Screw adjustment in AFM?

Jspec7m88

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Dec 3, 2005
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Nappy City
I was taking off my stock AFM as it was destroyed by water...and I was looking at it and had a quick question.

What does the adjustable screw on the bottom side of the stock AFM do?


Also, I know this is a wrong statement to be making in this thread but it relates...If anyone has a stock AFM for sale or to give away...let me know.:naughty:

Jeff
 

thevork

ShoarmaTeam Member
you can drill out the little iron plate they made over the screw.
then you can unscrew it and your afm will let more unmetered air pass.

The positive effect is that it will raise your fuel cut just a bit.

Don't do this unless you have enough mods to back this mod up like a wideband controller and maybe an AFPR to allow for some more fuel to get injected to compensate for the increased air flow.
 

jetjock

creepy-ass cracka
Jul 11, 2005
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Redacted per Title 18 USC Section 798
The truth is the AFM screw is for adjusting idle mixture. That's it's designed purpose and is why it's sealed. It does that by changing the amount of unmetered air versus metered and thus shifting the Karman frequency. This shifts the fuel trim loop one way or the other.

*Edit*: OK, I pulled my TCCS books out and it's a bit more complicated than that.
The screw is for adjusting idle mixture on the N/A AFMs. This makes sense as they use a vane type flowmeter. As airflow increases the vane presents less of a restriction in the AFM so the screw has most of it's effect at idle.

Unlike the N/A AFM the conductance of a Karman AFM doesn't change as airflow increases. It's "wide open" all the time. As such it would also affect idle mixture but not nearly as much as the N/A AFM. So the screw on a Karman is there to calibrate the AFM to a max specific airflow for it's size since the same electronics can be used in different size housings. This obviously explains why you can cheat fuel cut using it.

It also explains (to a point) why N/A AFMs have a code stamped into the housing that denotes the screw's initial idle mixture setting at the factory while the Karmans don't.
 
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Jspec7m88

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Alrighty, thanks. For the record, I have an AFC so I can at least see what's going on. I didn't need a drill bit...I just unscrewed it until I couldn't see it then popped it right out.:)
 

Jspec7m88

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Well, my car is running now...:)

My A/F's are right around 15, I did what (aye mate) mentioned in his thread about raising fuel cut w/ the stock AFM. I did that...and everything is running great. No codes, either.:)
 

Jspec7m88

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Dec 3, 2005
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Yes, it's doing great. I took her out for a test drive after I did it and under all conditions, all the numbers were good, I got 1.5psi more than I had and no more check engine light!:) Thanks for everything, though. I appreciate it.