YZF owners, techs: My (and a few others') bikes are experiencing intermittent loss of compression when the bike is cold. After that, the bike runs/starts normally.
The folks on thumpertalk's yz250f forum are discussing this, and I think we're stuck:
http://www.thumpertalk.com/bike/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=10&t=001661
The first time, on a 50 degree day, it started second kick, I coasted down a hill and dropped it. It spite of the decomp being in the off position, it would kick through with almost zero compression. I checked the decomp, it was fully seated in the off position at the cylinder head. 30+ kicks later the compression returned, it fired and ran fine (very strong) for the next hour.
The next ride a week later, 55 degrees out, it took 10+ kicks before compression returned, allowing it to start the first time, then it was fine all day. I don't feel any power loss or holes in the powerband. Revs, lugs, starts first or second kick. I just started it, 65 degrees outside, it had compression right away and started after about 10 kicks, then ran fine and will start after 1 kick.
Other factoids: 800+ miles of my-HS-pace trail riding, beginner MX use, 2 HS, 2 MX race weekends. On my 10th? oil filter, been using Mobil1 15w/50 per TT advice since mile 200. Depending on conditions, I rev or lug the motor. Bike is stock, stock pipe and jetting, fuel screw at 1.5 turns. I check the airboot at each filter cleaning with a white towel, no evidence of dirt getting past.
I was thinking: ? valve adjustment, ring wear, oil thickening due to temperature, or piston ring groove clearance problems due to the piston head hammering down. Some folks think it might be carbon buildup on top of the valve, temporarily causing a leak. Others have actually torn into the motor to find the intake valves wearing out the seats, but the exhaust valves are fine(!).
I don't want to grenade my bike, I'm -so- confused as to what's wrong. Suggestions, comments - help!
The folks on thumpertalk's yz250f forum are discussing this, and I think we're stuck:
http://www.thumpertalk.com/bike/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=10&t=001661
The first time, on a 50 degree day, it started second kick, I coasted down a hill and dropped it. It spite of the decomp being in the off position, it would kick through with almost zero compression. I checked the decomp, it was fully seated in the off position at the cylinder head. 30+ kicks later the compression returned, it fired and ran fine (very strong) for the next hour.
The next ride a week later, 55 degrees out, it took 10+ kicks before compression returned, allowing it to start the first time, then it was fine all day. I don't feel any power loss or holes in the powerband. Revs, lugs, starts first or second kick. I just started it, 65 degrees outside, it had compression right away and started after about 10 kicks, then ran fine and will start after 1 kick.
Other factoids: 800+ miles of my-HS-pace trail riding, beginner MX use, 2 HS, 2 MX race weekends. On my 10th? oil filter, been using Mobil1 15w/50 per TT advice since mile 200. Depending on conditions, I rev or lug the motor. Bike is stock, stock pipe and jetting, fuel screw at 1.5 turns. I check the airboot at each filter cleaning with a white towel, no evidence of dirt getting past.
I was thinking: ? valve adjustment, ring wear, oil thickening due to temperature, or piston ring groove clearance problems due to the piston head hammering down. Some folks think it might be carbon buildup on top of the valve, temporarily causing a leak. Others have actually torn into the motor to find the intake valves wearing out the seats, but the exhaust valves are fine(!).
I don't want to grenade my bike, I'm -so- confused as to what's wrong. Suggestions, comments - help!
