Honda GX270 9HP sputtering?

Hi, I have a Honda GX270 9HP on my go kart. It is almost brand new; it just sat in storage before I bought it. It idles just fine but when I hit the throttle, it sputters and pops. It will sputter for a split second, then it will "catch" and start moving for a split second, then repeats the process until I let off the throttle. I have taken apart the carb half a dozen times and cleaned it out, including cleaning the jets as best as I could. It seems to get worse the longer the engine is running. I appreciate any feedback.

Set the choke to half closed and see if it gets better.
If it does, then you probably have a vacuum leak in the gaskets behind the carb.
If that does not help then you might want to check the coil and spark plug.

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