The most important thing is to read your spark plug. As long as the insulator surrounding the electrode is completely golden brown then you are doing things right. The plug will tell you how the engine is running at different RPMS. Closest to the insulator is at idle and the further you look down the insulator, the higher the RPM. When you raise or lower the needle you change the fuel mixture and if the plug is black then you are feeding too much fuel in. If the plug is white or grey then you are too lean and on the verg of melting engine parts. Any modification is OK as long as you have the proper air fuel mixture and have the proper back pressure. It only takes a few seconds at full throttle to melt your engine so do research before doing any modification.