924Board.org

[FASTdan]

im struggling to adjust the idle mixture on my 2.0 924. There's a tube-like hole on top of the metering unit (right next to the fuel distributor) and im assuming you stick the correct tool down here to adjust a stop-screw on the metering flap arm. However, as i cant see into the metering unit i dont know what tool i need to be using??

haynes basic diagram makes it look like a flat blade screwdriver, but the description makes it sound more like an allen/hex key?

Also, how do you go about adjusting the overall mixture? is it a strip down job? Mine is running mega rich!

thanks
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1980 924LUX

[John Brown]

It's a hex; allen. I think 4mm. Pretty sure the tech section has info. Haynes accurately describes the process.
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John
80 931 - #931 44Cup
99 Escalade - tows track cars
gone but not forgotten: original 924.org car - 82

[924RACR]

Actually I think it's a 3mm, and you need a T-style allen wrench with a long handle to reach it. For little more than the cost of the "K-Jet adjusting tool" the parts places sell, you can get a decent full set of T-handle allen wrenches from Sears or the like - very useful on 924's.

Clockwise to richen, counter-clockwise to go leaner. It's very sensitive; as little as 3 degrees rotation can be off and less than perfect setting.

I usually tune it (once warmed up) for good throttle response (blip the throttle cam and see how quickly the engine spools up), then take it for a spin and confirm. Ideal is if you can drive it strong then shut it off and pull a plug to check color. I've found, through careful checking on the racecar, that #3 is the one you want to go by - seems like that cylinder tends to run the hottest, as expected, so will lean out first.

You know the drill, you're looking for a nice caramel color on the nose of the plug.
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

[FASTdan]

yeah i have a wide band lambda also which i do intend on using on the 924 at some point, however i need a quick fix in the meantime.

This 3-4mm allen key only does idle though right? Whats the basics of adjusting the running mixture? Some adjustment on a plunger of some sort? Sorry i only have a vague understanding of this system at the moment - much more used to webbers lol.

thanks for the help though!
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1980 924LUX

[chris24]

The idle is a screw on the front of the throttle, bypassing air round the butterfly intakes. Play with this one for the idle, aiming for 950rpm.
The fuel mixture is what has been described. Do what 924racr said to adjust the mixture, small increments and when you blip the throttle, make sure you take the hex key out of the adjuster else you'll do damage!
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1983 - 924 (185K miles) - not mint
1985 - 924 (148K miles) - mint
1990 - 944S2 cab (52K miles)

[FASTdan]

oh! right, so to clarify: the flat blade screwdriver adjustment on the throttle body is idle mixture aswell as rpm? i thought this was purely idle speed?
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1980 924LUX

[chris24]

Flat blade screw driver bit on throttle body - purely idle speed! It is just a throttle bypass and cannot affect the mixture really.

Hex bit on fuel distributor (part with the air box intake on it) alters the fuel mixture.
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1983 - 924 (185K miles) - not mint
1985 - 924 (148K miles) - mint
1990 - 944S2 cab (52K miles)

[gohim]

The first thing to do when adjusting CIS is to do nothing to the adjustments. You need to inspect the spark plugs, plug wires, ignition timing, camshaft timing, valve clearance, and vacuum lines FIRST.

CIS adjustments are VERY STABLE, unless expendable/maintenance items or ignition adjustment need replacement or adjustment.

The worst thing you can do is to start screwing with the setting when you don't have everything else to snuff.

NEVER make adjustments to a CIS fuel system when it is cold. The system has cold start, and cold running compensation, in the form of the WUR, cold start valve, and aux air regulator. Idle speed and mizture are adjusted AFTER the engine is at normal operating temperature.

Cold Start Valve is a 5th injector that injects extra fuel when engine is cold to aid starting.

WUR lowers fuel system feedback pressure to allow the airflow sensor arm to rise higher when the engine is cold, allowing more fuel into the engine when it is cold.

Aux Air Valve opens when engine is cold to allow more air into engine when it is cold to increase idle speed.

If you have already taken care of all the maintenance items listed above, the the exhaust is now rich with the engine at normal operating temperature, the adjustments were probably fiddled with when some vaccum lines were leaking, and now after replacement, with proper vaccum restored, the mizture is rich.

The opening on the air flow sensor box needs a long 3mm hex wrench to adjust the idle mixture. Mixture is adjusted while the engine is running. DO NOT PRESS DOWN while making adjustment. Adjustment is always made from RICH TO LEAN.

The default starting adjustment position for the idle speed and idle mixture are approxiamtely 2.5 turns out (counterclock wise) from all the way in (clockwise).

Idle speed is adjusted on the throttle body (flat screwdriver).

Check and ignition timing first. Then, adjust idle speed. Now find the engine idle sweet spot (middle of the adjustment range where turning the adjustment screw does not affect idle speed) turning the adjustment to the left and right to find the ends of the adjustment range, then setting the adjustment to the middle of the range. Now, go back and recheck the ignition timing and the idle speed, and readjust to factory spec.

Go around and around until the idle speed, ignition timing, and idle mixture are all in spec.

The hole in the air flow sensor box is a leak in the vaccum system. Make sure that you plug it before taking any critical mixture readings, and after you are done adjusting the idle mixture. The earlier 924s, and VW Golf, Jetta used a rubber plug on a stick, which can also be used on the later 924, which in the U.S. are supposed to be plugged with an "anti-tamper" ball bearing and aluminum plug which are supposed to be jammed into the adjustment hole after making the adjustment, and drilled or forced out after partially disassembling the air box each time an idle mixture adjusment is necessary.

[Iwan]

I've just adjusted my CO mix. haynes publish a figure of between 0.5-1.0% as being set by Porsche for the ratio. You need to make sure that your plugs timing etc are OK (alternatively - set the CIS - then adjust the timing then do the CIS again) before you start and that THE CAR is warm. You need a long 3mm allen key and a flat blade screwdriver.

You then need to make little adjustments to the Mixture - with allen key - anti-clockwise for leaner, clockwise for richer and also adjust the idle speen - aim for 900 RPM +/- 50 RPM. You may wish to blip the throttle in this process if you do - REMOVE THE ALLEN KEY FIRST - you can casue damage if you dont. Also dont press on the allen key when turning - it opens the airm meter plate.

Its a bit fiddly and there's no air mass meters on K Jetronic so err on the rich side (e.g .75% to 1%) as an over lean mixture will lead to burnt valves etc

[Meneer aard]

I have weird problem with the k-jet ??

First the valve for the idle mixture control when the engine is cold, for increasing of the rpm was broken, the valve didn`t open at all!, so it`s replaced. When the engine was cold with the broken valve, the engine would not running idle @ 900 rpm but at 500 rpm, but it never stalls :?

First time the engine runs with a cold start @ 1100 rpm, when the car is hot the idle rpm is 900-950. (settings with old injectors).

So i replaced the injectors (getting old, dirty), and adjust the idle mixture (with a hot engine). Now when the engine is cold the idle rpm is 900 and when the car is hot 1500 rpm!? But some times it wil drop arround 1100 rpm.

Is this a problem with the throttle cable? or the adjustment of the idle speed?
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Porsche 924 Euro 1977

[Paul]

Is your AAV working (Auxillary Air Valve)?
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White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

[Meneer aard]

[Iwan]

This can stick and it can be erratic - which may explain your problems. Aply copious WD40 to the linkage on the top of the throttle body and also soak around the shaft. Preferably do this when the engine is warm. In the morning, before you start the car, apply WD40 again and repeat. Keep repeating. It's also worht on a quiet piece of road driving so that you 'force' the 2nd throttle to open and close (hard acceleration to open) low rev change ups to close, throttle blipping etc.

Its worth a try.

The other option is that your air inlet plate is distorted, sticking or doesnt have a uniform clearance - this will impact on fuel delivery - have you checked it?

[Lizard]

just an FYI it is a 3 mm allen key.

although Vaughan, on some of the CIS mercs (79 450SEL) and also the CIS 928s they have a piece which bolts on over top of that hole and seals it and has a small spring to hold an adjustment screw up when not in use, that way you only need a regular allen key, if you are tuning your setup often which I could see being a possibility then I would recommend that you get a few and retrofit them to the 924/931 for easy adjustment
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3 928s,

[Neil@Bournemouth]

Hi all,
just a quick question:

I read above that the adjustment tube should be plugged with a rubber bung. Is this true for all 924's? In just worrying cos the tube on mine has no seal!

Cheers,
Neil.