Some time back, I replaced the Bing carb on my STOL 750/Jabiru 3300 with a Sonex/AeroConversion "AeroInjector" throttle body carb.  The AeroInjector's throttle body slide operates in reverse to most other carburetors in that the throttle cable pushes in to close the slide and reduce the rpm's to idle and the cable pulls the slide open for full throttle.  Typically, most builders use an AeroConversion throttle quadrant to reverse the cable direction.  By attaching the throttle cable below the throttle handle's pivot point, the cable moves opposite to the direction of the quadrant's throttle handle, i.e., pushing the throttle handle forward opens the throttle and pulling it back closes the throttle, or in other words, the throttle handle moves the same directions with the same response as all other throttles do.

I also installed a McFarlane "Vernier Assist" mixture control a while back.  This lets you push-pull the mixture cable as is used in most carb installations, but the Vernier Assist lets you twist the mixture knob to fine-tune the mixture adjustment.  This works really well and I can easily regulate my fuel flow as precisely as 0.1 gallon increments!

Since I liked the Vernier Assist mixture control so well, I decided to install McFarlane's Vernier Assist throttle control:

It works similarly to the mixture control, but has a knurled friction ring that you can twist to stiffen up the throttle control, but still has the Vernier Assist for fine-tuning adjustments of rpm's.  The problem was, it is a conventional throttle in the sense that pushing in opens the throttle and pulling back closes it.  I would have to reverse the throttle cable direction for correct operation of the AeroConversion throttle body carburetor. So, I decided to construct a firewall-mounted throttle reversing bell crank.  Here is what I designed and built:

I got a Mr. Gasket #1523 Carburetor Bell Crank Kit and constructed my bell crank from the kit.  It includes bushings for the bell crank which fit an AN-5 bolt with a metal lock nut.  I riveted the bell crank arms to the pivot wheel rather than use the kit's bolts. The vertical carb cable only needs about 1.3" of travel.  Obviously, it is desirable to have more travel on the throttle control, so I figured out the ratios of the two arm's lengths to give me slightly under 3" of throttle control travel.

I fabricated the mount from 3/16" 6061T6 and use 1/8" material for the brackets holding the terminating ends of the cables' conduits.  The horizontal cable runs from the throttle to the bell crank arm and is attached with a cable "B" nut.  The vertical cable is similarly attached to the shorter arm and is also attached with a cable "B" nut.  I bolted the entire assembly to the firewall.  It is a very solid and secure mount.  The throttle works very smoothly - there is no slop in the system at all.  Although the throttle cable easily pushes the bell crank arm forward, I installed a tension spring on the arm to assist the cable when advancing the throttle.  It also provides additional safety in that if the throttle cable broke or became detached, the spring would tend to open the throttle and not let it fall closed.

I'm very happy with the final result.  The ergonomics are excellent - when you lay your hand on the throttle, the knurled friction ring falls right under my fingertips.  I can easily slide the throttle in-and-out to roughly where I want the rpm's, then give the friction ring a slight twist to stiffen it up, and then twist the vernier assist knob to fine-tune my setting.

I thought this might be of interest to anyone who has to reverse the direction of a control cable.



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Comment by John Austin on February 25, 2020 at 9:45am

John Heather brought to my attention an important point:  the vernier-assist function works best with the friction twisted to the max.  If you have no friction on the throttle, the vernier-assist won't work. In practical use, I found that moderate amounts of friction will allow the vernier-assist to advance the throttle, but is less-precise or slips when trying to retard the throttle with the vernier-assist.  In my case, this is probably because I have a spring to bias the bell crank towards open throttle should the throttle cable break, and the constant spring tension assists the vernier-assist towards advancing the throttle whereas it slips in the other direction because it is "fighting" the spring.  Bottom line, if twisting the vernier-assist and it's not very responsive, give the friction ring a twist!

Comment by Kevin Powell on December 1, 2019 at 9:03pm


This looks great!  I saw the McFarlane vernier assist controls at Oshkosh this summer and thought, "Wow, these are nice! I want to use these.", and here you've done all the development for use with an Aeroinjector unit.  Very nice installation; thanks for this post!

Comment by John Austin on November 29, 2019 at 9:34pm

You're correct, Bob - both throttle and mixture function as normal push-pull cables.  There are no locks or release buttons.  The mixture is simply push-pull and then vernier-assist to fine tune (no friction adjustment, but usually not necessary for a mixture control).  The throttle is push-pull, vernier-assist, and has a knurled friction adjustment ring.  The nice thing is even when you stiffen-up the friction a bit, the vernier-assist still works.  I recommend buying them direct from McFarlane - mention you're an AOPA or EAA member and they'll give you an additional 5% discount. (Actually, I think they'll give the discount to anyone - they don't ask for any membership numbers, etc.!)

Try'em ... you'll like 'em!  :>)


Comment by Bob Pustell on November 29, 2019 at 9:19pm

A nice design, John. Well done! A question about the throttle and mixture cables you are using - confirm they have normal push-pull function? You don't need to push a release button in the middle to allow push-pull operation like most aircraft vernier controls? If yes, I like that a lot! An normal "button to release for push-pull" for mixure or prop RPM controls are not much of a hassle and work well, but a vernier throttle is a pain when taxiing or when flying in formation. Both those efforts require frequent power adjustments and needing to push that little release button in the middle of the throttle knob is a royal pain in the fanny. However for fine tuning cruise power and such a vernier function is da best. So, if these knobs provide unimpeded push-pull function but also provide vernier adjustment ability, I want them!

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