One of the better improvements incorporated into the 2-stroke engine occurred during the 1980s when power valves were introduced allowing a broader power band instead of all the power on the low end or all the power on the top end.
Power valves, also known as exhaust valves, open and close the exhaust port allowing the engine to produce steady power based on the roll of the throttle. So, whether a rider opens wide at the start or on a straight-away power valves ensure top performance at either end without taking you for an unnecessary thrill ride. Some of those older models offered two options: no power or maximum power and nothing in between.
Several scenarios result in clogged or sticky valves giving you a slight idea of what it was like to race in the 70s with a diminished power band but mostly just frustrate the heck out of you. A sticky valve takes longer to open or close making the bike hard to ride. Running too rich for long periods, using crappy gas or low quality premix prevents a clean burn and eventually carbon residue and oil muck up the valves thereby narrowing the exhaust port and/or inhibiting the valve opening action.
Clogged power valves love to eat spark plugs and you won't get much top end. If you're burning through plugs or having trouble opening wide once up to speed it's a good idea to check the valves. It's an easy fix at least in terms of cleaning the actual valves. Unfortunately, you'll also perform a top end rebuild so you'll need a kit.
Reaching the valves on most bike models depends upon removing the cylinder which means breaking the gasket seal. Don't reuse the gasket! Besides, dirty power valves usually mean a top end rebuild is in your near to immediate future anyway. (Note: Some power valve designs allow for cleaning without removing the cylinder but most 2-strokes require it.)
The cylinder houses the power valves which look a bit like airplane flaps (see diagram). You'll need to remove the bolts to extract the valves. But check your service manual before removal. Some valves only install a certain way so jot down notes and/or take photos as you remove and disassemble.
2005 Yamaha YZ125 Cylinder Head Diagram- #14 and #15 point to the power valves
Once removed, scrape off any carbon build-up using steel wool or a stiff toothbrush then apply contact cleaner or other similar solvent (brake cleaner, gasoline) to finish the job. You can leave them soaking in the cleaner while you prepare for the top end job. Reinstall the valves to spec and continue with the regular top end job.
You can adjust most power valves for lower or high-end speed but not all manufacturers use the same method. KTM uses a tensioner and spring contraption while Honda uses a cable operated system. Adjusting the valve enables you to increase power on the bottom or top end depending on your preference.
Eventually, all power valves gum up when not serviced regardless of the quality of gas and oil. However, regular tune-ups and using the correct premix goes a long way towards keeping the exhaust port open, the valves working properly and you riding.