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VFR750's Avatar
 
Join Date: Mar 2004
Location: Windsor, CT
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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:


You are now officially screwed.

Because you see the idler can't quite move up enough to get the tensioner in:


So then you rotate the crankshaft to "free the chain"



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:


Now you can safely remove the mechanical adjuster and install the pinned hydraulic tensioner:


Pull the pin


Now you can cut the tie wrap


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.
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'82 911SC, SSI, 22/29 tbars, 22F/22R Adj swaybars, Bilstein Sport, Elephant polybronze & monoballs, Cambermeister bar, turbo tierods, Carrera oil cooler, front brake cooling ducts, Sparco Sprint 5 & Recaro SRD PAX seat, Teamtech harness, DAS Sport rollbar.
Old 07-31-2016, 08:30 AM
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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
Old 07-31-2016, 09:08 AM
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The picture is in you 13th nightmare build.
Old 07-31-2016, 09:15 AM
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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.
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Old 07-31-2016, 12:21 PM
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Great info! I am just making up some tools to do the cam timing, will def. do this.
Old 08-01-2016, 09:57 AM
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Wow, OP, so clean, I am ashamed of myself :-(
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Old 08-01-2016, 04:58 PM
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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.
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'82 911SC, SSI, 22/29 tbars, 22F/22R Adj swaybars, Bilstein Sport, Elephant polybronze & monoballs, Cambermeister bar, turbo tierods, Carrera oil cooler, front brake cooling ducts, Sparco Sprint 5 & Recaro SRD PAX seat, Teamtech harness, DAS Sport rollbar.

Last edited by VFR750; 08-02-2016 at 05:42 AM..
Old 08-02-2016, 05:30 AM
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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.
Old 08-02-2016, 09:03 PM
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