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Valve Dynamics

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Virtually all posts talking about valve clearance are about the gap becomingg smaller. What if the gap was larger due to measurment error? Initially, I thought the valve stroke would decrease. After some thought, it seems that an excessive gap would create dwell between the cam lobe and bucket. Because the lobes seem to gradually increase acceleration of the valve, would I be correct in saying that a large gap could result in a relatively large impact force thus rapidly accelerating the valve downwards to the point where it doesnt follow the cam lobe or is it just that meaning it just wont travel as far into the cylinder?

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'loose' valves (but within spec) will act like a cam is of smaller duration, 

to a point increasing vacuum and cylinder pressure both favoring low end torque.

 

Myself trail ride a CRF250X and lug it rather than rev it and, always set up the valvetrain to the maximum tolerance values.

Another benefit is the longer the valves are seated, the more heat they transfer back into the cylinder head.

 

While too much lash is hard on the components, but IMO depending on design (roller follower or direct actuation) 

the engine would be terribly noisy way before the point where damage would become a concern.

 

Edited by mlatour
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Cam is ground with lash take up ramps, the clearance would have to be excessive before the it's off that area of the cam.  Here's a graph of a crf cam at 2 different lash settings for a visual of the take up and valve acceleration. Mlatour you can see your duration change, this a a 250x intake 49 state.

stock .005"

 

 

.005 lash.jpg

Edited by highmarker

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and .015" , which would be out of spec loose.   Both have valve accelerations that are within a "normal" range.    

.015 lash.jpg

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