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I guess I'm confused.
According to the instructions, open throttle, close choke, close throttle should hold the throttle open for fast idle. Mine just closes to the curb idle screw. (Sorry haven't measured the opening on curb idle)

That's why I curious about the theory behind the innards of the choke housing. What actually makes what happen.

I thought that the mechanism for fast idle basically acted like a switch when you fully depress the gas pedal. Press pedal to floor and fast idle is enabled if the choke is closed.

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Dave
If all you want to know is how the choke works, it is all mechanical by the spring inside the black plastic housing. When the spring is cold the choke is closed and as it heats up the spring opens the choke. I recently rebuilt the carb on my 51 and the directions with the kit, the service manual and the service bulletin were obviously written by different people as they use different names of the same parts and it takes a lot of close reading to figure them out. I spent a lot of hours rebuilding the carb which should have taken a lot less time.
Frank

Ok I'm going to try and make this as clear as I can.
I have attached a color coded graphic.

Green = Unloader
Blue = Fast Idle Linkage
Orange = Choke Cam
Purple = Choke cam "spring plate" (Not sure of the nomenclature)


Is there supposed to be some kind of linkage between The fast idle linkage and the choke cam where I have circled in red?

What causes the fast idle linkage to go down when the choke is closed? On mine there is no linkage.

For the fast Idle adjustment, the choke spring isn't in the picture. The choke thermostatic housing is removed for all adjustments.

So something has to keep the Fast Idle Linkage (Blue) down somewhat when the Choke Cam lever (Orange circled in red) is in it's most clockwise position.

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The green bit is supposed to have a tab that touches the bottom of the purple bit as the choke rotates closed. This holds it and the blue bit lower than they would normally be. When the blue bit is held low, it pushes down through that bent rod onto the throttle arm.

The orange doodle is only for unloading and is not directly germane to this discussion.

The usual problem with these carbs is that if someone takes their hand and forces the throttle back to slow idle while it is still on fast idle it will go: either the bent rod or the little tab on the green thing that touches the purple thing will bend out of position and voila, no more fast idle.

Ok, that makes sense.

What then, is the mechanism that cancels fast Idle and then makes it go to curb idle. I assume the tab on the green green piece has to be kicked out of position somehow to allow the blue piece to then travel up to achieve curb idle.

Dave, I haven't really been following this closely, so forgive me if I'm restating. But are you missing the fact that a small amount of engine vacuum is admitted to the choke housing which in turn draws in air which is heated by the exhaust manifold as the engine warms up. That heated air then relaxes the coiled thermostatic spring which allows the fast idle to be disengaged?

The purple piece is cam shaped. As it rotates with the choke shaft it presents a higher profile to the green piece. When the choke opens the purple piece again presents a low profile to the green piece which allows the blue piece to come up higher and you are back to slow idle.

Further, the purple piece is spring loaded to the choke shaft so that it can but must not rotate with the choke. This allows the choke to warm up and reopen while the engine is on fast idle without losing the fast idle. You are just going to have to look at the mechanism closely.

I guess what I am missing is what exactly is the mechanical difference between fast idle engaged and disengaged.

When the choke spring relaxes, the tab lets the unloader cam fall down somewhat. But what gets the unloader cam out of the way of the fast idle linkage to allow it to travel up to curb idle? Maybe I'm not seeing the forest for the trees.

Well, I took the carb off again and made the fast idle adjustment as prescribed then the unloader adjustment.

Now the engine starts into fast idle but will never slow to curb idle. The set-screw never touches.

I've tried giving it gas, a pedal tap, gunning the engine, but it won't slow down.

I think I may have found the problem.

On the inner tab, is it supposed to have a groove like this?
I suspect it's not making contact with the cam from being worn.

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