1963 Etype FHC – A Tour of the Throttle Linkage

Well, you know, it just doesn’t get any more exciting than a tour of the throttle linkage 🙂 But a guy on Jag-Lovers forum with a similar car to mine had an “event” where his throttle stuck wide open! There is a significant pucker factor when that happens! Here is a review of what I posted on Jag-Lovers to help him make a diagnosis. These are pictures of my 63 FHC. I know that the parts remain similar up through 1967.

Of course things start at the “gas pedal”, although the SPC calls it an accelerator pedal. Things to look for are fouling with the upholstery and is the return spring installed and in good condition. The return spring is hard to see but it is attached to the upper part of the lever reaching down to the pedal.

The accelerator pedal has a linkage that rotates a shaft in a sand cast aluminum housing that rests on the top of the firewall. This transmits your foot motion to another lever. Internally, there are some bushings that you want to make sure are in good condition.

Next, the lever is attached to a rod that goes up to a “bellcrank”. A bellcrank is basically a fitting that converts a physical translation of movement through a 90 degree angle. There is quite a bit of load on a bellcrank so they need to be in good condition. Here is a photo. BTW, that spiral wrapped cable leads to the heater box.

This bellcrank is mounted on the firewall, between the heater valve and the brake vacuum booster. From there, a long rod traverses the area behind the engine. It goes without saying that the rod should have a modest clearance to adjacent items so that it does not foul.

BTW both of these rods will accept a minor amount of length adjustement. Also if you look closely at the bellcrank, you will note that there are two spacers with wrench flats. The internal hole in the spacers is eccentric. In concept, you can rotate the spacers and obtain a modest change in the rotational position of the bellcrank. What you do not want to have happen is for the bellcrank to rotate “overcenter”, as then it gets stuck. Overcenter would be defined as the bellcrank rotating CCW such that the long rod is at or below the 9 o’clock position.

At the other end of the long rod, there is another lever. This one has several parts to allow it to straddle the windscreen wiper motor. It contains a rubber/metal sleeve type bushing. The strap that connects to the firewall has slotted holes, so some adjustment is possible.

At this point, the translational action of the long rod is converted to rotation of the shaft that passes over the manifold. As we will see in a second, there are 3 levers on this shaft that convert the rotation back to translation, pushing on the throttle arms of each SU carburator. There are several places in the shaft to check for binding.

I’m sure the designer had a need for this strange little connection but that need is lost in antiquity. As the mechanism gets some miles on it, I have found that the pieces get “beat up”. New ones might be in order as a routine wear item. Also visible in this picture are the L shaped rods that reach down to the throttle shafts on the carbs. They are held in place with the little black clips. Getting your fingers in there to remove and install these can be a trick. Make sure the clips haven’t been mangled, as that can introduction “stiction”.

This is a view out at the forward end of the throttle shaft. Another bearing here, subject to binding. The red is fresh grease.

Here is a view of one of the throttle return springs. The way the springs attach at their lower ends changes as you go from 3.8 to 4.2 cars but the concept is the same. Fresh springs are always a good idea. You can get a little more umph by rotating the arm on the throttle shaft to preload the spring a little more. Make a careful inspection in there to check for binding.

4.2 Linkage Differences

After I looked at my 1967 Etype 2+2, I see there are some obvious differences. Here is an image of the SPC plate.

The most obvious thing is that there is another vertical link on the passenger side firewall. Item 18. This results in Items 20-25 being much lower down in the engine compartment. The rods item 28 are shorter. This results in the linkage being closer to the top of the engine frame. There have been reports that with weak or collapsed motor mounts, the linkages can foul with the engine frame and bind. This is something to keep an eye on.

Last but not least, the condition of the SU carbs can play a big part in how well your overall linkage responds to throttle inputs. Here are 3 freshly rebuilt by Joe Curto SU carbs for my 3.8 car. On the right hand carb you can see the “butterfly” which is at the end of this long path of linkages. As the butterfly rotates, it allows more airflow. The SU carb cleverly takes care of adding the additional gasoline required to allow your engine to rev. Google “constant depression carburator”. Anyway, the shaft that the butterfly rides on must be in good condition or it can bind. Joe Curto offers a rebuilding service for just the throttle shafts at a reasonable price. These carbs have received his full restoration service and are a joy to look at.

As you can see, there is a lot going on with the Etype throttle linkage. Throttle binding or poor response can possibly be due to one component behaving badly or it can just be the cummulative effect of the sum of a bunch of small resistances. Fortunately, most of the major wear components can be purchased from SNG-Barratt. As noted at the beginning of the article, a stuck throttle is scary. It might be time to evaluate and bring yours back to new condition.