AFM Sticky Spot

[Captain Obvious]

I have heard stories about sticky AFM's in the past, but I have never held one in my own two hands. Until now.

So I've got this AFM here that has a binding sticky spot right in the middle of it's travel. I have looked it over very carefully and cannot see any visible explanation for the sticky spot. I'm thinking maybe the damper section of the vane may be making contact up in the belfry area, or the main shaft may be bent.

At this point, I'm toying with the prospect of taking the back cover off. I remember some discussion about doing this in the past, but don't remember if anyone has actually gone through with it.

Has anyone taken this back side cover off? Mine has two screws at the bottom and a whole bunch of glue around the entire perimeter:

Has anyone gone through the operation, and did the patient survive? I mean... This thing is worthless to me in it's current state, but it would be nice if this was more than just an autopsy.

[Captain Obvious]

There was some discussion about such a maneuver in the past, but I don't know if it ever happened:

https://www.classiczcars.com/forums/topic/67674-afm-loose-arm/

@chaseincats

And @kickstand80, since he's a big proponent of AFM modifications and adjustment. 

 

 

[chaseincats]

1 hour ago, Captain Obvious said:

There was some discussion about such a maneuver in the past, but I don't know if it ever happened:

https://www.classiczcars.com/forums/topic/67674-afm-loose-arm/

@chaseincats

And @kickstand80, since he's a big proponent of AFM modifications and adjustment. 

 

 

I actually did on the one with the wobbly arm just for the hell of it since it was useless in its current state.

Taking the back plate off surprisingly does nothing except give you access to the idle air mixture bypass passage to clean it if memory serves.  I remember opening it and being really confused how the sweeper arm is actually removed since I was sure that's how the rebuilders do it but no dice.

[Captain Obvious]

9 hours ago, chaseincats said:

I actually did on the one with the wobbly arm

Well?????  Pics or it didn't happen!!  

You're saying that taking that back cover off did not provide access to the sweep vane?

[Captain Obvious]

I've got some old pics that I've hoovered over the years, but it's not like talking to someone who has done it. But here's what I have:

This AFM has been cut open with a saw (photo credit unknown):

 

And here's another one that should show how the vane mounts inside the housing.  Photo credit on it:

 

 

[chaseincats]

16 minutes ago, Captain Obvious said:

Well?????  Pics or it didn't happen!!  

You're saying that taking that back cover off did not provide access to the sweep vane?

Unfortunately that's correct

[chaseincats]

9 minutes ago, Captain Obvious said:

I've got some old pics that I've hoovered over the years, but it's not like talking to someone who has done it. But here's what I have:

This AFM has been cut open with a saw (photo credit unknown):

 

And here's another one that should show how the vane mounts inside the housing.  Photo credit on it:

 

 

wtf mine didn't give access like that...maybe they're different per year?

[Captain Obvious]

Well it's clear that there were some changes over the years. The one I'm messing with (which should be from a 76) doesn't have all those screws, but instead has posts that have presumably been peened over as rivets, but other than that, I would expect the construction to be pretty similar.

Do you still have the one you opened? Got pics?

[chaseincats]

8 minutes ago, Captain Obvious said:

Well it's clear that there were some changes over the years. The one I'm messing with (which should be from a 76) doesn't have all those screws, but instead has posts that have presumably been peened over as rivets, but other than that, I would expect the construction to be pretty similar.

Do you still have the one you opened? Got pics?

I do but I closed it up.  I can crack it open tonight and grab some pics.

[kickstand80]

I have one that had a rub spot that caused the flapper to bind. I was able to use sand paper on the body to correct this. As I recall this AFM came from a 1976 car that had been sitting in a farm field for almost 20 years.

[Captain Obvious]

Well I went ahead and performed the surgery. Mystery solved. Goes like this:

I used a small carbide burr in a Dremel to remove the smoooshed off section of the posts holding the back cover on. Here's a pic of the burr and one of the posts ground off:

Worked my way all around the perimeter grinding off the posts. Also took out the two retaining screws:

Even after doing that, the cover was stuck fast. The adhesive / sealant they used is quite tenacious, After some experimentation, I found that it was susceptible and softened by carb / brake cleaner. I lightly sprayed around the perimeter a bunch of times waiting a minute or so between rotations for the solvent to work into the seam:

Doing that a couple times letting the solvent attack the sealant glue. Then some prying with a small screwdriver:

And presto... Cover is off:

 

 

[Captain Obvious]

And once the cover was off, the problem was obvious.

Here's a pic of the damper vane. I'm back lighting the gap between the movable vane and the case / body of the AFM. This is a pic when there is very little air throughput and the vane in the "mostly closed" position:

And here's a pic of the damper vane at about mid-scale. Note how the vane is rubbing against the inside of the case:

And then here is the damper vane "beyond" the sticky spot. Note that there is a gap again:

So mystery solved.... The case is bent. Pushed in at that spot. Like someone dropped it or hit it with something big enough to dent the case. Note that I looked it over thoroughly from the outside and did not detect the dent. I did find an anomaly on the outside of the case, but it's quite subtle.

Here's a pic of the outside of the case after some "block sanding" in an attempt to highlight the problem area. The clean spots were wiped by the sanding, but the darker spots (still dirty) did not:

So, mystery solved!!! I'm going to try to fix it, but ran out of time today. I'm thinking that a few well aimed taps with a ball peen hammer might push the dent back out enough for the damper vane to clear the case.