AFM Vane Details

[Captain Obvious]

So I'm giving my AFM the once over before putting it back on and I notice that the air metering vane looks odd...

Here's the outlet side with the vane closed:

As you open the vane you can see that the bottom edge of the vane is not straight:

So I'm guessing that it's not supposed to be that way, and it's bent as the result of a backfire into the intake or something?

And simply for posterity, here's a pic from the intake side:

[240260280z]

Bead blast it.

It looks bent but I never looked closely at one.

This one looks straight:

[Zed Head]

Your AFM has the backfire relief valve so that must have been some big backfire. Maybe it's a funky tuning trick to lean out the low air flow end of the range. I just looked at my spare AFM (MSA rebuild) and the vane is straight, edge-on like your picture, and sits flat when the vane is wide open.

[FastWoman]

Definitely a humungous backfire -- or several dozen of them. Here's the thing: Everytime the vane gets bent, the AFM meters leaner and leaner, causing worse backfires, etc. When I was in college and owned a '75 Z (which had the AFM without the backfire relief valve), I had a similarly bent vane. I was able to fit a block of wood up through the throat of the thing to use as an anvil, and then I was able to tap the vane flat with a hammer. It made a world of difference in how the engine ran. :-)

[Captain Obvious]

Maybe it's a funky tuning trick to lean out the low air flow end of the range.

That's exactly why I wasn't sure if it was supposed to be that way or not. Fluid flow is weird stuff and since I'm sure these AFM's were highly characterized at the factory, I wouldn't be surprised if that feature was intentional. But unfortunately, it sounds like it's not.

Everytime the vane gets bent, the AFM meters leaner and leaner, causing worse backfires, etc. I had a similarly bent vane. I was able to fit a block of wood up through the throat of the thing to use as an anvil, and then I was able to tap the vane flat with a hammer. It made a world of difference in how the engine ran. :-)

Crap. And that answers my next question of "OK, so it's bent. Does it really matter?" I was hoping that even if it was bent, it didn't have a great implication on how the car ran. Now I gotta do something about it...

Bead blast it.

Only if you come over and help me tune the system afterwards.

[Zed Head]

How did these AFMs with the bent vanes run at low throttle opening? I just swapped in a78 AFM that I had, to be sure that it was a good spare, and it works fine but seems extra lean at low air flow, with a little bit of a lean surge/almost a stumble. Not a huge deal because my gas mileage has improved considerably (only on my second tank though, so far) and it runs great at higher RPM and throttle openings, but now I wonder if the vane is bent.

Mine appears to be factory stock, with gobs of pale glue over all of the critical adjustments inside (surprisingly messy though). Now I have the urge to take it off again and check the vane.

[240260280z]

The strange thing is that the flap looks cast with reinforcements and I would guess it would crack before bending so nicely and evenly.

[Captain Obvious]

How did these AFMs with the bent vanes run at low throttle opening?

Mine runs fine at all throttle openings, but I believe I'm running a little rich. However, I'm still very new with the car and haven't even had the opportunity for a decent shakedown yet. I've been finding and fixing the problems as I discover them and haven't gotten to the point where I'm ready to really start "tuning".

I believe my AFM has been tweaked by the PO, and that may be why I'm rich. In other words... If I put the AFM back to stock I may end up lean.

The strange thing is that the flap looks cast with reinforcements and I would guess it would crack before bending so nicely and evenly.

Yup. I'm sure it's cast. At this point, I'm glad it's as flexible as it is. :surprised

[Captain Obvious]

I'm sure your really really not supposed to do this, but...

Can I take off the other side of the AFM to get better access to the vane? Might I even be able to take the vane completely out to work on it?

[FastWoman]

Obvious, I THINK you can do that. The waffle side has what looks like blobby little rivets around its edge. Those are actually screws, blobbed over with some sort of tough cement. The cement will soften very slowly in carburetor cleaner or perhaps lacquer thinner. I've never gone any further than exposing the screws, but it looks like Blue has removed the waffle in his photo. I bet it took a lot of glue softening to get the parts separated. Once inside, I suspect you'll find the vane rotates on ball bearings and that the whole thing is pressed together. Good luck with that one!

Seriously, put a stick (maybe a 1x2?) inside the throat of the thing, and eyeball it. I think I recall using a stick as an anvil and another stick on the otherside to tap out the vane, if that makes sense. It worked fine.

Edited April 19, 2012 by FastWoman

[Willoughby Z]

Capt.- You can take the back cover off. Been there, done that. You'll have to pick at the stuff covering the screws and use a razor around the edge.

Nothing goes "sproing!"...

BTW, my vane was curved. I don't believe yours or mine is bent from front-fire. I opened mine to find a cause for the vane sticking at WOT. I also think you'll find that if you were to flatten the arc on the lip, the vane won't even move on it's pivot, scraping against the sides of the chamber.

[240260280z]

Another way to prove the bend is correct:

"Mind the gap" between the flap and the body. If the flap is actually bent, the side gap between the bent part of the flap and the side wall of the throat should be greater than where it is not bent. From the photos, the gap looks even so the bend looks normal. It could be an aero improvement to spill air to the side and balance the forces to the edge at low rpm.

Using a feeler gauge could give empirical data.

Nice that Willoughby Z confirms a bend too.