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Old 10-07-2007, 11:38 PM   #1
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Default AFM adjustments spring vs. screw

im sure everyone has heard of adjusting the AFM, but i havent really found any concrete facts. i have heard of people takeing to top off and makeing the spring tighter or looser, but does this affect the entire RPM range or just idle? i already know the screw on the side is for idle only. so what exactly is the purpose of taking the top off and messing with the spring when you can just turn the idle screw? or does the spring actually adjust the whole rev range?

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Old 10-07-2007, 11:42 PM   #2
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Old 10-07-2007, 11:49 PM   #3
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so by that article, the spring adjusts the whole rev range?
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Old 10-08-2007, 12:59 AM   #4
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if you tune the mixture screw to see a perfect idle, then you dont need to screw with the tension.

the stock tension is tuned for our engine. make it looser and it will just get richer easier as little effort can force it fully open. conversly, tighten it and you lean out everything as it takes more load to fully open the flapper.

the flapper itself has a little wiper arm, when it reaches certain spots as it rotates it sends a different voltage (or resistance, cant remember) to the ecu to tell it there's more/less airflow.

but like I said, if you tune the mixture screw without touching the tension it will idle the injectors and fule the car like normal. you may be able to lean it out a tad with a little extra tension, but it's going to touchy.

what size injectors are you trying? like i said before I've idled 305cc injectors perfectly on the oem afm and the rx7 afm. I had no trouble at all. everywhere out of boost was exactly where it needed to be. you should not be blaming the afm on your situation. unless the injectors are too large to tune to 14.7:1 at idle, I'd look into the installation/tuning of the FMU.
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Old 10-08-2007, 01:24 AM   #5
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what size injectors are you trying? like i said before I've idled 305cc injectors perfectly on the oem afm and the rx7 afm. I had no trouble at all. everywhere out of boost was exactly where it needed to be. you should not be blaming the afm on your situation. unless the injectors are too large to tune to 14.7:1 at idle, I'd look into the installation/tuning of the FMU.
i have 310 out of an 89 probe gt. i wasnt blaming the afm on my problem, i was just curious why some people were adjusting the spring tension. just trying to learn a little more i plan on keeping the top of the afm sealed up
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Old 10-08-2007, 02:16 AM   #6
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why wouldnt you want to open up and tune the AFM? you will be running pretty rich otherwise with those injectors. With a wideband, it would be a breeze, but without, you can at least get it closer than the factory settings.
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Old 10-08-2007, 02:42 AM   #7
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why wouldnt you want to open up and tune the AFM? you will be running pretty rich otherwise with those injectors. With a wideband, it would be a breeze, but without, you can at least get it closer than the factory settings.
i am curently peged out at full rich all the time (even in vac) on my AEM wideband with the bleeder on the cartech all the way out. so i should mess with the tension spring?
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Old 10-08-2007, 02:54 AM   #8
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Well it should be easy then. Open the AFM and tighten it up until you see 13:1 at atmospheric pressure, as that should be about right if not on the rich side. Then turn that idle screw until you get 14:1 (where the stock ecu idles). It could save you up to 30 miles per tank, and that could ammount to $3 - $5 per fill-up, not to mention the benefits of running 12:1 vs 10:1 in boost. After tuning, i bet you will feel more torque from the motor, better spoolup, and overall better drivability.

Once you get your AFRs close to where it was stock, then tune the boost region with your FMU starting at rich and working towards 12:1, or 11.5 for cali gas (thats what im stuck with).

Take this for what its worth, as I have never messed with an AFM first hand, but I have a pretty good understanding of basic tuning principles with the miata (and of course most other engines would follow the same rules)
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Old 10-08-2007, 10:18 AM   #9
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Go too tight and you might not let the fuel cut switch stay closed at idle and have stall problems, especially if you have any small vacuum leaks.. Just a caution.
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Old 10-08-2007, 10:31 AM   #10
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Originally Posted by bryantaylor View Post
i have 310 out of an 89 probe gt. i wasnt blaming the afm on my problem, i was just curious why some people were adjusting the spring tension. just trying to learn a little more i plan on keeping the top of the afm sealed up

most people, when referring to adjusting the tension, are talking about the rx-7 AFM which must be changed to run properly on the miata.

I've done a lot of tinkering around with both, making 200rwhp with the stock ecu and afm.


Once the thing can be adjusted in such a manner that it can idle at stoich, then it should cruise at stoich as well. If you're seeing a good idle and a rich cruise, then maybe the FMU preload is too high and shooting the fuel pressures up too quickly below 0~hg.

the 310cc injectors at stock pressure can most likely fuel around 150rwhp. So keep that in mind when you have your preload set higher than average, plus the FMU increasing the pressure in boost. Tune your pressures lower to something like 45psi at 0 vacuum. and completely turn the needle valve open. See what happens then.
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Old 10-08-2007, 02:16 PM   #11
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alright, i will try turning down the base presure on the fmu and probally out s resttircotr in line too, ive read a piece of wd-40 tube in the vac line works great
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