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“In fact, the over-run valves were notorious for losing spring pressure and causing a high or erratic idle speed. The
practical solution was to just tighten the nut about 2 turns.” Basically, adjust it just tight enough that the valves are
closed at idle -- which can be confirmed by putting a finger over the hole in the filter housing with the engine idling.
“Setting the over-run valves by checking with finger over the hole is as good as any method. As long as they don’t leak
at idle but work when you blip the throttle and release it to create an over-run condition they will be OK. You will
probably hear them anyway. Again from memory I think they were meant to start to open at 20" Hg but by their nature
they are more a sort of progressive leak rather than a step change device so manifold vacuum could exceed this value to
a small extent.”
ACCELERATOR PEDAL -- EARLY MODELS: Mike Morrin says, “The Jaguar pre-HE service manual seems not to
have been updated from 1975 to 1981, which is fine for my car, but I have seen a few things which are obviously
wrong for most of the pre-HE cars. A minor example of this is the RHD accelerator pedal assembly: The only type
shown in the manual was only fitted to the first 200 cars.”
THROTTLE CABLE -- ELECTRICAL PROBLEMS: Ron Brock says, “The starter was acting as though I had a low
battery. After replacing the battery to no avail, I started in on the starter. The car would crank over fine sometimes and
other times it would barely turn. After receiving several "tips" about checking the electrical connections, I went thru
them from the battery forward.
“My problem became clear one night in the garage, I was attempting to start the car with the bonnet up, and it was just
dragging. It was then that I noticed a whiff of smoke coming off the back of the engine. I walked around to the front of
the car to determine what was "smoking," it was the throttle cable, and was it hot (by the way this also solved my
sticking throttle problem). It seems that the engine was grounding itself thru the throttle cable. I was unable to remove
and clean the standard grounding strap at the engine, so I added a second strap that I purchased from NAPA for $4 and
the car has been fine. Absolutely amazing. I would seriously recommend that anyone have starter/charger problems
explore this cheap and easy fix first.” See page 553.
THROTTLE LINKAGE BUSHINGS: There is a throttle shaft on each side of the engine, parallel to the heads, that
transmits throttle motion to the butterflies. The rear end of this throttle shaft, along with some linkage, is supported by
a plate bolted to the rear of the intake manifold. There is a rubber bushing in the plate for the shaft to turn in. This
bushing is probably shot -- British nonmetallic parts again. This bushing will dry up, crack to pieces, and fall out,
leaving the throttle shaft to wallow around in the opening. In fact, this is another example of a 100% failure mode in
the XJ-S; if you haven’t already replaced the bushings, they are probably bad right now. The effect on the throttle
operation is not good, as it tends to screw up the sync of the two butterflies with each other and with the throttle pot in
the turntable. There are also reports that this problem can have adverse effects on emissions tests, especially if one
bushing is gone and the other is still on the job.
The part number for the original Jaguar rubber bushing is C34388. However, I don’t think anyone in their right mind
would recommend the original bushing -- it’s rubber, for heaven’s sake. Rubber throttle linkage bushings are a relic
from cars with rigid linkages from the pedal to the carbs; since the engine moved around on its mounts, some means
had to be provided to connect the pedal on the car to the butterflies on the engine without the engine’s motion affecting
the throttle position. The solution was to have one rotating shaft with one end mounted on the car and the other
mounted on the engine; the pedal was connected to a lever right next to the mount on the car, and the butterflies were
actuated by a lever next to the mount on the engine. The pedal twisted the shaft which opened the butterflies, and
engine motion would rock the shaft back and forth without twisting it. Rubber bushings were needed at the ends of
such shafts to isolate engine vibrations from the rest of the car, and to allow the small angular motions of the shaft
without any binding.
None of this applies to the XJ-S. The motion of the engine relative to the car is dealt with via a throttle cable. The
linkages where the bushings are used involve shafts that are mounted at both ends from the same intake manifold, so
there is no relative motion. Totally rigid bearings will work fine.