Thinking it through, I'm going to need to know what my pedal throw is, what the effort on the pedal is going to be, and what the effort on the carbs is going to be.
What else?
For reference, below is a throttle cam
Thinking it through, I'm going to need to know what my pedal throw is, what the effort on the pedal is going to be, and what the effort on the carbs is going to be.
What else?
For reference, below is a throttle cam
Here's a rough idea of what I've got. In the last pic, the blue parts are roughly where the linkage for the carb is
Break out your slide rule (or a flexible tape measure)
Basically you want the arc of travel of the bellcrank to be equal to the pedal throw. (or cable travel in this case).
Since the throttle blades more or less operate in a 90-degree arc, you could take the length of throw and calculate a radius that should suffice. That is, of course, assuming that the bellcrank is directly connected to the throttle shaft and not some other mechanism.
So, since 90 degrees is one quarter of a circle.... if you have a 4" cable travel, multiply that by 4 to get 16" circumference.
16 / pi = 5.09 diameter
5.09 / 2 = 2.54 radius.
That would suggest a 2.54" distance from the shaft to the groove in the bellcrank.
What kind of throttle are we operating? You mentioned carb, but what are we hooking this up to? The reason I ask is it might change my recommendation. A Qjet with it's tiny primaries is great for the above idea (although you could just hook the cable to the existing linkage), but a 110mm round throttle body might like a progressive bellcrank. With a big round TB the difference in flow between 0 degrees and 10 degrees of throttle is massive. But the difference in flow between 80-90 degrees is almost imperceptible. Having a bellcrank that has an increasing radius does two things; prevents jerky throttling off idle, and also helps to make the right foot more tied to flow than throttle position. It makes the throttle move slower at lower angles and faster at higher angles because flow is not proportional to throttle angle.
This is really up to you. ( and the engine you're driving) I had mine tip in quickly from 1/2 throttle to full.
That's because with the long stroke engine (4.17 ) finesse wasn't needed the way you need with a short stroke.
Depending on chassis needs you might need more or less control at certain parts of the throttle.
Curtis73 (Forum Supporter) said:Having a bellcrank that has an increasing radius does two things; prevents jerky throttling off idle, and also helps to make the right foot more tied to flow than throttle position. It makes the throttle move slower at lower angles and faster at higher angles because flow is not proportional to throttle angle.
This is the bit I was forgetting how it works. I probably can sit down and write up an excel sheet that gives me a rough relation between throttle position and air flow, then work backwards to find the radius.
These are for Weber 2bbls, setup to be progressive carbs. 32/34 if I remember correctly? For 200ci Ford I6
Rough progress album. https://imgur.com/a/JgHx3
Ah. I'm going to say something that will sound derogatory, but it isn't.
Get a clew, man.
You can use a fixed-radius bellcrank on one carb and a variable radius on the other. A clew is a term we use in theater rigging where you attach several ropes to one ring. Like this:
The green circle is the Clew. In your case, a clew could be used like this:
Throttle cable to a clew. The blue line is the throttle cable from the pedal. Split into two at the gray clew, each going to different bellcranks. The left crank would be the progressive throttle, the right one linear.
Or if you want zero throttle input from the left carb until a threshold is reached, you could do two linear (circular) bellcranks, but the one on the left you would make smaller, and the cable for that carb could have a slot in the clew so it doesn't engage until later. Then when it does engage, the smaller radius on the bellcrank would cause it to meet full throttle with the right carb at the same time your foot hits the floor.
It also occurs to me that I'm supplying drawings from MS Paint while you have already supplied 3D CAD renderings. That makes me simultaneously chuckle and cry inside a bit. 
Also, because I'm a nerd and I'm feeling like Buzz Killington right now....
Clew is a sailing term. The Clew is the trailing point of a sail. If you think about it, the single tie at the trailing point of a sail is countering the forces of the multiple grommets of the sail as it rises on the mast. The word clew is also used to describe where all the ropes of a hammock meet the rope you tie to a tree to suspend it.
The origin of Clew is from old English and Scottish and it means to conglomerate or store together. A ball of yarn was a Cleiwen or a Clew. As in, it's where all the yarn comes together.
It is also the origin of the word Clue; you assemble individual facts until all the clues come together to one likely conclusion.
I think we might have crossed a wire. The Webers are already progressive carbs, but as far as I know they can be set up to operate with the primary and secondary in sync which is why I mentioned that. The way I have them, and want to keep them, the primary is opened directly by the throttle linkage and the secondary opens at about 1/3-1/2 throttle from a mechanical linkage.
I want the two carbs, fore and aft, to operate in sync. Primaries both opening at the same time, secondaries both opening at the same time.
The clew stuff is interesting, though. I wonder if there's a link between Clew and Cleave.
In for twin-carbed small 6. I'd like to subscribe to your newsletter.
In reply to barefootskater :
Checked my build thread of my first car, Susie? It is very sparing on most of the details I have of her, but its good enough for an overview.
https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/susie/162875/page1/
Mr_Asa said:Thinking it through, I'm going to need to know what my pedal throw is, what the effort on the pedal is going to be, and what the effort on the carbs is going to be.
What else?
For reference, below is a throttle cam
That looks like a 944.
In reply to dean1484 :
I believe it is. I googled throttle cam and it was the first decent image I could find.
Mr_Asa said:I think we might have crossed a wire. The Webers are already progressive carbs, but as far as I know they can be set up to operate with the primary and secondary in sync which is why I mentioned that. The way I have them, and want to keep them, the primary is opened directly by the throttle linkage and the secondary opens at about 1/3-1/2 throttle from a mechanical linkage.
I want the two carbs, fore and aft, to operate in sync. Primaries both opening at the same time, secondaries both opening at the same time.
The clew stuff is interesting, though. I wonder if there's a link between Clew and Cleave.
So you want to retain the delayed/mechanical secondary on each individual carb, but have the two carbs linked 1:1 as opposed to progressive? Am I understanding that right? Each carb as an individual is progressive primary/secondary, but you want 1:1 between both mechanisms?
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