that the throttle arm bottoms out on?

to be clear the throttle cable attaches to this arm and there is an allen screw that i though was a stop, but it depresses a little do da, that does not seem to have a function.

is this just to dampen the throttle closing?

 

Yes, it sounds like the throttle damper that you're talking about.
Although a picture would confirm it.

 

Throttle damper?

It's called a dash pot...

 

Actually, there are two (at least!) "little plunger things."


In the foreground of the picture is the "Dashpot", which does damp (not dampen) the throttle closing (both you and MudHippy are correct). If the throttle slams shut, the fuel left in the intake has no air to burn, so you get a fireball out the exhaust. Or at least a high-HC puff.

The one in the back is the throttle opener. When the vacuum goes to zero, the throttle is opened a crack. I believe this is to reduce dieseling.

The manual is a terrific resource: http://personal.utulsa.edu/~nathan-b...93throttle.pdf