To my other GT6 pages

February 14, 2020


Throttle Linkage


One of the things that contributes to the experience of driving a car is how the controls feel.  One thing that most people expect from a sports car is controls that are quick and responsive.  To a large extent the feel of a control is influenced by the linkage between the cabin control and the ultimate device it controls.  A sloppy linkage feels like a sloppy control.

Some people consider the throttle linkage arrangement on GT6 cars (also TR6) as a little Rube Goldberg-ish, especially on left-drive cars.  There are more than half a dozen joints in the linkage train that can get worn and loose, leading to slop in the pedal response.

One popular solution to this situation is to install a throttle cable kit that effectively eliminates some of the joints.  This is apparently effective, and not very expensive, but really just trades issues with articulated joints for those of cables.  Even though there are a lot of joints in the stock design, there is no reason they can't be tightened up and maybe even improved upon.

I'm only focusing here on the part of the linkage that lives under the carburetors.  It consists of a horizontal link that transmits linear motion from the throttle shaft behind the engine, a bell crank that converts the horizontal motion to vertical, and an arm that converts the vertical motion to rotary motion for the carb throttle shafts. 

My assembly was dirty, crusty, and what wasn't loose was frozen.  The vertical link was missing altogether.  I think it fell apart in disassembly.




The bracket that spans the carbs and holds the bell crank was in pretty bad shape.  It had deep wear marks from the crank, and seemed to have a lot of holes that weren't used.  It was pretty simple to just make a new one.  It's 14 gauge steel, like the original.





Next, I looked at the horizontal link.  It is adjustable length, and has a ball joint on the bell crank end.  The ball joint was a little loose, and though there is a provision for tightening it up, that adjustment was frozen.  I ended up damaging the joint trying to free it up.





I believe at least one vendor may sell a reproduction of the original ball joint, but I found a pretty good modern replacement for it.  It is not adjustable, but is all stainless steel, and has a rubber boot to help keep the joint clean.





I cleaned up, de-rusted, and re-plated the parts of the link.




I liked that new ball joint so much that I ordered a couple more in the next smaller size to make a new vertical link.




Then, on to the bell crank.  The pivot hole of the bell crank was loose on its shoulder bolt, and was the source of a lot of the slop in the linkage.




The first mod to the crank was to remove that staked ball.  The original vertical link just had a plastic ball socket that would snap onto that ball.  Those plastic pieces didn't age well.




I had also noticed that the hole where the horizontal link connected was wallowed out, so I welded it closed and redrilled it for a snugger fit.




Then, to address the pivot hole, and to even improve the design a little, I bored the hole for a pressed-in flanged Oilite bushing.  The flange will bear against the mounting bracket so the crank can't rub.




Then I could mount the bell crank with a new shoulder bolt.  I thought the bracket looked better black.  It's powder coated.




Then added the stop.  It limits how far the bell crank can rotate.




The last assembly was the lever arm for the carb throttle shafts.  It includes a lost motion arrangement.  In any linkage system like this, it is often wise to provide for a small, controlled amount of slop, euphemistically called "lost motion".  This ensures that the linkage is not bound tight, and makes sure that the carb throttle shafts can fully rest on their stops at idle.

The lever arm also had to have it's ball removed to make way for the new vertical shaft ball joint stud.




The steel parts were re-plated, with a chromate treatment.




Putting everything together.  The whole mechanism is tight and smooth (if that makes sense).




This seems like such an insignificant thing, but I really think it will contribute to a good driving experience.

It really only took a few hours of hands-on time, and I think the cost was around $20 for the ball joints and bushing.

Comments to Ed at mailto:elhollin1@yahoo.com


To my other GT6 pages