What's the Connection Between Valve Gap and Compression Release?

[RoysAtco]

Hi All,
I'm having issues with my side valve Briggs and Stratton 12hp 281707. I've been given help by the sites experts along the way. Cutting a long story short I've taken too much off of the inlet valve stem and have been left with a 15 thou lash. I'm told that this will affect the compression release which is why the engine is trying to yank my arm off when I pull start it. So my question is 'why and how does too much valve lash affect compression release'?

Thanks in advance.



Roy

 

[bertsmobile1]

At low speeds the inlet vale ( usually ) opens a tiny amount near TDC to bleed off some of the compressed charge.
At high spped this does not happen.
The actual distance the valve moves is less then 0.015"
So if you have 0.015" gap before the pushrod starts to open the valve then it aint going to happen because all that happens is the deecompression opening does nothing other than take up the excessive slack.
Usually it is a problem on OHV engines as they have higher compression ratios than side valve engines

 

[RoysAtco]

Thanks Bertmobile1 for that reply. Now finally I understand how it works and I'm grateful. I've now ordered a new valve and I'll be more careful when I fit it.
Some time ago I read that not all Briggs and Stratton engines have the compression release feature and you mentioned in your reply that it is usually an OHV issue. So can I still assume that my 1990 side valve engine has it, or am I barking up the wrong tree?

Thanks in advance.

Roy

 

[bertsmobile1]

Depends upon the compression ratio.
On the side bangers it used to be on the exhaust valve till the EPA made them put it on the inlet.
On the OHV's it is a bobweight on some side valves it is usually a funny grind in the cam itself.

 

[RoysAtco]

Ok thanks.
I have no idea what the compression ratio is except that it nearly broke my arm!
I've asked Briggs and Stratton if my engine has the release system. Hopefully I'll get a reply in the not too distant future.
Thanks again.

 

[dougand3]

https://www.cubparts.com/oemparts/a/cut/5b74db6087a8661288177b54/b-s-12-hp-281707-0411-01-part-1
Look at camshaft (46). Camshaft & Gear BS-212897, superseded to 697668. Better pics - https://www.google.com/search?q=697....69i57j69i61j69i60l2&sourceid=chrome&ie=UTF-8
There is not a boomerang bobweight like bertsmobile1 said, so must be some funny cam grind.
12hp rope pull engine - you gotta have some compression release. For now, rotate flywheel just past TDC for some easier pulling.