Is Mechanical Throttle Linkage worth it?

[skidd]

Probably! But, it depends!
For me it was, but not at first.

I'd been running a cable throttle since I completed by car back in the summer of 2018. I had a steel braided Knockoff cheap throttle cable from Amazon, routed nice and smooth in a simple U turn to the passenger side as my carb was reversed on my manifold.
I had been running a vacuum secondary carb. It was one of those Summit Racing Holley 4010 Knock Off Carbs. Honestly! A really great carb. My car ran fantastic!! The throttle feel was perfect. Not to light, not to stiff. Easy to modulate, no binding. It was really well setup.

I'd been wanting to try a good mechanical secondary carb on my car, but really didn't have a good reason to swap. Until my buddy with his classic mustang topped with a super basic Holley 4160 vac secondary carb. We'd been fighting and fighting to get it to run just right.
For a laugh, we threw on my Summit racing carb, and his car ran AMAZING! No surprise really since I'd already had it pretty well tuned on my car. So, I sold him my Carb, and ordered a mechanical secondary from Summit. The Proform 600.
It's pretty much the same as the Holley Brawler. In fact, all the parts are so exactly the same, I'd put $5 that Holley re-sells the ProForm under the Bralwer badge!
I did have to adjust the butterfly valves in their bores. Like almost every carb I've ever bought, the blades are never quite tightened down the exact right place. Simple fix at least.

Last weekend, I threw on my new carb.. and now the low-quality throttle cable showed it weakness. It just could not handle the added strain of the return spring on the secondary butterflies. The cable contorted and twisted under the strain. It didn't seem to matter where I set the cable to pull on the carb linkage, it just hated the extra spring load.

I thought I might just order myself a good high quality throttle cable. Ford? Lokar.. something. But, they were all in the ball-park of $100.
While the parts for a DIY Mechanical Linkage setup cost me just over $150. So, .. Mechanical LInkage , here I come at long last!

Spent the better part of yesterday and today building it and tuning it, and getting it just right.
Adjusting the pivot point on my home-made Russ-Thompson knock off pedal.
Adjusting the starting, and ending points of each arm for maximum torque applied to the linkage at the right time.
Pinning the arms to the 3/8 shaft in just the right place.
Tweaking the lengths of the linkage arms to get the progressive Throttle Feel I wanted.

The final result, was a fantastic feeling throttle cable, with once again the exact right feedback, and progression.
It doesn't matter how many return springs I add to the carb, the linkage can handle the load easily.
The fact that all the various geometries can be tweaked, and adjusted, allowing for fine tuning of both throttle opening rate and torque applied to the throttle shaft made a huge difference.

Like so many of us on here, I read post after post after post about how much better a mechanical linkage setup was. But, given how absolutely perfect my cable setup performed, I could never quite understand what the fuss was about.
But, this was due mostly to the fact that my vac secondary carb was so easy to operate. I would assume that a stock Ford EFI setup would feel equally as smooth and light with a cable, since it was just a simple Throttle Body that needs to be operated.

However, once a carb with more return-spring pressure was in play, the simple cable setup was just not up to snuff.

So.. if you have a light-duty carb or EFI, I can say with experience, that a throttle cable can easily be made to work and to work well.
If you have a carb that needs more oomph to move, I can now say with experience that a mechanical linkage is a game-changer!!


Additionally.. one of the biggest improvements I found when trying to get all the angles and rod-lengths just right... was the angle on the 3/8 shaft of the 2 aluminum throttle arms.
Initially I had them both aligned on the rod at the exact same angle. This worked ok, but not good enough for what I expected.
What I found was happening was that as my throttle pedal was trying to push the arm PAST the apex of it's travel arc, it was losing a bit of it's torque at the same time that the secondaries were kicking into full open.
I could easily feel the onset of the secondaries (which I liked), but the transition was very strong. More than I wanted.
What I did was clock the arms a few degrees (you can just make it out in the photos below). I clocked the throttle pedal arm to be "sooner" than the carb arm.
This resulted in the carb arm being slightly past it's apex, as the pedal arm was at the top of it's apex. Thus, the pedal was applying maximum torque to the 3/8 rod at that point, as the carb arm was in the middle of it's secondaries transition.
I also had the pedal arm pivot point set closer to the 3/8 rod to accelerate the movement, which reduced some of it's ability to apply max torque to the linkage. So, maximizing the torque transition was pretty valuable.

P.S. So far, the double-pumper is proving to be a good upgrade. Only a couple of times around the block so far, but.. 3rd gear has never wanted to get up and go like it does now!

Parts List: (mostly copied from various posts within this forum)
- McMasterCarr 6072K175 Ball Joint (comes with gold coating. I used steel wool to remove the gold finish, and clear-coat paint to protect it)
- McMasterCarr 8364T2 18" x 3/8 Stainless Shaft
- McMasterCarr 93250A060 & 93250A060 12" & 18" Stainless Threaded Rod
- McMasterCarr 6072K31 Ball Joints (Also had the gold finish. I polished off the finish and painted 2 of them)
- McMasterCarr 91864A045 BlackOxide cap-head screws
- Amazon Stainless Flange Nuts (https://www.amazon.com/gp/product/B009K5YJH6/)
- Amazon Stainless 3/8 Collars (https://www.amazon.com/gp/product/B0776RHZ2P/)
- Amazon Aluminum Throttle Arms (https://www.amazon.com/gp/product/B076VNSTNS)
- Amazon Throttle Bushing (https://www.amazon.com/gp/product/B00JRCSN06)

Pivot Ends through the frame square tube for added stability. And, allowed the one pivot to be mounted inside the footbox, closer to the throttle arm.

https://www.ludicrous-speed.com/zp/zp-core/i.php?a=Shanes%20Cobra/MechanicalThrottleLinkage&i=PXL_20230312_205155328.jpg&s=595
(https://www.ludicrous-speed.com/zp/albums/Shanes%20Cobra/MechanicalThrottleLinkage/PXL_20230312_205155328.jpg)

Simple SS threaded rod with a piece of shrink tube for asthetics.

https://www.ludicrous-speed.com/zp/zp-core/i.php?a=Shanes%20Cobra/MechanicalThrottleLinkage&i=PXL_20230312_205235859.jpg&s=595
(https://www.ludicrous-speed.com/zp/albums/Shanes%20Cobra/MechanicalThrottleLinkage/PXL_20230312_205235859.jpg)

my Russ Thompson KnockOff Throttle Pedal.

https://www.ludicrous-speed.com/zp/cache/Shanes%20Cobra/MechanicalThrottleLinkage/PXL_20230312_232601135.MP_595.jpg
(https://www.ludicrous-speed.com/zp/albums/Shanes%20Cobra/MechanicalThrottleLinkage/PXL_20230312_232601135.MP.jpg)

[MB750]

Nice write up, thank you.

My intentions were to use the cable (Edelbrock AVS2 carb, FFR throttle pedal), but my beef was with how many turns it would have to take before actuating the throttle. I try keeping the total degrees of bends in a cable less than 180. This comes from my motorcycle days. It keeps the cable from having too much friction in the sleeve. The orientation in these cobras have 360 degrees worth of bends (out the footbox forward, turn around, then back to forward to pull the throttle on the carb). Too many bends, so I will use the Forte linkage.

I like how you've got one of the heim joints on the outside of the first link. That's good distribution of leverage. Did you drive a bolt completely through the 2x2 or use a threaded stud into a tapped hole?

[skidd]

Like you MB750, I hated the idea of the S-Bend throttle cable. Which is why I ran my carb backwards for years. Allowing a single U-Bend. The Summit 4010 style carb makes that dead simple to do, since the jets are in the bottom of the float bowls, not in the side meeting plate like a typical holley. In other words, I didn't have to worry about swaping jet extensions.

As for my mounting, yes drove a cap-head bolt from the back side of the 2x2 box. Needed a 3/4" hole on the inside of the cockpit to fit the cap-head. Installed with a bit of blue lock-tight to make dang sure they are not going anywhere. Then, I just plugged the holes with some 3d printed plugs.

[CaptB]

My Forte's is awesome and it was an easy install.

[ggunter]

I know for me Fortes mechanical linkage was the way to go. After originally having the Holley Sniper set up with all four barrels opening the same time, I could never get the smoothness from a cable setup. I tried the FFR cable, (unsatisfactory) (then Lokar, better but didn't give me a smooth linear feel.) Then Forte's linkage with the mechanical adjustability gave me the smoothness I was looking for. Only way to go for me. Your results may vary.

[John Ibele]

Another happy custom mechanical linkage creator here. I also benefitted from parts lists and tips / techniques here on the forum. But in the end, as you say, the key to a linkage you're happy with is a LOT of fine tuning to get linear response, freedom from binding, and adequate pedal feel - all the way from idle to WOT. Definitely doable for a first-timer, but they need to be mechanically inclined, with a pretty good idea of how forces are transferred from your foot all the way up to the carb. Post #163 (https://thefactoryfiveforum.com/showthread.php?27168-John%92s-Mk4-Build-Thread-Windshield&p=477763&viewfull=1#post477763) in my build thread has some details, but very similar to Skidd's.

[CraigS]

I agree. A linkage was a huge step for the better when I did it. One other comment. I also went from a vac secondary to mechanical secondary. I had tweaked that vac carb to the best of my abilities over 5-6 years. I swear, the mechanical secondary added 30-40hp in the 2500-3500 rpm range. I figure the vac was not opening the secondary until way too late and probably way too slowly. Since there are no accel pump squirters for the vac secondary, it makes sense to me that they can not be opened quickly, or they would create a big lean bog.

[MB750]

I did a lot of research on carbs before I pulled the trigger. I knew I wanted mechanical secondaries and a manual choke (no, I'm not 75 years old). I checked out the tried and true Holley 4bbl, but settled on the Edelbrock AVS2 because I really liked the annular accelerator pump jets in the venturis. They spray out a bunch of really small holes rather than one larger one. This atomizes the fuel better, which keeps the AFR's more in check.

I wanted mechanical secondaries for the same reason Craig said above. When I hit the gas I want all 4 barrels puking as much fuel into the engine as possible. Pretty much every motorcycle carb since the early 80's has a vacuum secondary of sorts. Technically it's a vacuum slide that keeps the velocity of the air greater to atomize the fuel better, but it acts the same as a vacuum secondary. I know this AVS2 carb does have a little vacuum flapper type thing on the top, but that can be tuned just enough so the huge gulp of air can be mitigated a bit to prevent a lean hesitation.

I'll also be tuning with a wideband o2 sensor. As long as I know what passage in the carb is at play at any time, I can tune problems out.

[J R Jones]

I agree. A linkage was a huge step for the better when I did it. One other comment. I also went from a vac secondary to mechanical secondary. I had tweaked that vac carb to the best of my abilities over 5-6 years. I swear, the mechanical secondary added 30-40hp in the 2500-3500 rpm range. I figure the vac was not opening the secondary until way too late and probably way too slowly. Since there are no accel pump squirters for the vac secondary, it makes sense to me that they can not be opened quickly, or they would create a big lean bog.

Craig,
Back in the day my Shelby GT350s ran a Holley 715 with the square center pivot floats. They worked well for me and I had no problems.
At one point with the race car, I went from the (mechanical?) secondaries to a gear drive linking both throttle shafts. Holley also made an add-on accelerator pump kit. It was a bigger diaphragm pump and extended sprayer nozzle tubes to reach into the rear venturis. An add-on double pumper.
jim

[skidd]

@MB750. That Summit carb I sold to my buddy has Annular Boosters. Pleasantly Surprisingly Great Carb! I actually considered going to an AVS2 as well, my neighbor has a good bit of experience tuning them. Alas, I have far more experience with Holleys, so I stayed with what I knew.
Besides, I already have a complete jet kit, accel pump cam kit, vac-secondary spring kit, small assortment of squirter nozels... etc.
And unfortunately, I have to jump up to over $800 to get a good Annular booster equipped Holley (Quick Fuel SS-650-AN)
now. since I live in S. Texas, I'm strongly considering swapping in a Manual Choke kit. Cable or Solenoid.. not sure yet. But, one step at a time.

[Danomatic]

Its value will be priceless when you snap a throttle cable leaving you stranded somewhere. Truth

[Ford & Jeep Fan]

I'm firmly NOT a fan of mechanical linkage. Remember the Why GM went to cable linkage back about 1970?? people got killed when the driver sided engine mount would fail and the engine would rise enough to pull the throttle wide open.
That does not mean I'd use just any old store brand cable. Lokkar or a OEM cable would be my minimum standard..

[MB750]

I'm firmly NOT a fan of mechanical linkage. Remember the Why GM went to cable linkage back about 1970?? people got killed when the driver sided engine mount would fail and the engine would rise enough to pull the throttle wide open.
That does not mean I'd use just any old store brand cable. Lokkar or a OEM cable would be my minimum standard..

I see your point, but when I'm presented with exceptions like this I run the odds on either:

My cable throttle being an issue
My motor mount breaking and causing a WFO situation

I'll stick with mechanical linkage.

[CraigS]

skidd go w/ a choke cable. This is how I did mine w/ your exact carb SS650AN.
181570
This is another shot while still electric showing the beginning of my swiss cheese flap. I did that so I could get more increased idle speed w/o going way too rich.
181571
I don't have a picture but after I got the cable working I was able to dremel a small slot in the edge of the black plastic cover for the cable to pass through. Made the carb look a lot more normal. BTW, if you ever want to go to a 750 just call QF. They will sell you a main body for your 650 that turns it into a 750 for a lot less than a new carb. I did that when I did a 408 stroker shortblock.

[Jim1855]

Just my opinions and experience.

Mechanical Linkage. Yes. It allows leverage and movement ratios to be adjusted to your application. It becomes easy, although potentially time consuming, to adjust how quickly the throttle pedal actuates the carb or EFI butterflies. This is all about throttle modulation which can determine how easy the car is to drive, all personal preferences. With a cable about the only place you can change the pull ratio is at the throttle pedal with various locations for the cable connection. Can there be problems, yup, know all about that too. Still the mechanical is the only option for me.

Chokes. Why? Are you driving in the cold? The choke blade and horn are just restrictions to the airflow. Double pumper HP series are my favorites. I found that a pump and hitting the key were all that I needed. But I did also need to feather the throttle for a minute or two, usually while I got belted in and checked systems & gauges, once driving there wasn't a problem.

Elevation. Yup it's possible with a carb. Not perfect but ran from Michigan's 700 foot average to the Colorado fourteens multiple times. Usually dropped a few jet sizes at the Front Range. Never had a problem getting to Idaho Falls.

But, we are all able to make our own decisions.

Jim

[rponfick]

I did like Skidd and Craigs and reversed the carb and used a gentle bend in the cable. One thing to remember, which I didn't, is when the car has sat for a while and you need to dribble in a little gas to start, don't dribble in the wrong throat barrels, as they are now reversed. Dribbling fuel in the secondaries does not work as well.
And, I had been asked does the carb know it is on backward, and not run?
Ralph