Great timing Geradcaca...I to have this issue and have just started working on it ('59 Giulietta with '74 5-speed). Seems that the cause is the mechanical clutch linkage. It changes the amount of clutch engagement as the car moves in reverse. The momentary loading/unloading cycling appears to cause the jumping/bucking. The faster you move, or the more the incline, the greater the shudder. The hydraulic clutch in the later cars allows the driveline to 'float' independently from the vehicle and is not affected by the slight body to driveline movement.
I just purchased this from Centerline: Trans Mount 101/105 Part # TM725
Original style transmission mount for 101 Giulia 5-speed cars, and 105 Giulia cars, including GTV, Duetto, and TI/Super 1965-68.
Can also be installed as a heavy duty upgrade on 115 cars 1969 and later with very little increase in harshness.
For competition use (GTA) round holes on both sides of the mount may be filled with metal dowels for added stiffness.
Here are the dowels mentioned (from Alfa Stop):
Aluminium reinforcing plugs for gearbox rear crossmember support silentbloc
I was guided to the problem soulution(s) by this 2004 post on the AlfaBB:
Duetto: Clutch Shudder in Reverse Gear
(be sure to read the linked posts from the Alfa Digest).
An excerpt from one of the Alfa Digest links referenced in the post...
"Betcha somebody put in a later gearbox at some point, with its later transmission mounting bushing, as this is exactly what I experienced with the '67 GT Junior I used to own when I put in a later gearbox. The problem: The '67s have a mechanical clutch release, with a cable that is pulled by a bellcrank when you press on the pedal. Pulled tight, the clutch releases; left slack, the clutch engages. With the original gearbox, the bushing was designed to make this work in both directions.
The problem is that the later gearboxes have a softer bushing with a different shape, because the later 'box uses a hydraulic clutch. Going forwards, this is no problem, because the rear-wheel torque "does the right thing," keeping the cable taut when you're on the pedal. But going backwards, the rear-wheel torque as you start to engage the clutch slackens the cable, which causes a sharp engagement of the clutch -- the first jerk. This causes the cable to go taut again, which makes the car jerk backward, which makes the cable go slack, which makes the car jerk, which -- and it keeps up till you put it in neutral."
This post from the Alfa Digest
in 2002 was the very informative. Edited from the Alfa Digest...
Subject: judder in reverse
From: "Watry, Andrew (LNG-MBC)" <[email protected]
Date: Wed, 13 Feb 2002 14:33:47 -0500
Re Steve's query.
To some extent they just do that, but there are a few things to check, in what has been my experience the order of severity:
1. Motor mounts (on the engine) have good strong rubber and are tightly bolted top and bottom
2. Trans mount (at rear of trans) in good condition and installed right way up
3. Trans mount bolts all tight?
4. Clutch is in good shape as in not roughly engaging or juddering on its own?
5. Throttle linkage works smooth and OK, and car has low smooth idle speed? High idle can contribute to problem.
6. I assume as a 67 it still has the mechanical clutch. Does it have cable or rod actuation on the last link from the bellcrank to the TO actuation arm? (a viciously debated Digest thread) Rod may cause less compliance and therefore more oscillation cycles.
7. There is a relatively simple rubber-bumper hood stop "solution" that can be threaded into the trans mount, effectively arresting the fore-aft motion of the gearbox. Some say it works. It was written up in one or more club newsletters, which I don't have handy.
My current Super had incredible reverse judder when I bought it, because, as it turned out, one motor mount that was not well seated on the cross member and was not bolted in. Even after fixing that, it still has slight judder in reverse, especially uphill, as have other Supers I've owned. For reasons probably relating to the cumulative effect of the above factors, some cars are worse than others.
I hope this helps