Exh valve opens slightly on compression stroke

[Crazy8 55]

 On my new-to-me C-121 I was going thru a few things on the K301 12 HP engine and I see something very odd happening with the exhaust valve. I had checked and reset the valve lash to .008 on intake and .017 on exhaust. I then removed the head to inspect the cylinder and valves and then turned the engine over by hand to observe the piston and valve movements. The odd thing I found was that in the middle of the compression stroke, you can clearly see and feel the exhaust valve opens ever so slightly and then closes again before TDC. I have already reset the valve lash so I don't know if it was doing this before. Any ideas on what's going on?? 

[stevasaurus 22,759]

I think what you have there is the Automatic Compression Release doing it's job.  When cranking the engine, it turns over slow enough that the ACR opens the exhaust valve slightly to make turning over the engine easier.  When it is running, the ACR does not do this at operating speeds.  In other words, your engine is just fine.  

Edited May 7, 2023 by stevasaurus

[Crazy8 55]

Could this be some sort of compression relief for start up?

[Crazy8 55]

Oops I added my comment before I saw your reply Steve. That's great to hear. Engine OK.

 

Thanks,

 

Crazy

[squonk 41,136]

Yes there is a ramp on the cam that opens the valve at low RPM. Held open by a tiny spring. Centrifugal force over comes the spring and allows the valve to act normal over 600 RPM.

 

 

Edited May 7, 2023 by squonk