Adjusting Screw on AFM

[Gannon]

I have noticed that on my Hotwire airflow meter, there is a little plastic plug that if you remove it, there is some sort of adjusting screw.

Does anybody know what this does?

[Xtreme_RX]

In the Flapper models it adjusts idle CO%

[Gannon]

In the Flapper models it adjusts idle CO%

So basically an idle mixture screw.

Mine has been tampered with, (not by me) i wonder why.

[steptoe]

ask Adrian, discopotato03, think his manuals cover the hotwire

[discopotato03]

Goes rooting through a sea of Nissan and other Manwells .......

Got it , this is covered in the first or (1) General Section of the 87 manual under 1-5 Periodic Maintenance Services .

The screw on the LHS of the loom plug is an idle mixture adjusting one .
Procedure :

1) Neutral gear selected .
2) Warm up engine sufficiently until cooling fan starts to operate .
3) Connect tachometre to ignition coil and battery and insert probe of "CO"
metre into the tail pipe more than 40 cm/15.7 inches .
4) Make sure that the ignition timing is corectly adjusted .
5) Plug purge hoses after disconnecting them .
6) Adjust both idle adjusting screw of the throttlebody and idle mixture using
DRIVER (Special tool) .

From this I gather that the adjustment screw is just a variable air bypass passage to trim idle mixture when new . The manual has a table of models from different countries and a CO percentage column in the table .
Not surprisingly all the catalyst equipt models have a diagonal line through their relivant boxes . I'll bet this is because the idle mixture is pre set at the factory and the screw hole plugged with an aluminium plug to keep sticky fingers out .
For the viewers at home you can easily remove this plug by carefully drilling a small hole through the middle of the ally plug , screw a self tapping screw into it and pull it out with vice grips .
Most well equip servos with tune up servicing have CO metres and people who can generally set a suitable ratio .

She who must be obeyed has decreed out of time .

Cheers A .

[discopotato03]

Just to round off , the early 3 plug cars (vane type AFM) also have a screw which does the same thing - trims idle mixture or air/fuel ratio and same deal with the tamper plug .

I had my car checked over not long after I bought in in Melbourne and drove it home to Sinn City . It was a little off colour and subsequently found to be down on compression pressure which is not surprising after 250,000 k's . They removed the AFM's alloy plug and made a minor alteration to the idle mixture so that it ran a little better . Its now creeping up to 270K and the usual high mileage cooling issues are the only major engine probs so far .

So all you have to do (if concerned) is take your car to someone who can measure its idle mixture (and set base ignition timing at the same time) and set them to an appropriate figure .
The reason why ignition timing and AFR have to be set together is because altered timing alters AFR because combustion efficiency changes with timing .

Cheers A .

[Gannon]

Thanks Adrian

I had a bit of an issue of surging while accelerating, so i swapped AFM's and it improved. The first AFM had the plug removed and you could see that the little screw had been turned. But if its only idle, it cant have been the problem