where the throttle cable and a choke cable; fuel enrichment on this carb, must have the rubber boots installed and in good condition. These rubber boots prevent a significant manifold vacuum leak which greatly impacts the performance of the engine. The manual with the engine does not point out the importance of these rubber boots. Checking the status of these seals should be part of engine troubleshooting guides in my opinion. By using the 3 psi pressure test as part of a engine check, including checking the throttle cable and choke cable boots, manifold vacuum leaks could be eliminated and verified before the rider hits the road. This low air pressure did not seem to simply pass through the two cycle engine affecting the test. Other carbs will present a different situation; but these rubber boots may also be a critical seal on other builds. Good luck with your carb problem & happy riding.