Valve question

[Sc120sc]

working on a stuck exhaust valve on my newly acquired 147 I got the valve working with some PB Blaster and a dead blow hammer. Didn't take alot and its moving up and down fine.

I see the valve opens a very tiny bit then seems to close again before opening fully. Is that how it should be? See no reason for it to work any other way. Is that like a compression release?


I have been rotating the motor by hand to let the valve loosen up. I have noticed if you constantly rotate the motor non stop the valve travels fine. BUT sometimes if you stop turning and the valve is in the open position it sticks open and requires a tap to go down. After the tap it "snaps" down.

[green407]

Yup, that's the ACR (auto compression release). The engine needs to be spinning at about 600 rpm before that disengages. Totally normal operation.

[finsruskw]

DO NOT go beating on it when open or you may break the tang on the ACR if it is engaged!!

Some are hardened and will break and then you will be looking for a new cam!!

Best to remove the valve, clean the stem and guide and replace.

[Sc120sc]

Originally Posted by green407

Yup, that's the ACR (auto compression release). The engine needs to be spinning at about 600 rpm before that disengages. Totally normal operation.

What about that valve seeming to "stick open" if I am not keeping motor turning?

[R Bedell]

What about that valve seeming to "stick open" if I am not keeping motor turning?

"Best to remove the valve, clean the stem and guide and replace."

[ol'George]

Those particular engines did have valve sticking issues sometimes.
Either remove it and clean the carbon off it, paying particular attention to the guide area or just replace the valve as they are relative inexpensive.
As mentioned do not hit it as the compression release tab on the cam is easily broken, then you have a complete tear down for cam replacement.
And yes, the compression release is holding the valve open .010 give/take on the compression stroke till the very last few degrees of rotation BTDC.
then it closes jast as the ignition fires the plug.
At anything over cranking speed, the centrifugal force causes the tab to
become inactive allowing full compression.
simple mechanical design and it works well even in todays 'lectronic excesses.

It might be noted that a guide reamer works well cleaning valve guide as carbon also and sometimes, contributes just as much as a carboned valve stem.
Lacking a guide reamer, a drill of the correct size used by hand
can work but carbon is hard.
IIRR 5/16" is the guide size but various engines/sizes/manufactures use other guide sizes.
-----TMI (too much information)

[Sc120sc]

Originally Posted by ol'George

Those particular engines did have valve sticking issues sometimes.
Either remove it and clean the carbon off it, paying particular attention to the guide area or just replace the valve as they are relative inexpensive.
As mentioned do not hit it as the compression release tab on the cam is easily broken, then you have a complete tear down for cam replacement.
And yes, the compression release is holding the valve open .010 give/take on the compression stroke till the very last few degrees of rotation BTDC.
then it closes jast as the ignition fires the plug.
At anything over cranking speed, the centrifugal force causes the tab to
become inactive allowing full compression.
simple mechanical design and it works well even in todays 'lectronic excesses.

It might be noted that a guide reamer works well cleaning valve guide as carbon also and sometimes, contributes just as much as a carboned valve stem.
Lacking a guide reamer, a drill of the correct size used by hand
can work but carbon is hard.
IIRR 5/16" is the guide size but various engines/sizes/manufactures use other guide sizes.
-----TMI (too much information)

Its funny the valve only sticks if you stop rotating by hand and valve is fully open. Otherwise it operates fine.Goes up and down a million times.