The Stearman engine control rod installation for the W-670 is pretty straight forward from the front cockpit throttle quadrant to the firewall. The mixture rod goes through the upper-inboard hole in the fire from the fustto the second belle wall, and the throttle rod goes through the lower outboard bole.

Now it starts to get a little nonintuitive and the line drawings in the manuals that try to show the 3-D representation of the rods and bellcranks aren’t much help, and tend to make one’s eyes cross.

Fig.1 - Two engine control rods crossing as they go from the first to the second bellcrank.

Fig.1 – Two engine control rods crossing as they go from the first to the second bellcrank.

The key bit of information that one needs to know is that the two control rods that go from the first bellcrank on the engine side of the fu-ewall, CROSS one another before they get to the second bell crank. Then the two rods from the second bellcrank, CROSS again before they get to the idler arms. The mixture control rod attaches to the top bellcrank as it comes through the firewall, then goes next to the bottom of the second bellcrank, and from there to the top idler arm.

Fig.2 - Orientation of the bellcranks so that the mixture rod is next to the firewall.

Fig.2 – Orientation of the bellcranks so that the
mixture rod is next to the firewall.

The bellcranks on the fuewall are positioned so that the mixture control rod is positioned closest to the firewall, and the throttle rod is positioned away from the firewall.

The reason seems to be that having the mixture control rod from the bottom of the second bellcrank to the top idler arm provides more clearance above the engine mount tube element.

Another confusing factor is that the controls can be made to function with the rods going straight across and not crossing over twice. This is the way that Boeing did it, and it does provide better clearance for the mixture rod.

 

Fig.3 - Arrangement for the Carburetor Heat Control Rods.

Fig.3 – Arrangement for the Carburetor Heat Control Rods.

Fig.4 - Control arm on the heat control valve shown pointing back towards the firewall for “cold” operation. Arm points forward for “Carburetor Heat On”.

Fig.4 – Control arm on the heat control valve
shown pointing back towards the firewall for
“cold” operation. Arm points forward for “Carburetor Heat On”.

These pictures and details are for the W-670 installation. The carburetor heat control rod goes through two bellcranks and is also pretty straight forward. However, it is possible to mis-position the bellcranks so that the motion of the rod leading to the heat flapper is reversed. This results in having carburetor heat on when the control handle is in the cold position. I would rather not go into how I know for sure that this can happen.