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AustrianVespaGuy
AustrianVespaGuy

FINALLY, EFI Throttle Linkage!

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This has taken me a full DECADE of trial-and-error-and-refinement but I have FINALLY cracked this awful nut: how to properly make a good throttle linkage for an EFI conversion!  Note that you'll still need to tailor this to your particular manifold/throttle body/mounting points/etc., but I PROMISE this will get you 90% of the way there no matter what setup you are running and I also GUARANTEE that's it's better than what you've already managed to cobble together!  So let's get started! As always, first step is to disconnect the battery, and then understand what we're trying to do and the challenges involved:

 

A.) We want a PROGRESSIVE throttle opening, just like that old 32/36 carb we ran back in the day.  That is, we want quite small amounts of throttle opening at the beginning of the pedal travel (for idle, off-idle, and cruising), and then 'progressively' more throttle plate movement towards the end of the pedal travel, so that tapping full power from cruise doesn't take that much time or effort.

B.) We want the (stock) pedal travel to MATCH the throttle opening, i.e. pedal up = throttle closed, and pedal down to the floor = throttle 100% open.  Sounds obvious, but if you've ever tried to run a plain cable straight from sidedrafts or a TB, then you've realized that the throttle(s) tend to open up all the way and hit the stop when the pedal is only about 2/3 of the way down.  Works, but sucks, and we're better than that!

C.) Form AND Function: this thing needs to work, work well, be smooth, not bind, not require a whole lot of messing or adjustment all the time, and look like it belongs in there, just like the rest of your EFI conversion, rather than some weird contraption that you cobbled together out of random bits you had lying around.

D.) Not cost a bloody fortune; we're DIYers!

 

So, in order to accomplish all of that (particularly A&B), we unfortunately need a 3-section linkage setup:

1.) A cable at on the throttle body side (basically ever EFI TB I've seen is set up for cable actuation).

2.) A throttle rod on the pedal side; that's how the 2002 was designed and again we want to avoid issues with mis-matched travel.

3.) A bellcrank mechanism to connect the two halves together AND to give us that progression for the good throttle resolution and response.

 

Let's start with the cable on the TB side, and while you can make the OEM cable for your throttle body or a bicycle cable work, I particularly like this generic throttle cable, because it comes with that adjustable eye on the end with the little set screw to clamp it to a bare cable end, and this one piece (regardless of the exact cable you use) is rather integral to this whole setup:

https://www.speedwaymotors.com/Mr-Gasket-5657-Throttle-Cable-Kit-Stainless-Steel-Braided,287962.html

image.png

 

You WILL still be responsible for making sure you can get the other end with the nub on it to somehow mate up nicely to your throttle body though!  The cable stops (preferably adjustable and with both ends mounted to your intake manifold to avoid any throttle movement due to the engine shaking relative to the chassis) I will also leave up to you to find the best spots/mounting methods for your own car.

 

Next, let's get to the real meat of the whole thing, the bellcrank.  I've seen (and made, and used) all kinds of various bellcranks, with varying degrees of success.  But just the other day I found THE THING that made me revisit this whole linkage topic (yet again), and which culminated in me typing all this right now.  And here she is, for a staggeringly affordable $20, and you can even pick your color!

https://www.speedwaymotors.com/Aluminum-Bell-Crank-with-Bearing-Center,256119.html?sku=91055300-BLK

image.png

Note that it's pre-tapped with 10-32 threads; that'll be important later.  For mounting it, you'll need (x2, one for the pivot and one for the cable eye) 1/4" x 1/4" x 10-32 shoulder bolts.  I got the from McMaster-Carr, P/N 91259A171:

https://www.mcmaster.com/screws/shoulder-screws/alloy-steel-shoulder-screws-9/

As to WHERE to mount it, I re-used an L-bracket from a previous linkage iteration that I had on top of my brake booster support, but for most people I think the best method would be to use a piece of flat bar and come straight up from the big bolt at the top of the pedal box that holds it to the booster support.  If you happened to have things apart, I would think you could also drill/tap the mounting hole straight into the upper part of the booster support itself, but this looks pretty hard to do with the bracket in the car.  And if like me you're a proper engineer who only owns Metric tools on principal, fortunately just like -40 degrees, SAE and metric happily meet here, and M5x0.8 is actually the same thread as 10-32! Or just use a thru-hole and a nut, lots of ways to make this work. Here's an interim shot of my new bellcrank in and hooked up to the throttle cable:

image.png

Finally, let's do the throttle rod.  For the ends, these nice little ball joints are cheap, thread right into the bellcrank, and fit nicely through the pedal arm (secured with 2x jam 10-32 jam nuts). 2 required, one for each end of the throttle rod:

https://www.speedwaymotors.com/QA1-BJGR3-Grommet-style-Staked-Ball-Joint-10-32-RH-Thread,260909.html

image.png image.png

 

The rod itself is easy because it's a straight shot, but it needs to have 10-32 (or M5x0.8) threaded ends to screw the ball joints on to and it's critical to get very close to the exact right LENGTH, so measure from the pedal arm to the 2nd-from-the-outer hole in the bellcrank first!  (Use that second hole so that you can 'tune' the travel a little in either direction by moving to either the middle or outer holes; in my experience, you'd be very unlikely to use the inner 2 holes).One option is to use 10-32 threaded rod, but I couldn't find any locally, so I just bought some 3/16" plain steel rod, cut it to length, and tapped both ends.  Perhaps though, hindsight being 20/20 and all, an even nicer solution would probably be to buy this swedged tube at the right (~16"?) length, an then use the male 10-32 linkage studs (one RH thread and one LH thread!) in both ends:

https://www.speedwaymotors.com/Swedged-Aluminum-Throttle-Linkage-Tubes-3-16-Inch-10-32-Thread,3944.html?sku=94055301-16

https://www.speedwaymotors.com/Speedway-Steel-3-16-Inch-RH-Male-Throttle-Linkage-Rod-Ends-with-Stud,34302.html

https://www.speedwaymotors.com/Speedway-Steel-3-16-Inch-LH-Male-Throttle-Linkage-Rod-Ends-with-Stud,29691.html

I'll let you try that route if you want, but not worth me placing a second order now that mine's already done! ;)

 

And here's the final product! Again if you're just starting fresh, I'll bet you could make it even cleaner than mine, but either way, I FINALLY have a throttle linkage I am genuinely, completely happy with!IMG_20200819_125243.jpg

 

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Nice job and great write-up! This might just come in handy for me when I get around to doing this. Thanks for sharing!

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