After growing weary of wasting entire test sessions "finding my needle" and missing the opportunity to swap props or pipes during race day, I recently installed a 3rd channel mixture control in my boat.
At first, I simply bolted a control arm onto the knurled adjustment screw on my OS Max 9B carb. This worked OK, but I noticed that often I couldn't get it lean enough or rich enough with the whole throw of the servo arm. On the flow meter, I could go from say 7.8 to 7.0 CFH propane @ 40" water: a range too narrow to compensate for a prop change or a different fuel tank pressure due to a different pipe.
So, while visiting Marty before the Hydro Masters, we installed an OS in-flight mixture assembly.
The brass body screws into the carb. The black plastic housing holds the needle and threads onto the helical groove in the brass piece. A black plastic arm splines onto the housing for the pushrod connection. As the plastic housing rotates, it slides the needle in and out of the carb's fuel orifice.
The assembly comes with two needles: one coarsely tapered, and one finely tapered (the fine tapered needle has a groove cut in the adjustment end).
With the coarse needle, rotating the assembly ~100 degrees would move the flow from less than 4 to way over 10. I knew immediately that this would cause each click on my transmitter to be too touchy on the water.
So, we installed the fine needle. That produced a much more appropriate range; say 10+ to 9.5, but even with both the needle and the plastic housing screwed in all the way, the flow was still over 9; too high for my engine. Range is key, but offset is also important.
The cure was to machine a little off the end of the brass helical guide and deepen the hole chamfer.
The needle was bottoming on that piece rather than the fuel orifice. With the brass piece a little shorter, the needle could reach farther into the carb's hole and meter those low flows.
Using a mill, we took a little off at a time, until finally we could get the needle to flow around 4. Backing off the setting a little bit put it in a really nice range: about 5.5 to 8.5, which is centered around where my engine likes to run, and 6 clicks translates to about 1 CFH on the meter.
All this work really paid off on the water. In a few passes, I can dial in the needle to where I want it, no matter what my prop choice, pipe choice, air conditions, % nitro, phase of the moon, etc…. With the good sensitivity, not too coarse and not too fine, and proper offset, it is a really sweet setup.
One More Trick:
People running wheel radios like my Futaba PCM1024 have a tough time using the 3rd channel needle, since the auxiliary channel knob is on the top right of the transmitter. You must take your hand off the wheel to needle! Marty suggested a fix: simply unsolder the wires from that potentiometer, and switch them with the thumb wheel adjacent to the throttle trigger that currently sets the rudder dual rate. Keep your hand on the wheel and dial the mixture with your left thumb! I haven't had a chance to do this yet, but I can't wait.
-Brian Callahan
Until next time......
