About 2 weeks after I had fixed the head cover rubber plug leak another one showed up. Small drops of oil would be on the starter and the carb had oil on it. My initial thoughts were perhaps one of the bolt gaskets were leaking - what made this leak different than the rubber plug was no oil was on the back of head at the air passages these were bone dry. Last thursday I had enough of this and start pullling things apart. Pulled the seat, drained and pulled the fuel tank, the chrome head covers, and looked over the situation. The head cover and all the bolts were dry, the vent hose and throttle cable were covered in oil - so what the L is going on here? About this time I looked over at the fuel tank I had set on the floor and a 2 inch puddle of oil has ran out of the end of the speedometer cable onto the floor. 2 cups of coffee later things started to make sense - I could understand how a little oil could ride up the inside of the speedometer cable as it turns but not that much- so it must be getting pushed up the cable - how? - the only way I could think of would be crankcase pressure. As I looked over the route of the air vent hose - sure enough I had routed over the throttle cable that cause a slight flatting of the hose, and I remebered stuffing the stainless steel screen into the head cover fairly hard so it wouldn't fall out when I was putting the cover back on - so I pulled the hose off the tube and I could see the screen and pushed this down. Wiped everthing off, put it all back together, pull the hose off the air box to see how much air the engine pumps out the vent hose - fired it up and dang the savage it quite an air pump! It could be I have a bit more blow by @ 16.5k miles but given the size of the single piston and no bigger than the crankcase is I'd bet a new engine pumps quite a lot of volume as well - It doesn't smell like piston blowby nor is it smoke so I'd guess it's normal. Anyway it's gone 200 miles since then without a oil drop anywhere.      

You can make couple of conclusions. The first is that You did an awesome job sealing the head and the plug. The second is , as Greg mentioned , there is a lot of pressure and it has to go somewhere , and we have to be careful when routing the hoses. And the third very important one is that when the engine "blows thru that hose, the next thing it does is , it "sucks" back with the same power /as Greg mentioned/. Just imagine what happenes when some "mechanics" pull the hose off the air box and let it hang down so it doesnt soil the air filter. It is like a vacuum cleaner hose that suck road dust in the engine. The oil becomes like a liquid lapping compound.