re: bucking in reverse gear

[Dana Loomis <dloomis@domain.elided>]

--- stephen hood <hoodmeister@domain.elided> wrote:
>> I have a 67 Duetto that has a peculiar reversing
>> problem.  The driveline causes a severe oscillation, or bucking when
>> reversing.  

And Scott Fisher answered: 
>Betcha somebody put in a later gearbox at some point,
>with its later transmission mounting bushing... 
>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.

Brilliant!  My '66 Giulia Super started "bucking" in reverse after I put a
1750 engine in it a few months ago and I've been meaning to investigate.
The gearbox is the original one, but while it was out of the car I did
exchange the 35 year-old rear mounting bushing--which was covered with
filth and about the consistency of Jell-o--for a good one I had on hand.
The new bushing was, of course, the later type for a car with a hydraulic
clutch.  

This also solves a little mystery.  Someone had shoved a bolt into one of
the holes in the old rubber bushing, and I wondered why.  I'd bet that the
car had developed that bucking problem as the old bushing started to
deteriorate, and the bolt, which gives the bushing some extra stiffness,
was a quick quick fix.

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