The last lingering issue with my AC set up is figuring out how to boost the idle speed when the compressor is engaged. I really wanted to have it automatically go to a set speed and stay there. Unfortunately that is very difficult to do with sidefraft carbs. First, you are moving 4 large throttle plates, in my case 4 x 40mm. Even a small variance results in a significantly different idle speed. Second, anything powerful enough to snap the throttle open from rest results in tension in the system. When this tension is released, the throttle goes to a different position.
My first solution was to use a cable starter solenoid from a lawn tractor. It worked well in that it had no problem opening the throttle. Tension build up led to raising the idle to 1200 rpm with the throttle at rest but, if you blipped the throttle, the idle then went to 3500!
Next I tried a hand throttle using a vernier cable control from Aircraft Spruce. Again, system tension resulted in idle speed changing if the hand throttle was set with the throttle released.
Frankly, I gave up. I have now fitted a standard automotive AC idle solenoid with an adjustable positive stop. It’s not powerful enough to move the throttle by itself but it will hold a consistent speed. Most of the time you are driving on the throttle and it doesn’t matter. If I’m sitting at a light when the AC engages, I just have to blip the throttle to allow the solenoid to work.
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