Whenever you use a prop that's too small for the engine, it will not put sufficient load on the thing. The engine will give you fits because it'll be overrevving. The carb is sized to provide appropriate mixture for a range of rpm, and the engine will be trying to operate outside that range. The consequences are greater than just a hard to handle engine. It'll be doing to itself all the bad things engines do when they're over revving.

All that said, the recommended prop sizes are very often a lot wider range props than you probably think. Look up the prop sizes the mfg of your engine suggests before you decide your 10x6 is too small. You might be wrong. The range of props for any engine is very often quite wide. A lot of the engine experts at our favorite flying fields often have only one favorite prop. And some of them often suggest that only one prop is "proper" for certain engines.

And that said, the size prop that actually suits the use might actually be a 10x6 or one outside the usually recommended range. The airplane often has more to do with the proper prop than not.