By adding or removing the copper gaskets under the engine head you change the amount of compression in the combustion chamber and the performance of your engine. By adding more of these gaskets you can lower the compression, this does several things to your engine. First it retards the timing that means the piston now travels further up the cylinder before the air fuel mixture reaches full burn. This is because compression in needed to explode the vaporized air and fuel at the top of the piston. Compressing the molecules causes them to heat up and ignite, it also makes the fuel "droplets" into a vapor. By retarding the timing the engine should run cooler but will probably produce less power depending on your current amount of compression. Lower compression also stops/prevents "pre ignition knock" or dieseling. This is caused by too much compression causing the fuel to ignite too soon.

By removing gaskets or using a thinner one you can increase engine compression. This advances engine timing, this can be good or bad. Good because it causes the air fuel mixture to ignite a little sooner so that by the time it reaches full burn and is at its maximum energy producing potential the piston is near or at top dead center (tdc) allowing full travel to create power and making the most use out of the kinetic energy in the spinning parts. It can be bad if you have too much compression thus causing the air fuel mixture to burn too soon and cause the engine to ping or knock. More compression will normally cause higher engine running temperatures but produce more power. It also makes a finer mist / vapor out of the fuel giving a better burn and more power.

Before changing compression check engine rpm then after changing compression see which ones give better results. Then try flying the plane and see if you notice a difference. A new Carb needle setting may be required .