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I'm almost done rebuilding my 4100 and I cannot find the fast idle lever. I looked on Mike's Carbs and the only fast idle lever I see is the plastic peace that sits right behind the choke housing (see picture). From my understanding, that's not it. Any idea what this part is called (also pic attached)? If so, know where to find it?

Also on a side not.... the dashpot is not necessary when you have a manual (T5) transmission right? It was only for autos to help with the cruising when gears switch automatically.... right?
 

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Looks like you have your finger on the fast idle cam, it is just flipped around backwards. The dashpot is to prevent sudden deceleration so the engine doesn't stall. I don't think it's necessary on the manual transmission cars because if you let off the gas suddenly, there is still a manual connection between the engine and the drivetrain until the clutch pedal is pressed then engine RPM falls off naturally. All of my automatic cars have one.
 

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Somebody may have removed the Fast Idle Lever. The nut in the diagram is the only thing that holds it onto the throttle shaft. You'll probably need to source a used one somewhere.
 

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I'm almost done rebuilding my 4100 and I cannot find the fast idle lever. I looked on Mike's Carbs and the only fast idle lever I see is the plastic peace that sits right behind the choke housing (see picture). From my understanding, that's not it. Any idea what this part is called (also pic attached)? If so, know where to find it?

Also on a side not.... the dashpot is not necessary when you have a manual (T5) transmission right? It was only for autos to help with the cruising when gears switch automatically.... right?
The Fast Idle Lever is the plastic part you are pointing to. The piece in yellow you highlighted in the picture is the Fast Idle Adjusting Lever. It sits behind the choke under the plastic lever. It appears to be in place in the picture I can just see what appears to be it under the plastic lever. The reason you can’t find one listed is because it is considered part of the choke not the carb. If you need a one you have to get it from a parts carb or used off eBay like this ad.

https://www.ebay.com/i/193019686781?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=193019686781&targetid=593772286733&device=t&mktype=pla&googleloc=9011694&poi=&campaignid=2086088304&mkgroupid=77258593396&rlsatarget=pla-593772286733&abcId=1141016&merchantid=6296724&gclid=CjwKCAjwldHsBRAoEiwAd0JybVGJRkZEgD-kgPXRUVhjXoVRnbT3TH0RPGhy2uWHjaIXaurhOjRL6RoCg2wQAvD_BwE

Or a carb rebuilder like this one:
65-69 Autolite 2100/4100 Carburetor Fast Idle Cam Limited Quantities Call Before Ordering! [CAG-FICL-4100] - $19.99 : Champion Carburetor, Your Carburetor & Parts Specialist

Note that there are different versions used in different years.

The purpose of a dashpot is to prevent the engine from stalling with a rapid deceleration. With a manual transmission it is not needed because when you press 8n the clutch all the load is removed from the engine. With an auto Trans the load remains until the rpms drop low enough for the torque converter to no longer load the engine.
 
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