Transfer Case Seals.
Explanation? (Dr. Science Mode On) Okay, a two cycle engine first admits the fuel and air to the crankcase, air is pressed (or sucked) into the crankcase when the piston rises., when the piston falls back down (being pulled by the con-rod) the fuel/air mixture is compressed in the crank case, as the piston falls eventually a port into the crankcase is opened allowing the compressed fuel/air mixture to be pressed into the cylinder. Before the piston gets to the transfer port it has passed the exhaust port and the hot exhaust is expelled from it's own pressure. While the exhaust port is still open the fuel and air mixture helps expel the exhaust gasses. Quickly the piston begins to rise closing off the transfer port and exhaust port. At a few degrees before top dead center the spark plug fires causing the mixture to ignite and a flame front causes rapid expansion of hot gas that forces the piston down. First the piston passes the exhaust port, then about 30 degrees of the crank rotation the transfer port is opened. The cycle is complete. The piston falls, rises, kabang, falls, rises kabang. Two stroke. If done right you get a bunch of power out of the displacement of the engine, you also get a bunch of heat too.
Other things, the fuel and air mixture must remain in the crankcase (or transfer case), so the path back to the carburetor must be blocked. Reed Valves (think Yamaha) a rotary disk valve (think Kawasaki) or piston port (Think McCullough) is used to keep the mixture in there. (Mr. Science Off)
See it here..
Steve's Gas Engine Shed - 2 Cycle Theory
(Mr. Mechanic mode on) if you have any leaks in the transfer case the engine will become hard to start and will exhibit odd and sometimes unpredictable low speed operation. There are seals on the crank shaft witch deteriorate over time, sometimes with the gas we get to day a very short time, if the engine has reed valves they can get damaged too. The other two valving methods are much more robust.
So, that's what I know...
- Bee