I have a 1998 yz400f that I bought about a year ago. Guy before me raced it but took good care of it. Besides fork seals, brakes, this that and little stuff its ran perfect the last year. Anyways, riding it easy the other day and shut it off to bs, go to start it, low to no compression. I didn't even have to use decomp lever to turn it over. After pushing that bike 2 miles up and down hills to get it to my truck I was ready to open up the gas tank and drop a match in there. After some extensive online research I ended up here making this post.From what i read sometimes an exhaust valve gets stuck open. Okay, tear the cam towers off and try to turn it over. Same thing, low compression(except at this point all valves should be closed and i shouldnt be able to turn it over) While turning it over with hand over exhaust i can feel it pushing and sucking in air. Okay, and exhaust valve is stuck, nope, tore head off and all valves are seated, or at least what the eye can tell. Top of piston and valves have bad carbon build up (although I had the motor apart last year and cleaned everything up). My most logical explanation is that there is build up where the valves should be seated and its just enough of a gap that i cant see but its letting air escape. My question is this, (since my manual doesn't go this far in depth) what is the best way to clean the valves/take them apart in order to clean them. Pushing them down and holding them down with my thumb is out of the question because I already tried. Putting the cam tower back on and using the lobs to hold them open is an option but I i was hoping someone on here would have a better solution (maybe how to take valve assembly apart) I know the bike could at least use new piston rings but i don't have the cash right now, however, worn cylinder/bad rings/etc wouldn't explain the sudden loss of compression. Valve clearances are good and it was timed correctly prior to taking it apart. First four stroke I've ever owned so I'm stumped on the disassembly of the valves. Despite extensive adventures across the web i couldn't come across a "how to" on this.