I bought a stock 04 about two months ago.I put in cam,filter,& pipe. I broke in the cam & pulled it the garage to change oil. I started with the tranny. I took out the drain plug & oil blew out all over fender & nerf bars.Me being new to the bike I thought maybe I was supposed to crack the fill plug to relieve pressure. I changed oil & check valve clearance & took it out for a XC practice. I got home cleaned it up
& noticed the vent was off the valve cover! I really don't think I would have forgot that. I put it back on, took all breather hoses off & blowed out again.They were all clear again.Since this I've only rode around the house for a few minutes.I would crack the fill plug & there is pressure relieved!I'm stumped any help appreciated!

it sounds like you hooked the tranny vent tube to the carb nipple near the needle cover. the nipple on the carb is plugged.

that isnt where the hose goes. the hose plugs into the black plastic cover that goes under the gas tank. the hole in the cover is on the right side of the bike. the hose, when installed properly will do a 180 and be pointing down.

its a common mistake and causes this symptom all the time.