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-   -   How to Avoid Common Cam Timing Problem (http://forums.pelicanparts.com/911-engine-rebuilding-forum/923692-how-avoid-common-cam-timing-problem.html)

VFR750 07-31-2016 08:30 AM

How to Avoid Common Cam Timing Problem
 
I suspect we have all done this: Timed the cams "properly" and then found the right hand side WAY out of adjustment. And hopefully you found it before you ran it, or otherwise avoided bending the exhaust valves.

The problem is caused by letting the idler arm drop when you remove the clamp or mechanical tensioner for the hydraulic tensioner.

I PROVED that just be letting the idler arm drop, the chain WILL drop off the lower edge of the intermediate shaft sprocket, and you will UNWITTINGLY rotate the crankshaft a "small" amount to get the tensioner on. This is the dreaded cam timing jump happening. BTDT.

You pulled the tensioner and let this happen:
http://forums.pelicanparts.com/uploa...1469981374.jpg

You are now officially screwed.

Because you see the idler can't quite move up enough to get the tensioner in:
http://forums.pelicanparts.com/uploa...1469981440.jpg

So then you rotate the crankshaft to "free the chain"
http://forums.pelicanparts.com/uploa...1469981494.jpg


The cam did not move, but now the crankshaft is off exactly one chain pitch. Or about 20 degrees (!) retarded. Which means your exhaust valves are REALLY open as the piston is coming up. Carnage is likely, unless like me, you had deep pockets in the pistons. "Deep Pocket$" can only help so much. ;)

Use Tie Wraps after you have set the timing correctly:
http://forums.pelicanparts.com/uploa...1469980944.jpg

Now you can safely remove the mechanical adjuster and install the pinned hydraulic tensioner:
http://forums.pelicanparts.com/uploa...1469980965.jpg

Pull the pin
http://forums.pelicanparts.com/uploa...1469981132.jpg

Now you can cut the tie wrap
http://forums.pelicanparts.com/uploa...1469980977.jpg

Left hand side is not as critical because the loose chain lays over the top of the intermediate shaft sprocket. The tie wrap just makes sure the chain always stays in place between the cam sprocket and intermediate shaft sprocket.
http://forums.pelicanparts.com/uploa...1469981985.jpg

boyt911sc 07-31-2016 09:08 AM

Picture of the left side......
 
Mike,

Very good tips. Do you have a picture of the left side with the tensioner released? I like to use yours as a reference. Thanks.

Tony

VFR750 07-31-2016 09:15 AM

The picture is in you 13th nightmare build.

john walker's workshop 07-31-2016 12:21 PM

TDC #1 before removing the tensioners and don't turn the crank when they're out. If all the rockers are in place instead of just two, like was suggested somewhere, the cams stay put.

boosted79 08-01-2016 09:57 AM

Great info! I am just making up some tools to do the cam timing, will def. do this.

DRACO A5OG 08-01-2016 04:58 PM

Wow, OP, so clean, I am ashamed of myself :-(

VFR750 08-02-2016 05:30 AM

Thanks! I spent an incredible amount of time cleaning. I was, and still am afraid of dirt and grit contamination. Too much invested to let a moment of sloppiness ruin it.

I want to be very clear as to what happens when you let the idler drop down. The cam shaft does not move at all. What happens is the chain goes slack and drops down and towards the intermediate shaft sprocket. A kink forms in the chain. The slack chain actually tries to engage a different tooth on the sprocket. Just one tooth pitch over (down and to the left)

What happens is you don't realize that when you rotate the crankshaft, to get the hidden kink out, you don't rotate the cam, but the kink really doesn't go away. What you really did was reset the number of links between the two sprockets. And the idler locks this new count into position.

And why do you rotate the crankshaft? Becuase the kink prevents you from rotating the crankshaft in the opposite direction. It's jammed. The stage is set, you are about to screw up the rhs timing.

Without being able to see into the case when this is happening, it is hard to get a picture of the actual kink. But it's there. And the results of my experiment proved to me what went wrong on my build.

Wayne's book mentions the risk, but my only criticism is this caution is buried in many long paragraphs, and frankly I missed it. I think a dedicated paragraph and highlighted caution would be better.

Walt Fricke 08-02-2016 09:03 PM

I recall the occasional kinks from the slack piling up in there, but don't recall that ever causing a jumped tooth when I futzed around releasing it.

But on reflection, my kinks have been from when I was putting on the cam wheels, with no idler yet, or at least without doing anything with it, so no timing was involved yet.

I don't use mechanical tensioners to do the tensioning when setting the cams. Since I don't have to remove anything to install the tensioner, I'm not going to have the problem.

But always good to know something more.


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