Crankcase Breather - VENT IT!
What do I do about the vent in my rear cylinder (The Crankcase Breather)?
In your Honda Shadow motor, pistons push air and gasses on both ends of the cylinder. Moving anything takes some amount of energy. Pumping pressure out of an engine through a small breather also takes energy, because the breather hole is so restrictive, but it has to be so. When crankcase pressure builds because the air and gasses inside can't easily vent, it will escape wherever it can. This is the cause of many small oil leaks on older engines. I have had people call me who had plugged their breather and then experienced head gasket leaks, oil spouting out of places it shouldn't be, and poor performance almost immediately.
To fix this, you have to exhaust the area inside the crankcase; the area below the pistons, thereby removing the combustion by-products, which can blow by the piston rings. After only a couple of power strokes, the case pressure rises. The perfect breather would allow crankcase pressure and blow-by gas to expel while simultaneously providing low pressure inside the crankcase. Honda has designed their crankcase breather to go to a small tank (the triangular opaque tank) to catch the fuel and such that get past the rings in blow by. You don't really need this tank, all you need to do is run a foot or so of hose to the ground from your crankcase breather or you can kick it old school and run it so that it drips over your chain. No, this is not a really super great method of chain lube, but better the oily blow-by gets on your chain than elsewhere. You may have seen some people put a mini filter on the crankcase breather port, but it will eventually end up getting clogged with the blow-by and not allow the pressure to vent properly, thus causing the aforementioned problems.