A forum for owners of Yamaha TTR250 trail and enduro bikes!

Hi Mark

Possible causes are:

• valve(s) not closing or a valve/valve seat damaged
• head gasket blown
• hole in piston 
• spark plug loose

I have had a situation where, after changing a spark plug, a bit of dirt got into the cylinder and got trapped under the exhaust valve resulting in no compression. I am not saying that's what's happened in your case but you gotta be looking at something unusual from the symptoms you have described 

If I was in your position I think I would firstly take the plug out (making sure no dirt drops into the cylinder in the process!) and spin the engine over. If you can put your thumb over the spark plug hole and keep it there then you definitely have a problem.

To eliminate the piston and rings as the cause, pour in a few cc of oil and turn the engine over again. If you suddenly get back compression then it's likely something as happened to the rings - not heard of a TTR burning a hole in the piston - yet....

If the oil makes no difference then the next easiest thing to check is that the valves are adjusted properly and all fully closing. It is remotely possible that, on shut down, the cam chain jumped on a camshaft sprocket thus messing up the valve timing.

If the valve timing and clearances are OK then next up is the head gasket. In my experience a blown head gasket that results in total lack of compression will be fairly obvious as you will hear a chuffing sound as you turn the engine over and it might be accompanied by oil/petrol leaking out.

Good luck and let us know what you find.

Brian