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On Probation
Location:
North East NSW
Registered:
December 2002
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Re: 4ac - easy question
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Fri, 15 April 2005 08:32
 
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well, actually that's an integral part of tuning the idle. it's not really hard. you just need to find out wether or not it's a fuel bleed (in is leaner, out is richer) or an air bleed (in is richer, out is leaner)
get a notepad and make some columns. idle speed, idle mix, result. now, using a screwdriver (or 5c piece if it's hard to get a screwdriver in there) wind the idle mix screw all the way in and count the turns. then bring it back to where it was (count the turns out), just so you know how far it is out to start with
now adjust things, make precise half turns of the screws. write down everything you do, and what changes it makes to the engine. this way, you can go back to settings that worked if you stuff up
your table should look something like this
SPEED - MIX - RESULT
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3 out - 0 - stalls
4 out - 0 - stalls
5 out - 0 - idles 1200rpm
5 out - 0.5 in - idles 1100rpm
5 out - 1 in - idles 1000rpm
and so on. those are totally unrealstic numbers, but you get the idea. tune in full turns, then in half turns, then in quarter turns. once you have two quarter turns between which your settings are good, tune by hand
if you have a vac gauge, you'll be able to get the car idling at many different states of throttle and mixture, you should try to achieve the idle with the highest manifold vacuum, something like 18in/Hg is good for a stock motor. anything under 15in/Hg is a pretty crappy idle, and it will use alot of petrol
alot of service manuals will tell you to tune to "best lean idle", where you raise the rpm, lean out the idle mix, then lower the rpm, lean out the mix, etc - till you get a factory rpm idle with the leanest possible fuel delivery
i personally don't like this, as the idle circuit of the carb affects your fuel delivery right up till about 2500rpm, and you want a bit of fuel in there to make power, and make the car go smoothly, more than the minimum anyway. i also tend to run a rich idle because i don't use a choke in my cars
a 4a-c carb diagram, or a gregories/haynes/toyota manual should be able to tell you specs to start from, and which way to tune the idle mix screw. good luck
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