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· Master Story Teller & Fabricator
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Discussion Starter · #1 ·
I just received a GT gas pedal to experiment with, so that I can try to decrease how quickly my GT's solid linkage opens the throttle on my single side draft carb set up. I'm about ready to try to rebend it or go nuclear with a chop off and weld on something that will decrease how much the linkage moves, but before I do that, the thought occurs to me that there are two, possibly 3, adjustment points in the GT solid linkage set up: The threaded rod at the gas pedal, the other holes in the gas pedal, and the rod along the firewall.

The thought occurs to me: Why does the rod along the firewall have adjustment potential?

I can understand the fine tuning potential of the threaded rod when, say, switching from the Solex to a Weber or when adjusting the height and range of throw of the gas pedal, but the one along the firewall seems to be screwed all the way together on every GT I can remember. Opel worked out the length needed for the other rods that aren't adjustable, why is the firewall rod not also a set length?

Okay, now let's consider the current set up on my car. I'm using a single side draft carb on a Midikit manifold with the new chromed throttle linkage that recently became available, plus a special length linkage rod made just for the SSD set up that goes from the passenger side pivot to the carb. The special rod for the SSD has an added feature that the normal length rod that the chromed linkage set comes with does not: It's also adjustable.

So, why have I started this thread? Well, the linkage opens the throttle on the carb WAY too quickly. Unmodified, the first 1/2" of gas pedal movement(at the pedal rubber) would have you at 60mph in no time. There was no fine feathering of the enhanced throttle response of the side draft. Totally annoying when limping along in traffic. So I drilled a hole in the gas pedal lever, where the threaded rod mounts, 3/4" closer to the gas pedal's pivot and this improved things greatly. Now I can push the pedal a whole inch before I'm at full throttle! Yes, I'm exaggerating.

But I want even more feather to my throttle response. Double what I currently have would be perfect. So, this has me thinking about modding a pedal and other things. Now, they sell this linkage and special rod for guys with SSD's, right? I ask Do all those guys also have touchy gas pedals? Do they have to keep one foot on the brake to dampen the sudden acceleration? Wouldn't it have been smart for the linkage rod maker's to make the various levers on the linkage longer or shorter as needed to soften the throttle response? Or make them adjustable?

There's that adjust word again. Thinking about how the rate of movement changes depending on where in their rotation arc the levers are at, I'm drawn to why that firewall rod is adjustable again. What is it that I, personally, want? Well, almost all the acceleration and speed occurs during, say, the first 1/4 of the throttle opening(personal observation), after that it's mostly all watching your speedo slowly go over 100mph. I don't go there. Mostly I drive around at 35-40mph average speed. I want my fine speed adjustment during that first 1/4 of the throttle opening and then after that I don't give a fig.

So, here's a question: Has anyone messed around with various adjustment positions of the solid linkage and what sorts of different behavior have you discovered?

I'm going to go play with my gas pedal and take some pics and make some drawings of my linkage so that I can try to find an adjustment method that will lead me to throttle response heaven.

Talk about any related subject you want......
 
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Gordon
I would go step by step and screw the throttle linkage at the firewall for a longer way :yup:
Also the vertical linkage next the pedal is adjustable!
But i also think you have now much more power as your 2.0 without piston rings.
So you must find a new feeling for the BEAST now!!!:drive:
 

· Super Moderator
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Gordo, whenever I build a custom throttle linkage I always give myself multiple options for pedal travel and return spring tension because every situation is different and every driver has different likes.

Example below.
 

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· Master Story Teller & Fabricator
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Discussion Starter · #5 ·
Yeah! Now we're talkin'!


:yup:


Below are some pics of the angles that my linkage is currently at. I've got some readjusting to do. Right now the pedal fully opens the carb with 1" of space between the floor and the pedal. There's no over-travel protection in this system. What happens if I squash the pedal to the floor that last 1"? Stress the linkage? Break the carb? However, the long lever at the back of the driver's side firewall hits the side of the footwell with a nice slapping sound at about full throttle, so that is my over travel protection.

This is hard to describe to those that don't have the adjustable linkage at the carb, but you can simultaneously adjust the spring tension and the lever position in relation to the rod that goes through that L-bar that turns your throttle. I like a firm gas pedal, so I have the spring pulled up a bit at idle. The spring lever and the lever that moves the firewall rod are locked together, though, so by lifting on the spring I'm shoving the firewall rod to the right, which is probably why the big lever is slapping the footwell. The solution here is a shorter spring going from the body to the spring lever, then I wouldn't have to have that spring lever lifted and the firewall rod shifted to the right.

Like I said, I've got some readjusting to do and maybe some modding, I wanted to get your thoughts.

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· Master Story Teller & Fabricator
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Discussion Starter · #6 ·
I've drawn up a schematic of the possible linkage lever positions for the carb actuator rod. Imagine that you are looking at the rod from the carburetor. The rod is the center of each diagram and the lever I have drawn in red is the lever that is moved by the firewall rod. The carb's throttle has about 90* of rotation, so the lever sweeps over a 90* arc. How fast the throttle opens depends on where in the 360* the lever starts and ends at. As you can see in the drawing on the left of each picture, if the lever starts at point A and rotates 45* to point B, the horizontal movement of the firewall rod is 4 "units". But if the lever starts at point B and rotates to point C, the horizontal movement of the firewall rod is 8 units. Other scenarios follow in the other pictures.

I initially named my photos "Fast early throttle, slow late" ; "balanced" ; etc., but now as I post this I'm not so sure. Remember, it's the firewall rod that moves the lever, not the lever moving the rod, so I may actually reversed in my thinking. What I am searching for is the scenario that will give me the slowest early throttle opening during the first half of my gas pedal's movement. I shall post the pics for you and I to ponder......









 
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V (Velocity) equitation is simple: V=d (distance)/t (time). To get variable speed of throttle opening you can change the time (how fast you are pressing the throttle pedal) or the distance. In your case, you want to have variable distance between carb throttle shaft and where the firewall rod is attached.

Smaller distance will give you quicker response, longer distance will give you slower response. This is achieved via off-center pulley as shown in the picture. As the pulley rotates, it changes the radius and the throttle response. For that purpose you need to change the linkage to cable system.

http://www.weberperformance.com.au/product_info.php?products_id=837

I have also attached picture of Otto's similar solution.
 

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· Super Moderator
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I have also attached picture of Otto's similar solution.
PJ, that part is just a stock Manta/Ascona throttle linkage piece. I've used them on some of my custom linkages as well.
 

· Master Story Teller & Fabricator
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17,656 Posts
Discussion Starter · #10 · (Edited)
I got the throttle linkage and the carb to behave like a normal street car. I started at the carburetor and worked my way back to the pedal. I readjusted the carburetor linkage rod so that it's lever which is actuated by the firewall rod is rotated all the way horizontal to the passenger side. This, however, caused the spring lever to point virtually straight down. I rebent the linkage spring to be about 3/4" shorter to keep the linkage slightly under tension when the throttle is closed.

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This worked pretty well and the pedal/throttle response was almost normal, but I still wanted more. Alas, there was no more adjustment possible with the linkage in it's stock state. So I decided to lengthen the driver's side linkage rod lever that is moved by the threaded rod and pedal. I had a Cannon side draft linkage lever that normally clamps to a rod and chopped off half of the "clamp" and squeezed the remaining rod hole so that it fit nicely over the base of the pivot ball on the driver's side linkage. Then I removed the linkage ball that the threaded rod attaches to and bolted the Cannon lever to that location. I enlarged the two remaining holes in the Cannon lever and mounted the ball for the threaded rod in that new farther out location. I had to bend the flat part of the gas pedal lever(carefully!) sideways so that the threaded rod moved up and down in a straight angle in relation to the now angled pedal lever.

Success! Lots of pedal travel and nice feathering of the early throttle actuation.

:veryhappy


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