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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.