Decompression valves, how do they work???

[Erik]

Recently there was some discussion on the board concerning the decompression valves. Below a drawing and a small piece op text from the BOS:

When I look at the drawing and the picture of the Scott cylinderblock I do not see how this valves would work. As the valves only seem to end in the actual exhaustport (see yelow circle) they would in my opinion just act as a way for the exhaustgasses to escape when opened. The compression would not be altered in any way. Or am I horribly wrong???

The text says that the valves open an additional port above the exhaust port. But what does this mean? Or should my cylinder actually have an extra port in the wall? If so, I would understand the working of the valves.

Who can enlighten my troubled mind?

Cheers,
Erik

[dave bushell]

Hi Erik
Yes you are horribly wrong!
The half compression valve covers a 5/16″ diameter hole that is centered in the cylinder wall 3/4″ above the top of the exhaust port. When the valve is opened, it allows the compressed mixture to vent via a similar sized hole in the top of the exhaust port (circled in your picture) thus lowering the compression.
Hope that helps.
Dave

[Erik]

Thanks Dave!

Thing is that both the cylinder blocks I have (Flyer) do not have any holes in cylinderwalls at the place you mention… That must be the reason for me being so puzzled!

At least one of the blocks was sleeved so that could be a reason…

Cheers,
Erik

[Erik]

Hi Dave,

I checked both my blocks this evening and none has any holes in the cylinder wall! I appears both are resleeved… Maybe by discovering this I am one step closer to the cause of my engine blowup of last year…

Cheers,
Erik

[dave bushell]

Erik
A thought has crossed my mind and actually stopped briefly! You may have slightly later blind head barrels (Flying Squirrel) that, although they used the same castings as the earlier ones, did not use the half compression feature and therefore the barrels were not drilled above the exhausts. I think that the h/c valve position holes were blanked off by suitable sized purpose made bolts.
Dave

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