Before you buy one, see if you can borrow one from someone.  A shop or
someone who has an extra.  I bought a spare and it was crap but you can find
them on Ebay or TheSamba.  Get it from someone that you can trust
though--lots of dicks out there it seems to me.  The drilled out plug
doens't mean anything, most have been opened to adjust the bipass a bit,
only a few have never been adjusted.  I don't think the spring should be
moving like you describe but it is hard to see what you are describing.  I
ran mine with the cover open and it stays pretty steady and moved as I
changed throttle position like it should.

>From: Michael Diehr <md03@XOCHI.COM>
>Reply-To: Michael Diehr <md03@XOCHI.COM>
>To: vanagon@GERRY.VANAGON.COM
>Subject: Re: 1000 mile rebuild recommendations?
>Date: Fri, 11 Aug 2006 09:08:56 -0700
>
>I think one or both of the previous mechanics have already messed
>with the AFM -- the black plastic cap can be removed with gentle
>pressure, and the adjuster screw plug has already been drilled out.
>Maybe it's just a crudy rebuild and can't be adjusted back into spec?
>
>Also, I notice that at idle, the spring bounces all over the place,
>but the idle seems to smooth out a bit if I hold it steady.  Is that
>normal for it to vibrate like that?
>
>Thinking maybe I should just buy another AFM and do the debug-by-
>parts-swap method...?
>
>On Aug 10, 2006, at 4:07 PM, Kenneth Lewis wrote:
>
>>Michael,
>>   Just because the AFM is rebuilt doesn't mean it's good. Possibly the
>>spring is not tensioned correctly. I wish there was a good
>>procedure out
>>there to tune them. Here are some things I have learned by
>>experimentation.
>>YMMV.
>>
>>   First you need a good voltmeter and O2 sensor. The three throttle
>>body
>>alignments outlined in the Bentley should be checked. Carefully pry
>>the
>>black cap off . If you don't mess it up too bad it will go back on
>>and stay
>>without glue. Drill out the plug so the allen screw is accessible.
>>Use an
>>allen wrench and determine the base setting of the allen screw.
>>That is,
>>find out how many turns until it bottoms out so you can return to
>>where you
>>started, just in case. Write it down. Likewise mark the black
>>plastic wheel
>>so you can return it to it's original starting place.
>>   The engine has to be up to operating temperature. Disconnect the O2
>>sensor and hook up the voltmeter to it. If it is running lean the
>>reading
>>should be near zero. Ideally the voltage should toggle equally
>>between zero
>>and (about) one volt.
>>   Off the top of my head I can not remember which way to turn the
>>wheel to
>>enrich the mixture. I noticed that after a few notches in the right
>>direction the engine RPMs picked up and smoothed out. Unfortunately
>>the
>>spring tension that equated to the best sounding idle did not give
>>the best
>>reading from the O2 sensor. This is the part where I wish I had a good
>>procedure written out. I fiddled with the spring tension and idle
>>screw (on
>>the throttle body) until I got a happy balance between idle and
>>good O2
>>sensor output. Use the allen screw to fine tune the O2 output voltage.
>>Hopefully the output is good at higher RPMs, otherwise you need to
>>adjust
>>for that.
>>   I run through this procedure every year before vehicle emissions
>>testing.
>>Good Luck,
>>Ken Lewis
>>http://neksiwel.20m.com/ (more O2 info here)