fuel mixture screw access

Yes once removed there is a 3mm? Hex adjuster down in on the arm. You will need to replace the screw cap. Soak it, shock it and sharpen your flat blade. Good luck…..

 

Why do you need to adjust the mixture screw? Seems the factory made it difficult to access for a reason.


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It’s non-tamperable because that was required by then-current emission control regulations; the idea is that it is set at the factory and then locked in. Forty-plus years later enough will have worn/aged/changed that the original factory settings may not be right anymore.

The exhaust gas analyzer should have the last word here.

 

The factory seal is gone. What is left is just a screw. It may be tight but just unscrew it.

 

I have the same. Once the screw was removed we used a long Allen “key” (it was very lean). Note it’s very sensitive- my mate the mechanic grew up with these and I noticed that he was gently shaking the key whilst turning for adjustment. It was hooked up to the MOT gas analyser.

 

The adjustment is very small, 1 or 2 degrees of turn on the hex screw makes a suprising difference. The hex screw pushes down on the lever that affects the fuel distributor plunger, it is that lever effect that makes very small adjustments count.

These cars are rarely adjusted correctly, and not so many shops have gas analyzers anymore. It can be carefully played with and set by ear and feel if the car is otherwise functioning well.

 

Andrew - please describe how to set by ear as most of us have no access to a gas analyzer...In my case, 85 Mondial QV Euro, which has a completely rebuilt FD, WUR, etc., I set the mixture "by ear" but it smells rich yet the spark plugs show no evidence of richness (e.g. black sooty, etc.) so I am hesitant to set it leaner for fear of burning a valve, detonation, etc. - Here's what my plugs look like:

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Real shops have them. Nearly impossible to do many types of diagnosis except by trial and error without one. A Ferrari dealer called recently for advise and I told him he needed to read the exhaust gas ratios to figure it out. He had no idea what I was talking about.

 

At 2.5 % it should run fine. A car needs to be running correctly to run well with a lean mixture. Rich mixtures are usually to cover something else up.

 

Pointless to try if you want it to run right. And reading CO on its own is a waste of time. If any one gas tells you anything important it is CO2, not CO.

 

You might want to get a Gunsen Exhaust Gas analyzer G4125. Amazon carries them, not too expensive. It measures CO only, but if the car is working properly that will be the key measurement to set the mixture. Not as good or accurate as a proper shop gas analyzer, but as noted those are not so common anymore. These measure CO pre cat, since you do not have a cat it will be ideal for your car.

To adjust by ear/feel, you use a T handle hex tool, so you can have a good visual with the orientation of the T to know where you started from, and if you mess things up you know where the original screw setting was. You can rich or lean 1/16 of a turn either way, and see how the car idles, and responds with an off idle pull of the throttle. Too lean tends to cause a lag when the throttle is pulled, too rich can cause the idle to be a bit less smooth while off idle is less affected.

The way these Bosch systems work is heavily tilted towards getting low emissions at idle. Off idle, the car will run well enough within a broader range of mixture settings, and as noted the cars tend to run a bit better when a bit rich. So you are really trying to get the set up best for a smooth idle, and immediate off idle response. If your mixture is too far off, it can also affect starting the car.

Adjusting mixture will affect idle speed, so you then adjust the air bypass screw to refine the idle back to the preferred rpm, typically about 1,000 rpm. The Bosch system needs good airflow to function, so idle lower than 900rpm is not really working for the unit (these latter rpms are for the 3.2, I am presuming the QV's are similar). I have my 3.2 at 1,050rpm. that way when ac kicks in it is closer to 1,000rpm.

Your iridium spark plug looks like the gap may be a bit wide? Some people indicate the cars are best to use a conventional NGK plug.

Signs of running rich can sometimes be traced to a distributor cap contact, the rotor carbon button or a wire or extender failing. Might be worth double checking some of the ignition items. The newer reproduction dist cap sometimes don't allow the wire to seat deep enough to have the brass pin that retains the wire properly make for a good connection. Can't really check some of these ignition things without taking the cap off. But you could put an ignition strobe on each wire and see if there are any intermittent "misses" on any specific cylinder. Would probably show on a spark plug being a bit more sooted up than others, but might be subtle enough to not show that.

 

Thank you - will look at the Gunsen product. As for the plugs and plug gaps, they are the Iridium BPR8EIX and as per the instructions, they were not gapped but used as delivered...Thought everyone liked and recommended the iridium plugs...?

Car starts fast, has no hesitation off-idle and generally runs well but as noted before, the exhaust smells rich. So, best course of action will be to either get an analyzer or take it to someone that does

 

I use nothing but Iriduim plugs and have been very happy with them. The mistake people make is believing a wives tale that they cannot or should not be gapped.
Nothing could be further from the truth and if they bothered to read a little they'd see NGK tells you to gap them.

 

Thank you. Should the Iridium's be gapped to the factory spec for the car from the OM? e.g. 0.024-0.028...?

 

Rifledriver - would also be interested in your opinion of the Gunsen Exhaust Gas analyzer G4125 mentioned above as an effective tool to set mixture on Bosch K Jetronic - thanks

 

I would never waste the money. Measuring CO only is a waste of time. Setting CO to some nebulous specification with no understanding of what you just did to HC and CO2 can be counterproductive. As I said if you are going to only read one it should be CO2. it is a combustion efficiency indicator.

 

Yes. I know no good mechanics who don't love them. If you want to save a few dollars you can get one of the lesser types of NGK but zero downside and a lot of up side with iridiums.

 

10-4 ty on responses to both

 

I would love to have access to an exhaust gas analyzer but I do not. I was at a show a year ago and had a well known mechanic ask me to start my car.... pulled off the air box and using his finger applied a small amount of pressure to the air meter flap. This caused the engine to speed up (and smooth out) just a bit. He grabbed a screwdriver and a 3mil allen key and by adjusting richer (blocking off the adjuster hole with a finger and giving the throttle a quick rev between tries) until it just barely stopped speeding up when pressure was applied to the flap he proclaimed it better than it was. I used the same procedure (by making it lean on purpose first) after I replaced my injectors a couple weeks ago and plan to do again if I can get my funky WUR business figured out. This is a method I have read many times on the Mercedes K-Jet forums.

CLEARLY it is no replacement for an exhaust gas analyzer but I'd love to hear what the experts here think of this method.

 

Its great if you want it perfect at idle and no where else. It just shows ignorance of the effect the O2 sensor has at idle.

Donnie Callaway was a very well known mechanic. He was also an idiot.

 

Heh, very true! Well it certainly wasn’t Donnie and my car doesn’t have an O2 sensor (81). Is there a better way for a home mechanic to get it into “close enough”?

 

Fuel composition has changed so much I tend to take the book spec as a guide though my experience is more with carbs. The gas read out at MOT when I got the car was only just within the HC range (very high) and low on CO, yet previous MOTs showed it as very different. The idle speed went up as it was adjusted the mixture so we brought back to about 1000. I have no sense of smell but it sounded "smoother". We checked the readings at higher rpm but of course there is no load when stationary. However I let the garage owner /mate set it. (I do carbs for him)
Subsequent 1000 mile trip showed no signs of smoke or hesitation and it pulls well. Starting is excellent when cold and good/adequate when hot.
As an aside the cold start injector (into the plenum) is disconnected. I asked the supplying workshop (not my mate the garage owner who spotted it) and was told they always do this as "they have a tendency to run rich". I am a bit sceptical but so far there appear to be no adverse effects.