Hi DKVEURO,
If the warm-up compensator is allowing too much control pressure on the top of the timing piston the piston will go down further than it should, allowing the mixture to be too lean. Pressing down on the air plate counter-acts the pressure the warm-up compensator is giving the top of the timing piston by providing a pressure against the bottom of the timing piston.
Keep in mind that at idle, the throttle is closed and virtually NO air is getting into the engine via the throttle. The idle air circuit provides idle air and idle speed control by modulating the air quantity via the idle speed air distributor. So what we are controlling here is the idle FUEL quantity by slightly pushing down on the air plate. When the air plate goes down the timing piston goes up because of the pivot point in the middle of the arm.
This test is an easy way to check out the warm-up compensator. They usually fail by allowing too little pressure at the top of the timing piston.
Hope this explanation clarifies the issue for you!
