You just leaned out your fuel mix. Puch air boxes are pretty restricted, and removing it allows more air to be blended with the gas. A lean mix will often result in increased rpm and power, however it may also result in catastrophic engine failure. The fuel is mixed with the engine’s lubricant (2T oil), and a lean condition delivers less lubricant to the cylinder and piston walls, resulting in increased friction and heat until the engine seizes.
That being said, 23mph sounds a little slow, depending on your hp rating. If you have the 2hp version, you should be able to go 30mph. If you can’t, with the air box on, you should thoroughly disassemble and clean the carburetor. Verify float height is correct and Check what jet size is in it while you have it apart. If yours is the 1.5 hp version, it’s top speed would be 25mph. 23 isn’t far off. Removing the air box would give you a lean condition as I described, and could be dangerous for your engine. There are instructions in the wiki for converting from 1.5 to 2 hp, if that’s the case