Not surprising. The forces on a camshaft are not symmetrical. Think of the
resistance from a single valve and spring. As the cam is opening it, the
spring is resisting, essentially pushing back against the cam, in effect
trying to rotate the cam backward. Once the peak of the lobe is passed, the
force of the valve spring is trying to rotate the camshaft forward.
Now put all eight lobes into place and you can see that the camshaft lives a
very frustrating life, constantly being subjected to conflicting forces.
Smokey Yunick claims to have done studies with high-speed strobe lights that
show that camshafts actually reverse their motion briefly at points in their
cycle.
So don't worry about that specific phenomenon. The correct way to check
timing chain slack is to rotate the crank backwards (opposite engine
rotation) enough to put slack in the manifold side of the chain, then check
for contact with the L-bracket.
Gordon Glasgow
Renton, WA
www.gordon-glasgow.org
> -----Original Message-----
> From: owner-datsun-roadsters@autox.team.net
> [mailto:owner-datsun-roadsters@autox.team.net]On Behalf Of Cohen, David
> M
> Sent: Wednesday, March 26, 2003 10:56 AM
> To: datsun-roadsters@autox.team.net
> Subject: strange timing change discovery on my '68 2000
>
>
> Hi all,
> I have a very strange problem. When the motor is cold, the top timing
> chain is tight on all the rotation except when the #1 cam lobe is in the
> 2 o'clock position. At that point the chain loses enough tension to
> pull the chain all the way into the L-bracket. Once the cam passes this
> position there is tension on the chain again. Any ideas what the problem
> is or is this normal?
>
> Regards,
> David Cohen
> 510-354-4005
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