AFM Vane Details

[Captain Obvious]

Thanks Willoughby! You may have just saved me from "fixing" something that was never a problem in the first place! So the jury is still out on whether that bend is intentional or the result of a misfire. Glad I didn't jump the gun and start straightening it yet!

Blue, Yes that gap appeared to be consistent to me as well when I was looking it over. I didn't measure it, but it eyeballed pretty even up the length of the side. That, and the consistency of the bend itself is why I was originally questioning whether the bend was intentional or not. So, I'll measure the side gaps, and if they're even along the whole side(s), I'll assume the bend is fluid dynamics black art. More news as that develops.

Fastwoman, Yes, your dueling anvils make sense. I was just thinking that if nothing went sproing, it would be easier to adjust on the bench. Although, it seems I may not have to be doing anything with it. That would clearly be the best answer!

[FastWoman]

Erm.... There's no bend in either my '78 or '81(ZX) AFM vanes. I'm certain because I inspected very carefully for exactly that. I'd be utterly astonished if the bend were intentional. The bend in my '75 was a different shape, BTW, because there were no stops. (See Blue's pic. That's a '75 AFM.) There was no mistaking that the bend in that AFM was from backfire, as the vane did not align with the housing. Both the stops and the backfire relief valve were attempts to solve the bending vane problem.

Just tap it out. Nothing will explode, and all will be well. I promise.

[Zed Head]

Took my 78 AFM off yesterday to check it out and the vane just had a very slight tweak on a corner. I shined a flashlight through it when closed and decided that there is plenty more leakage possible around the edges of the vane than the small tweak provided by not seating, especially since the air flow is perpendicular to the bend (the air gap is under the edge) once the vane moves off of its bumpers, so left it alone and re-installed it.

As I noted above though, this 78 factory AFM is adjusted much leaner than the MSA rebuilt AFM I've had since I got the car running. I got 23 mpg on my second fill-up, compared to ~19-20 mpg typically with the MSA AFM (re-manufactured by Fuel Injection Corporation). Aside from the slight hesitation/stumble at low air flow, I like the old dirty AFM better than the shiny rebuilt MSA unit. I wonder if Fuel Injection Corp. adjusts them rich to be safe or if there's just a lot of variation.

Now I'll be checking all of the AFMs I see for bent vanes. Maybe we should start a list.

Edited April 19, 2012 by Zed Head

[Captain Obvious]

I didn't get the chance to pull the AFM back off the car to take a better look at that bend in the vane, but I'll reserve judgment on the intentionalness (just made that one up) of the bend until then.

In the meantime, I mentioned earlier that I believe my PO messed with the AFM settings. The glue glob on the lock screw for the clock spring gear has been pulled off the head of the lock bolt and some of the plastic gear teeth have marks on them (presumably from a screwdriver blade).

So, question is... Is there any way for me to tell where the AFM was originally before anyone messed with it?

[FastWoman]

Obvious, check the Atlantic Z website for info on how to calibrate (or check the calibration of) the clockspring. There's a method involving beer cans full of water. :-)

[Willoughby Z]

Obvious, check the Atlantic Z website for info on how to calibrate (or check the calibration of) the clockspring. There's a method involving beer cans full of water. :-)

And string. Graduated cylinder. Paper clip.

Find a cigar box to keep all your newly acquired scientic treasures. Make sure you stow your yogurt cup in there...

[FastWoman]

Funny!

[240260280z]

Hope (it works) Chest.

[Captain Obvious]

Today I inspected the gaps along the sides of my AFM vane and found that the gaps were slightly wider towards the tip than near the fulcrum. In other words, it appeared to me that the vane is in fact bent by accident, and not designed that way from the factory.

So, with that in mind, I performed FastWoman's double anvil straightening method, and here are the results. Not perfect, but about as good as I could get it without taking off the waffle side and pulling the vane completely out:

Here's my anvils. The metal one was my backup and went in through the downstream side for support. The wood block was my bending instrument and was inserted into the upstream side and hit with a hammer. The tube taped to the back side anvil helped direct the force nearer the tip of the vane because I couldn't get on it square due to the shape of the AFM body.

Results? Didn't notice any change. Still runs fine. :laugh:

So after I straightened the vane, I also played around with the vane position by moving the counterweight. The RPMs dropped when I tricked it lean and the RPM's went higher when I tricked it rich. In other words, it seemed to like more fuel, but not less. More tuning will be required.

[Captain Obvious]

Forgot to mention...

I ran my idle mixture screw all the way in as I counted turns, then I took it completely out, cleaned it up, and then put it back to where it was originally.

Anyone have a good starting point for where the mixture screw should be? I know they will all be different, but a good ballpark starting position. I think mine is way too far out. I'm barely catching any of the sealing rubber and I'm assuming my PO messed with it. Might explain why my idle likes more fuel, but not less.

[Wade Nelson]

There's a good video on Youtube about rebuilding a MAF.

Same basic MAF design as this one, I think originally a Robert Bosch design. Basically the little wiper (a potentiometer) wears a track out in the resister it rubs against. With a little work you can move the wiper up or down to scrape a new line against the resistive material.

It would be a VERY good idea to have a spare MAF in hand before attempting this, and if you're not mechanical, keen eyesight, and patient, fuggetaboutit!

And if swapping your MAF with a known good one doesn't change your "issues" then chances are your problems aren't with the MAF to begin with. I've wasted far too many hours blaming MAFs for various stumbles, hesitations, and problems when it turned out to be something else.

[Zed Head]

I've seen someone, maybe FastWoman or SBlake, suggest 5 - 51/2 turns out from all the way in as the factory starting point. I set my 78 AFM there and it was fine. I tried running it in a little to work on that flat spot I described but it just made the exhaust smell gassy so I ran it back out. I usually tune mine by exhaust smell vs. idle quality, unless I have to get emissions tested, then I run it out until I pass the test.

Don't forget that the engine is on the idle enrichment circuit when the TPS is at idle. Kind of throws a wrench in the tuning the spring by the fingering the vane method.

Edited April 22, 2012 by Zed Head