reed spacer?

Yes they work
By placing the reed spacer between the reed cage and the engine you increase the crank case volume and by moving the reeds back further you move the power band lower. By placing the reed spacer between the reed cage and the manifold you move the carburetor back and this moves the power band higher up.

The overall HP stays the same. A spacer does nothing to add HP. It only takes the HP you have and moves it higher or lower depending on what you need or want.

someone MX racing will likely want more low end so set it between the reeds and the engine.
someone riding dunes will likely want more top end so place it between the reeds and the carb manifold

Try it both ways just make sure you use new gaskets each time
 
this thread has been covered hundreds of times. i have found with stock reeds the reed spacer will give you more power. with aftermarket reeds, it does nothing. i used mine with my vforce reeds and i can honestly tell you i have more power without it. i still have it if you want it. i will give it to you