bosch cis mixture adjustment

If your car is at a shop that is familiar with any Bosch CIS system, this adjustment is a "no-brainer". There is a pressed-in aluminum plug in the fuel distributor that can be removed by drilling a shallow 4mm hole and, with an "easy out", the plug can be removed. Care must be taken with this procedure, as if you drill the hole too deep, you take the risk of getting pieces of aluminum from the drill process into the lower plenum housing.

Once the plug is removed, the mixture can then be adjusted by using a 3mm allen...or the proper "long 3mm allen tool" than virtually all repair shops should have just for this purpose. As the "mixture hole" is in the air flow stream, a vacuum leak will occur when the plug is out for misture adjustment purposes. Hence, make a tiny adjustment (right richens-left leans) and then temporarily plug the hole...allow the mixture to stabilize and take your readings. Remember, the base mixture is set with the oxygen sensor disconnected.

Regards,
David

 

3mm allen wrench, extra long. Get a set, one will fit.

 

Adjust here (red arrow)
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Mike,

before making bigger adjustments to your mixture, test the various pressures in the system. There are quite a few smaller things that influence the working pressures, which all have to be good before the mixture is right through the rev range.

 

The sloted screw is a plug. Tap on it with a drift and hammer softly to loosen it, then it should come out easy. The mixture screw is 3mm allen clockwise=richer, counter clockwise=leaner. As you put the allen wrench down in the hole you will feel it engage the screw about 1" or so deep.

Larry Fletcher
CIS Flow Tech Llc.

 

Check the CO reading with the gas analyser in each of the 4 tail pipes - you will probably find that two of them give a very low reading (the inner two on my car I think). Its because at idle the airflow out the exhaust basically all comes out the outer two pipes (because of the low volum of gas/ path of least resistance etc) ). Blip the throttle & the inners start doing their fair share as well.

I.

 

But CO is a percentage of total flow so should be the same for all four. I once had a tester add up the CO % numbers for all pipes and then saying the total was too high...

 

These adjustments are a little touchy; remember....lean out first, then back to rich for your final read.

 

Indeed Bert, the point is that (on my car anyway) there is seemingly not enough gas coming out the inner two pipes at idle to give an accurate reading. I can only tell you what I saw on the gas analyser readout!

If you put your hand over the pipes there is indeed hardly anything coming out the inner two at idle. Bring the revs up a bit and it all evens out. Its just the lower volume of gas following the path of least resistance.

 

+1!

ALWAYS adjust from lean to rich.

NEVER rev the engine with the tool in place.

Good idea to remove the tool, seal the hole, blip the throttle and check the CO after it settles between each adjustment. Shouldn't take much. I used to pay attention to duty cycle, too.

 

Not uncommon for volume and pressure differences between sides and between pairs. Measure from the outside pipes.

 

Do you have to re-plug the hole after you are complete and if so How do you re-plug it after you have removed (and likey destroyed) the cap with the philips-like slot in it?

Thanx

 

David left a step out in his explanation (or he used the word "plug" twice for two different things). What needs to be destroyed (at least on some US models) is a tapered aluminum plug that blocks physical access to the slotted screw plug that does the actual sealing. After the adjustment, the slotted screw plug goes back in to do the sealing (and the aluminum tapered plug is not replaced -- although it could be if you had another one).
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Thanks Steve for catching this...

Let me clarify a bit more.

The mixture adjustment allen screw is 3mm and is hidden from plain view by either a:

-Pressed in aluminum plug
-Or, a 5x.80mm screw

The aluminum "Anti-tamper" plug is destroyed in the removal process. Usually, the "screw-type" anti-tamper can be easily removed and reinstalled after the mixture adjustment is done.

Regarding the aluminum plug, you have two choices...as something must be put back in the "hole" after the mixture adjustment is done, else you'll have a good sized vacuum leak.

Choice one: The original equipment Ferrari aluminum plug. Ferrari part number 121571*
Choice two: A "CIS stick-pin" from the old VW/BMW CIS days which resembles a small rubber stopper, with a steel "pull pin". This is what I typically use, as they are easily removed for future adjustments. These "stick pins" were OE on most BMW's and CIS-equipped cars back in the 80's. BMW part number: 13511265321


David

 

THANX GUYS!!!!!

Now, I am tracking. All is clear. Appreciate it.

Mark

 

Exactly! Good add'l info.

It seems more often than not, these tube covers aren't in place. I found that the cap plugs used to protect the crank post on casement/awning/skylight windows fits perfectly over the tubes and makes for easy rem'l when adjust'g. There are a variety of colors that co.'s use, I contacted my skylight co. because they used red ones, they sent out a 1/2 dozen to me free of charge.