1975 AFM check out

[Dave WM]

I picked up a spare AFM at the local JY, it test ok per the FSM (voltage test/resistance test/pump micro switch) but I still wanted to test it in the car. No point in having a spare part if you don't know if its going to work. Anyway a little background I was doing some vacuum testing, and was at about 17 in/hg with the orig AFM after fully warmed up. I added the 500ohm pot in series with the temp sensor and was able to pickup RPM and increase vacuum by added resistance (enriching fuel to air mix). The plug was still in place on the AFM for the idle air bypass and I did not want to mess with it since this is not really a problem, car drives great and starts fine. So while testing the spare AFM I thought it would be a good idea to check out the effect of the idle air bypass. car starts as normal (quick) and idles well. vacuum is up to 18 in/hg which is what I could get with the pot set to max resistance, but this time adding resistance seemed to have no effect on the idle or the vacuum. My only assumption is the orig AFM was indeed lean, whereas the replacement was more ideal and enrichment did not really effect anything (a bit richer but not enough to mess up the vacuum). The other thing I noted was the air bypass had NO noticeable effect on idle or vacuum. that is from 0-at least 10 turns out. I did make sure the air passage was clear but I did not actually test the screw on the flow. So my assumption is the effect is very small and would prob need a AF mix gauge to be able to pickup up on it. For now I just set it back to 1.5 turns out and left it. going to test drive to car to see if there is any noticeable difference in engine performance, but will most likely re install the orig AFM and just know I have a good spare. Was wondering if anyone else has messed with that bypass screw? when I get it off I will try blowing air thru the little inlet at the mouth of the AFM and see exactly what effect the screw has and what its adjustment range is. I always assumed air bypass screws where typical 1.5 for middle adjustment.

Edited March 19, 2018 by Dave WM

[Captain Obvious]

I've messed with it a bunch. Couple years ago Blue and I also measured the mixture effect that screw caused. He gave my car a rectal and monitored the mixture numbers while I changed the screw setting. I've got notes around here somewhere, and when I put my finger on them, I'll post them.

And I'm not surprised that screw did not have a direct effect on manifold vacuum. That screw is upstream of the throttle plate, and while it might result in a tiny tiny change in flow resistance, the majority of the vacuum is going to be developed across the plate in the throttle body.

Sure, messing with that screw could 1) change the mixture, which in turn could 2) change the RPM, which might 3) change the vacuum reading... But not a direct "flow path restriction" related effect.

[Zed Head]

I've messed with it quite a bit.  It only has a noticeable effect if everything else is right, meaning close to lean.  I had a method where I would close it down until I got a miss then back it up 1/2 to one turn, before taking it in for emissions testing.  This would be after a couple of years between emissions tests where various modifications had happened.

 

 

[Dave WM]

I was going with the vacuum since I noted a direct correlation between the pot adjust and the vacuum (on the orig AFM).  Was thinking if the air bypass can change the mixture the same as the pot does (at idle adj the bypass to richen vs using the pot to richen). I also tried just touching the vane weight on the replacement AFM (cover not sealed) and movement either way would drop both idle and vacuum). My guess is my orig AFM is just a touch lean at idle.

 

Edited March 19, 2018 by Dave WM

[Dave WM]

Took it out for a test drive, same as before, only issue is a slightly smoother idle, and seemed like the BCDD was holding revs up more than normal, keeping it higher than the original, but still working by cutting off at less than 10 mph. I am going to reinstall the orig anyway, its just prettier... I will measure the depth of the bypass screw on the orig, the plastic plug was in place and everything looks like new, was difficult to turn so did not want to mess with it. After measuring I will see where its set at by comparing the replacement AFM which is not as hard to turn.