Early 911 hot rod - wobbling/oscillating gear lever

[Magnum]

Morning all

I have a strange affliction that manifests during motorway driving. My hot rod early 911 has a 3.2 and the accompanying 915 gearbox with newly changed linkage bushes. Limited changes from standard include a WEVO Stock 911 PSJ (i.e. solid coupling) and a lightweight wood gear knob.

When driving at constant motorway speeds my gear lever develops an oscillation (i.e. fore and aft vibration movement) whereby it bounces about 50-60mm at the gear knob with a particular frequency. This can be stopped with light hand pressure, but clearly not safe to do so!

I have made small linkage adjustments to change dynamics but to no effect – I know the answers are likely to be the non-Porsche PSJ but has anybody had this and implemented a fix?

[hot66]

Engine / gearbox mounts old or fresh ?


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[Gary71]

Try putting the (heavy) original knob back on?

[Mike911scrs]

HI Magnum

some bits to check/look at, let us know how you get on?

jack up the car at the rear and put on axle stands, slacken off the gearbox mountings, the 4 on the box and the 2 mounting to the car/rear cross member, now do the same with the engine mounting in the engine bay (you did get the mounting on the right way up?), with all the bolts loose start the engine and run for a few min' to let it settle, then tighten all the nuts/bolts back up,

while you have the car up in the air, check the drive shaft's are running OK and the CV joints are tight and running square with the flanges, I have noticed the some well used gearbox out-put shafts are starting to get out of shape and need a clean up to get the CV joint back in line to tight to the flange

the linkage joint is not the prob' with the movement, it has just amplified it

you may just have the rear wheels out of balance or a soft rear shock'

reagrds mike

[911hillclimber]

As you know, I have the same combo car as you.have none of these probs.
Let us know!

[Magnum]

Thanks all, especially Mike for the constructive suggestions

This is my first classic car on which I haven't instantly changed engine & gearbox mounts - i normally do so immediately, however, the car was so well maintained it didn't occur to me - silly actually.

Anyway it just so happens I have a lightweight starter motor and lightened engine mounting bar to fit. I also have some spare new engine mounts so will fit these to the gearbox cross member (which I suspect must be in worse condition than engine mount mounts).

So i now have enough jobs in the same area of the car to motivate me! Just need the weather to cool down - I think an early morning start!

I'll feed back when done!

Cheers all

[Magnum]

Hi all - feedback (partial at least)...

Interesting couple of hours tackling a variety of jobs.....you'll note there is always at least one "first problem" . I couldn't wait to get stuck into this, so on a sweltering Friday afternoon (bunking some work obligations), my first job was to replace the original starter motor with a Powerlite product, simply because lighter is better....well it is a lightweight.

The "first problem" after implementing the physical swap (my first starter motor job) was a befuddling electric challenge whereby I couldn't connect up the battery terminals without heavy sparking.....not good (or indeed welcome as I'm no electrician). After then cursing heavily whilst removing newly fitted starter and looking carefully, it turns out the connecting eye on one of the "fat" cables had been bent when manhandling these to the new starter motor fixing position/stud and it was touching the body of the starter motor...... so that was fixed quickly (and with expectation) after the initial confusion and bitter disappointment. That sorted it and so the new starter motor saves kilograms, works like a charm and makes no grinding or other unwelcome mechanical noises.

Whilst the car was up on axle stands I elected to replace the existing engine mount bar (with a new lightweight version, cos that's better right) and also the rubber engine mounts, as well as "re-set" all the mounts to mitigate vibration to remedy my oscillating gearstick....

I duly managed to find a new "first problem" on this part of the job, which arose when the mounting end of the engine mount bar (ie through which the main engine mount bolt slides and screws into) snapped off when trying to undo said bolt.....it was a sign of poor quality I thought, however before that thinking, was panic about how one removes a little piece of broken engine mount bar from a large bolt in an (what I thought was) inaccessible location - unwarranted panic actually as mole grips dominated!
Needless to say I couldn't determine 100% whether this was an existing crack or newly formed - there was no rust or different colouration on the crack surface so my money is on newly formed. Anyway on removal of the existing engine mount bar it became very apparent that it was a poor quality item - the plate steel was thinner than replacement and the bottom edge was not folded over (as is the top) to act as structural reinforcing/stiffening, and it was clear this was more flexible than should be the case (or is desirable). A rare and complete justification for the unjustifiable act of buying a lightweight version!!

When installing the new lightweight engine bar from Design911, I encountered my "first problem" for this item - the mounting ends were not threaded internally at the bottom (as was the original) for the main bolts to "attach", thus i had disengage from the job unexpectedly (is there anything more irritating, when you're set on completing a job, dirty, greasy and sweating like a pig?) and then rush off to procure longer bolts, washers & nuts..... bloody irritating. Then measuring bolts, cutting etc. all made extra work.

In the end all round success - starter motor all good and the new engine mount bar + rubber engine mounts have stiffened up things nicely. In fact the very joyful and unexpected consequence is my downshift into 2nd is vastly improved with none of the occasional baulking, which in hindsight must have been caused by flex/movement of the engine/box under deceleration and not a gear linkage adjustment issue which I thought to be the case (after I had fiddled previously when installing the solid gear linkage coupler/PSJ).

As for the oscillating gearstick, I have yet had the car on the motorway but I'm hopeful!

[MikeB]

Well done, Magnus, glad you got a result.

One tip if you are working on anything electrical, disconnect the earth lead (- ive) from the battery and you won't get any unexpected sparking

[911hillclimber]

That last report made me smile.
When I installed my 3.2 in 1996 I too found out about the treads in the factory bar.
I drilled mine out so the long bolts which my 2.4 had would just drop into place and then spin a nut on. I never could get an easy engagement of the bolts into the threaded ends.

I've just removed the plastic 'racing' engine/box mounts for factory rubber getting everything finger tight before bolting up firmly in a quest to remove the NVH that I had with the stiff mounts.
I hope the gear change quality will not take a step backwards.
Apart from saving a few Kg I found no benefit to the lightweight starter.

Hope the shakey gear stick is now stable.
I've seen a crude fix for that, place O rings around the base of the lever by the top plates so to encourage the stick to be upright in neutral.
when the lever is in gear the one set of O rings are in tension so taking up gently any free play and hence vibration.