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I understand it is spring losded and as the revs increase centrifugal force moves the weight radially outward, but what does that actually do for the engine? I it a counterweight? If so, why does the intake cam not have one?
 

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That's part of the automatic decompression release. When the engine is turning very slowly, as when it's being started, a chromed pin sticks out of the shaft near the lobes. The pin holds open one of the exhaust valves for an extended duration. Once the camshaft is spinning up to speed the chrome pin is retracted.

Does anyone know why it's called a "decompression" release? Does it hole the exhaust valve open durinthe compression and combustion strokes?
 

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It is called a "decompression" pin because it releases some of the compression to make the engine turn over easier.

If you have ever kicked over a big thumper like an old XR500R you would realize the importantce of this. To start those bikes I used to kick it slowley till it got to TDC pull in the decompression lever kick it over one time and it would start right up.


What it is doing is automatically holding the exhaust valve open, therfore bleeding the compression off though the exhaust.
 
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