Clutch judder front end shaking from stop

[mike sampsel]

Sometimes from a stop the front would shake and vibrate depending on how (what rpm) I let the clutch out. I just put in a new clutch, fingers, pp, pilot ... blah blah blah.

I found the bushings in my accelerator pedal had totally disintegrated. And the bush on the accelerator rod was broken. So I replaced the ones up front with bronze ones, and replaced the one near the coupler behind the seat too.

Also, I loosened the nut on the clutch clevis, pushed the pedal forward (not sure I needed too, but did push it forward), then I turned my clutch cable (from the rear of the car buy the tranny, thanks Bruce for the idea) into the clevis about 5 turns. This is what it took to allow for the 1.2 mm, then turn 15 mm nuts until you get 1 mm. Before this I never got to 1 mm. I also went from timing at 7 degrees before TDC to about four degrees before TDC.

The result is I can’t get the car to shake anymore! WooHoo. Not sure what fixed it, just glad I don’t need to pull the motor to clean my clutch. Never a shook in reverse, and I was pretty pristine with my clutch during the install too.

Guessing the 20/21 cams (with the advance I had) caused enough vibration at low end to cause some issues with my floppy accelerator rod play. Assuming the 20/21 are about as shaky as one can be with CIS.

[Trackrash]

Loose or worn transmission mounts can cause that issue.

[911pcars]

Grease or oil on the clutch facings can also result in that symptom.

S

[lrodri64]

You need to increase the “bow” of the clutch cable cover using washers.
Try it. I have fixed a lot of 911’s and beetles.

[911tracker85]

You need to increase the “bow” of the clutch cable cover using washers.
Try it. I have fixed a lot of 911’s and beetles.

details?

[David Inc.]

I had the same issue as the OP, and it turns out it was because the helper arm was seized on the fork shaft so there was no free play or adjustment. Once I had that lubed up and properly adjusted like he did all the judder went away.

[mike sampsel]

Originally Posted by mike sampsel

Sometimes from a stop the front would shake and vibrate depending on how (what rpm) I let the clutch out. I just put in a new clutch, fingers, pp, pilot ... blah blah blah.


The result is I can’t get the car to shake anymore! WooHoo.

Have to print a retraction from my woohoo above

So when I take off in first with some motion (not a stop) never shakes.
Take off at 1,000 rpm from stop never shakes.
Take off from above 2000 rpm let out clutch no shake.
Between 1,000 - 2,000 rpm and let out clutch: sometimes shakes

i was very careful about not getting any grease on the new clutch, however, I first put some white lithium grease in the pilot bearing and then added the sach style grease. Wonder if the white lithium melted and some ended up on the clutch?

Drive it I will.

[T77911S]

how did the surface of the flywheel look.

any "burn" marks or hot spots. they will have a blue tint to them.

[Jeff Higgins]

Originally Posted by lrodri64

You need to increase the “bow” of the clutch cable cover using washers.
Try it. I have fixed a lot of 911’s and beetles.

A lot of people miss this one. The clutch cable housing from where it exits the tunnel to where it terminates on the transmission is known as the "bowden tube". This section of clutch cable housing absolutely must have a bit of an arch to it - think "bow" in the "bowden tube". If it lies in a straight line from the tunnel end to the transmission end, it will cause a clutch chatter.

The reason for this is, and the reason it must have a bow in it, is that the engine/tranny rocks to one side as the clutch is let out and resistance is encountered in getting the car moving. The motor/tranny "torques" to one side. The bow in the bowden tube serves to allow that to happen without changing the effective length of that tube - it remains "anchored" on both ends.

If the tube is flat, one end or the other will unseat from its perch, changing the effective length of the tube. This effectively changes the clutch adjustment, which causes the clutch to release a bit. This, of course, stops forward progress, unloads the engine/tranny, and allows it to settle back down from its "torqued over" position, effectively changing the length of the bowden tube again as it re-seats on both ends.

Now the clutch starts to engage again, the engine/tranny torques over again, the bowden tube unseats again, and the clutch releases again. This all repeats so quickly that it appears to us as the chattering clutch from hell.

Make sure there is a good couple inches of bow in that bowden tube.

[gomezoneill]

Been working on these cars for a long time. Never heard about this. Interesting to know.

[911pcars]

Originally Posted by mike sampsel

........... i was very careful about not getting any grease on the new clutch, however, I first put some white lithium grease in the pilot bearing and then added the sach style grease. Wonder if the white lithium melted and some ended up on the clutch? .....

White lithium grease is better for stationary lube areas (door locks, hinges, etc.). Not especially suited for high temps., high loads and high rotational viscosity strength. If not fortified, it's also water soluble (turns to mayonnaise consistency), not an issue within the clutch area, but for areas exposed to the elements.

Sherwood

https://www.machinerylubrication.com/Read/340/high-temperature-grease

[Nick Triesch]

My clutch was just replaced. But also the flywheel!! It had grooves in it. It was shot. No more shudder!!!

[mike sampsel]

Originally Posted by T77911S

how did the surface of the flywheel look.

any "burn" marks or hot spots. they will have a blue tint to them.

My flywheel was machined down (per the advice of the mechanic who rebuilt my tyranny) and was still within spec thickness. So it's not this.

The bowden tube has a bow (pretty sure its a couple of inches, I'll get some pictures of it to demonstrate my understanding of the bow).

Based on today, I'm leaning towards the difference in the engine stability at 1,000 - 2,000 rpm with the 20/21 cam and my paranoia. As I could not get it to shake much today. Basically just being a stickler (I hope).

Seems if I did have oil/grease on the flywheel it might make itself known by issues at other RPM? Like why not at 1,000 rpm too? I will continue to report, but I do think the replacing of the bearing sleeve for the accelerator "reverse lever" helped (quite a lot too).

I did add sachs high temp grease inside the pilot bearing (should have cleaned all the white lithium out). Wish I had another Porsche 911 with 20/21 cams to compare too. I know my cam'ed Datsun has a spot where the car surges some, just because the cam is not as smooth as the stock ones. The new 911 is easier to drive than the cam'ed Datsun!

Since I seemed to find some studder (minor) when I accel'ed in second from 1,200 - 2000 in second today (car already moving and cllutch fully engaged) I'm leaning towards the 20/21 cam at the 1,000 to 2,000 range is not as smooth as the stock one?

Maybe I'm nuts, and certainly worthy of being known as making a mountain out of a mole hill

Many thanks and keep the information flowing please.

Oh, and I've got 700 miles on the engine since rebuild, it's a lot different the original 3.0 SC. Pulls really well from 4,000 to 6,000 (although I've not been to 6000 more than about 5 times now). It's been an adventure

[mike sampsel]

Originally Posted by Trackrash

Loose or worn transmission mounts can cause that issue.

I checked for tightness (looked fine), mounts new Uro Parts to the rebuild. I'll assume these are not an issue.

[mike sampsel]

So, is the bowden tube supposed to be at least an inch from the transmission at all points? Thinking this is not possible?

Here is a cut an past from Peter Zimmermann's post

.... nuts to gradually tighten the clutch cable, keeping in your vision where the arc of the Bowden tube is, and concentrating on the gap that you’ve set to 1.2mm. The Bowden tube should rest about an inch from the side of the transmission ...

Specifically post number 9 here

clutch cable adjustment help

Pictures of my bowden tube to come. It touches the tranny at a point? Not an inch from the transmission in all places?

[Jeff Higgins]

Looking at the bowden tube in the plan view (looking down from the top), it would not matter if it touched the transmission for its entire length. The arch, or bow, that we are referencing is visible in the side view.

Here is an example of correct routing. Notice it goes over the top of the throttle bellcrank. Some folks mistakingly route it under, eliminating the vertical arch, which is where the trouble begins. On newer cars, there is often a clutch cable mount in front of the throttle bellcrank that serves to hold the cable in this position.

[mike sampsel]

Originally Posted by Jeff Higgins

Looking at the bowden tube in the plan view (looking down from the top), it would not matter if it touched the transmission for its entire length.

Thanks Jeff,

The picture is great.

[mike sampsel]

So I neglected to take a photo of this strap holder placement on my Bowden cable.
Does it go on the nut circled?

[Jeff Higgins]

My car is older (1972) and does not use that strap, but it does look like that nut would be the likely place to mount it.

[T77911S]

clutch problems (clutch chatter) only happen when you are starting from a dead stop.
if the clutch is engaged and you are moving you are feeling something else.

grease on your clutch is going to slip, period. and at all RPM