Throttle Linkage for Dual Carb Slant 6
This is for a 1964 Plymouth Valiant with a 225 cid "Slant 6" engine running a custom dual Webber carb setup. It's a pretty wild looking carb setup that should provide plenty of power and some real "pop" when you open the hood to show off. As cool as it is, it has one very serious problem - it has no commercially way to hook it up to the factory gas pedal in an early Mopar A-body. Ah, the joys of custom carburetion! There isn't even a throttle cable mounting bracket on the engine; we'll need to fabricate one ourselves - most likely from scratch. The only good news here is that the dual carbs are already hooked to each other with the right linkage - all we need to do is get the single throttle attaching point on the forward carb hooked up to a gas pedal inside the car so it all works right. Who would have though 24 inches could be such a long way to travel?
The original setup used a funky mechanical rod and linkage system that stuck through the firewall and directly actuated the original carb - it is unlike anything else I had seen before. The throttle rod had a short bend in the front end that went towards the passenger's side of the car. That bend was connected directly to the rear of the carb, which was situated "sideways" from a normal carb installation on a V8 where the linkage is on the side of the carb. As the throttle rod arm rotated clockwise (when viewed from the front of the car) that short bend at the forward end of the throttle rod would open and close the throttle on the carb. The throttle return spring and automatic transmission kickdown linkage connected to the rod from a small "V" shaped set of levers welded to the rod at about the same place as the rear of the engine block. On the inside of the firewall, the rod was supported and weatherproofed by a small plastic and rubber bushing mounted on a small stamped steel "hat" bolted to the inside of the firewall - the three mounting bolts on that hat are what you can see sticking through the firewall around the place where the rod comes through. There is a small lever bolted onto the end of the throttle shaft that points off to the drivers side with a small ball stud on it. A rod was clipped onto that ball stud that ran down to the gas pedal and the bottom of the gas pedal has a pivot that is bolted to the floorboard. When the gas pedal is tilted forward, this causes the rod to pull down on the ball stud attached to the lever, causing the rod to rotate. The rod is supported at the carb by the carb linkage and at the firewall by the small rubber and plastic bushing. The system was engineered so that it produced an approximately 90 degree rotation of the rod, and opened the butterfly on the carb.
The new carb setup will in no way work with the original throttle rod arrangement. The throttle linkage on the new carbs is more like what you would expect and is placed on the side of the carbs, not the rear, and thus is 90 degrees off from the original carb. Our original attempt to fix this involved trying to swap on a later-model A-body gas pedal setup that uses a cable to hook up to the carb instead of the solid rod, but it didn't really fit the firewall right in this car. It placed the pedal in a rather funky position elevated off the floorboard and would have required some firewall surgery to make it all work right. So much for the "easy" fix. The pictures below give a great overview of the problem that needs to be solved here. You can see the dual carbs, the linkage connecting the carbs, the linkage attachment point on the front carb, the original throttle rod just hanging out next to the intake manifold, and the three mounting bolts coming through the firewall where the original throttle rod comes through.
After pondering all this for a while, I decided to try and figure out a low cost and easy way to custom fabricate something that would make this all work. My fundamental goal was to use the factory gas pedal arrangement all the way up to the point where it entered the engine compartment - it works just fine, fits the interior of the car nicely, and I like the old "floor mounted" gas pedals the pivot at the bottom. That meant everything from the firewall to the carb hookup point was fair game. Another very important goal was to not require modifications to the firewall, intake manifold, and carbs. That means no drilling of new holes and that I needed to use existing mount points for everything I needed to bolt down. It was pretty obvious that I would be using some form a throttle cable to control the carbs, so going with that line of thinking I had four basic pieces of the system to figure out - A mounting point on the engine for the throttle cable, a throttle cable, a connection between the factory gas pedal inside the car and the throttle cable that would be attached to the firewall, and another mounting point for the throttle cable at or near the firewall. Based on the descriptions and pictures above, can you see a solution in your mind? It's really all there right in front of you - all you need to do is picture a few custom machined parts to make it all work as if it was put there by the factory. :-)
Engine Mount for Throttle Cable
First up was the engine mount point for the throttle cable. I had already built a custom engine mounted throttle bracket for my 1975 Suburban, so I'm very familiar with how that end of the system works and how to build what I needed out of some basic metal bits. In this case, there are two changes from the previous design. First, I will be mounting the bracket to the rear of the carb instead of using the two on the same side as the linkage. This is because the front mounting stud by the linkage is very height constrained due to the shape of the carb casting. Second, the throttle linkage on these carbs is reversed from the setup on my previous installation, so the throttle linkage will have to go below the base and the springs above the base.
Note that you can easily add multiple parallel slots and use them to hookup various things to the carb linkage, such as cruise control, transmission kickdown linkage, or whatever else you needed. This is exactly what I did on my Suburban and the final hookup at the carb is via a simple piece of all-thread with spacers, a few washers, and some nuts. Simple, but functional.
Here's the actual bracket that resulted. After studying the Lokar cable more, I realized it has a lot of adjustments at one end already, and thus I could skip the adjustable L-bracket and just create a fixed bracket by bending the metal base up and welding in the needed supports. The photo sequence shows the bracket as it evolved while I was testing it - the amount of flex in the simply unit was disturbing enough to warrant more support, and once I broke out the welder and was committed to grinding shaping stuff, I just decided to do it "right" and make it plenty strong by adding as much support as I feasibly could.
Next up was the cable itself. After some poking about, I found some really sweet "cut to length" throttle cables from a place called Lokar - they market some nifty stuff to the "Hot Rod" crowd and those guys are all about custom fabrication and cool looks. The part I need is just a phone call away, so the design work is done here and I have one part of the system solved.
Throttle Cable to Gas Pedal
This is the most challenging piece of the puzzle. I needed to figure out a way to hook up the new throttle cable to something similar to the original throttle rod, but somewhere around the firewall. The actual throttle cable hookup (see next design section) would be simple once I figured out this piece - it's basically the same idea as the engine mount for the throttle cable - so I wasn't too worried about that. I was originally hoping that at least part of the original throttle rod arrangement would be used and that I would be using an "arm" mounted on the rod to hook up the throttle cable to.
The thing that stood out as a big problem to be solved was that the original rod was supported in only two locations and it used them to center itself and generally work properly - one was on the inside of the firewall where a rubber/nylon bushing is used as a pivot point for the linkage. The other was the hookup on the carb itself. Since that second mounting point was now gone, I had to figure out a new way to support and control the throttle rod where it comes out of the firewall. Also, this new system had to control the "in and out" movement of the throttle rod at the firewall - previously the carb attaching point had prevented the rod from sliding into and out of the firewall and messing with the gas pedal linkage on the inside of the firewall. Binding of the linkage would be a very bad thing and having the gas pedal move around under load would be disconcerting to say the least. I wanted to re-use the gas pedal, actuating rod, and lever that attached it to the throttle rod, so some the hookup from the lever to the throttle rod had to be the same and in the same approximate position from the firewall. Another problem was that the original throttle rod was not level/square with the firewall - it was inclined at about a 15 degree angle because the carb was higher than the mount points on the firewall. This made re-use of the original bushing and support piece inside the firewall questionable, if not impossible.
I knew I would need a bearing of some sort to support the throttle rod and allow it to turn smoothly, but getting my mind around the logistics took some doing. The epiphany I had was that the existing throttle mounting bolts for the inside support bushing bracket on the inside of the firewall surround the throttle rod and poke through the firewall into the engine compartment. They go through a horseshoe shaped brace that is welded to the firewall that has three threaded holes in it. I could use these three threaded holes combined with longer bolts and nuts for supporting the replacement mechanism and thus giving a way to mount whatever I needed to on both sides of the firewall without making any changes to the firewall. This, some form of mounting plate was easily doable, and I could commence with finding a suitable bearing. I found Boca Bearings on the web - you just specify the ID, OD, width, and style you want - and selected part #R1614-ZZ, a nice sealed bearing for about $9. That bearing became one of the "fixed" points in the design - like the firewall, it cannot be changed very easily - so I worked through the rest of the design with that in mind.
All this left me with the following design considerations:
At this point, my original plan was to re-use part of the original throttle rod and to try and hack it up to make it work. The diagrams below are my original plans for mounting a sealed bearing on the rod, and using a small collar with a set screw along with a simple mounting plate to hold it all in place. The collar/bearing/plate arrangement provides support in both the fore-aft and the up/down/sideways directions for the throttle rod with relative ease. The collar with setscrew and mounting plate should be reasonably easy to make at home. I had originally decided that idea #2 on the second graphic was the way to go. I left the drawing for idea #1 here because they contain valuable details for the L-bracket and throttle cable hookup and for comparison purposes. A "bumped out" area of the plate can be appropriately slotted and an adjustable L-bracket mounted to it so you can mount the throttle cable and adjust the distance from the cable mount to the "third arm" pickup point. A subtle but important detail on the second graphic is that the throttle cable mounts to the firewall side of the "third arm". This should place keep the throttle cable out of the way of the return spring and the automatic trans kickdown linkage, as well as better align the inner throttle cable with a reasonably sized L-bracket that the outer throttle cable shell will mount to. Better alignment means less frictional loss, less wear and tear, and a smoother throttle overall. That's all good stuff, and it packages the linkage closer to the firewall to keep it out of the way.
I had planned on a simple hookup to the throttle linkage right where the original throttle rod has a V-shaped piece of flat metal welded to it to provide a mount point for the throttle return spring and for the kick-down linkage for an automatic trans. If I simply weld a third piece of flat metal to the existing V arrangement, then I have an easy to get at point to mount a throttle cable to that will work like a carb linkage in reverse. As it rotates, the end of the arm will travel along an arc. I could put a bracket a bit away from it that the outer part of the throttle cable mounts to, and then mount the inner part of the cable to my new third piece of metal. The rod will turn and the cable will be pulled in and out based on where the gas pedal is. After this is done, the original rod can be cut off just past where the V shaped piece of metal is welded onto it - we don't need the rest of it. Lastly, I would need to make the mount point on the "third arm" adjustable so I can adjust the distance from the throttle rod centerline. Why? This controls the amount of "travel" in the throttle cable so you can match it to your carb setup - see the third drawing for more information. If this had worked, the side benefit to this idea was that there would have been no need to mess with the kick-down linkage as the relationship between the gas pedal and the kickdown linkage would not have been altered in any way.
I was also thinking that I could machine a small cover to fit over this entire thing and mount to the three extended mounting bolts. That would make it look much "cleaner" and keep some more dirt and crud off the linkage. You'd have to leave the proper cut-outs for the various bits to exit the cover and not rub on anything, but that's only three slots. You could also opt to polish/plate/paint the parts that are there and leave them exposed. I decided to take the "exposed" route with my first attempts here, partly because I need to keep an eye on things over time and partly because I think nicely detailed mechanical linkages look neat. I'm just an engineer at heart, I guess, and pulleys, levers, gears, and bearings are cool!
The plans have changed a good bit since then as I found problems with the original design, but the details on distance vs. angle vs. throw are incorporated into the final design. The lesson here is don't be afraid to do some brainstorming and put some ideas down on paper. Sometimes they work, sometimes they don't, and sometimes the act of writing it down and diagramming it out help make the a better solution very obvious to you.
The next pieces of the puzzle came after I purchased the bearing. These are both detailed in the graphic below. The first issue was that the only readily available bearings had an inside diameter slightly larger than the throttle shaft. That meant the collar for the setscrew needed to have an extension that is a smaller diameter that will fit inside the bearing and give it the required support so it does not flop around on the shaft - this type of part would require a metal lathe to create. The other issue is that the outer diameter of the bearing was not fully supported by the plate I have planned - it could have easily slid around as it was only supported by the pressure of holding it against the firewall. To fix this, I was planning on welding three small "ears" onto the back side of the plate to locate the bearing and hold it in place once this is all assembled on the car.
I ran into several snags while working through this that caused me to change my design. I found out that the existing throttle rod had a bend in it that seemed to be somewhat intentional, and that would have made the inside piece move in odd ways of it was fixed at the firewall. Also, the fact that the inside support and bushing was inclined at about 15 degrees made it all not work out that way I had hoped, and I had to scrap my idea of using any piece of the original throttle rod and make my own from scratch. At some point I also realized that the pull that the gas pedal would place on the throttle rod would be non-trivial, so I decided to add a second bearing on a new inner support piece as well to provide a solid mount for the throttle rod. Lastly, I realized that the way the bearing would sit against the firewall would not provide the support I was intending because the hole in the firewall was bigger than I had originally thought. This meant I would need a second inner plate to bear against the firewall to seal the unit, and provide support to the bearing as well.
After lots of head scratching, buying a mini-lathe so I could do this right, turning and mocking up some parts to see how they work, and more head scratching, I arrived at the design below for the throttle shaft portion of the overall linkage. I think this design is more than adequate for the task at hand. The diagram is not to scale, but rather, is a combined drawing of all the separate - and more or less impossible to decipher for anyone but me - drawings I made by hand as I created this. There are a few more pieces in here that I originally intended, but the overall package is quite compact and only sticks out from the firewall about 3/4". They parts are also reasonably easy to create once you figure out how to use a lathe properly. Note that this does not detail the throttle cable brackets on the engine or what goes on the throttle cable mounting plate that gets mounted to the firewall as part of this - I'll document that after I get it made and working. I'll also take some pictures of this in action when I get a chance.
The outer mounting plate will need to have three mounting holes that matched the mounting holes in the horseshoe bracket on the firewall and have a properly sized hole in it so that the throttle shaft sticks through with minimal clearance. It would have a recess in it sized for the bearing and about half the depth of the bearing, and would be cut such that only the outer race of the bearing touched it and the inner bearing race was free to spin.
The inner mounting plate was needed to have three mounting holes that matched the mounting holes in the horseshoe bracket on the firewall, as well as a similar recess for the bearing on the side that mated with the outer mount plate - the recess depths needed to be sized so that the plates touched and fully encased the bearing when mounted on the car. The firewall side of the inner plate needed to have a bulge that fit inside the horseshoe shaped mounting brace on the firewall but still rested against the original rubber bushing around the throttle rod hole. This combination of details should provide simple and adequate sealing of the throttle rod assembly against the firewall. Also, the center hole in the inner plate needs to be sized to clear the collar and setscrew assembly.
The throttle rod itself has to be cut down in several steps, with the largest section being on the outside. The part where the bearings ride would be turned to a diameter matching the ID of the bearings, and a simple collar with setscrew machined to slide over the throttle rod. This collar and setscrew assembly clamps the bearing onto the throttle rod at the proper location once the throttle rod is inserted through the output mounting plate. The diameter of the outer section of the throttle rod is relatively unimportant as long as it roughly matches the size of the hole in the outer mounting plate and is clean and round. The diameter of the innermost section must match the diameter of the round hole in the gas pedal lever, and then the innermost section of the throttle rod much be filed to create a "half-moon" shape that will fit into the original gas pedal lever. At a suitable location outside the outer mounting plate, and small lever will be welded to the throttle rod to provide an attaching point for the throttle cable.
The inner bearing support is machined like a cup with an outer flange large enough to fit over the same mounting bolts as the inner and outer mounting plates, and the inner area bored out to provide a "non-contact" clearance for the throttle rod. A small recess in the end away from the mounting flange - similar to the bearing recesses in the inner and outer mounting plates - would center and hold the inner bearing. A bearing plate would then be machined to fit over the bearing and hold it in place via small bolts that went into drilled and tapped holes in the end of the inner bearing support. The throttle rod would stick out through the inner end of the inner bearing support and the gas pedal lever would be attached there.
Firewall Mount for Throttle Cable
This is basically the reverse of the engine mount plate for the throttle cable. You have a mounting plate with an elongated area on it. The elongated area is in line with the lever you want to hook the cable to and has a slot for mounting an L-bracket to it that in turn has the throttle cable mounted to it. In this case, the mounting is via the same three bolts that hold the inner and outer throttle rod mounting plates to the firewall, the elongated area goes out towards the driver's side of the car, and the lever will be sticking off the top section of the the throttle rod and will pull from the passenger's side to the driver's side as the gas pedal is depressed.
Here are some pictures of the firewall throttle bracket and the throttle shaft assembly. As with the engine throttle bracket, I decided to use a fixed mount instead of an adjustable L-bracket and use the adjustment capability in the Lokar throttle cable instead.
As designed, there are a number of parts here, some of which must be installed in a specific order for them to work. To help keep confusion to a minimum, here is the assembly procedure for this pile of parts.
This stuff is all pretty readily available from various sources. Probably the hardest piece to obtain is the 3.5" diameter steel rod you will need for the inside bearing support - solid steel this big is heavy and hard to cut.
Basically, you need a mini-lathe plus a few other common machine tools are all you need. You do not need a mill to do this project. Also, the welder is only needed for one piece - welding the throttle shaft arm onto the end of the throttle shaft. This can be done by any reasonably competent welder and can easily be outsourced to a local welding shop if you do not have a welder.
Fill in construction details here.
Page last updated 01/02/2009 01:51:39 PM