308qv mixture adjustment

I believe those are actually "cover" screws and the adjustment screw for each port lies underneath, but these are not adjusted to set each cylinder's mixture individually -- they are set during the assembly of the fuel distributor to give equal fuel rate delivery from each port at any plunger position. You don't want to disturb these -- if your fuel distributor is not working correctly (i.e., it isn't giving equal fuel out of each port), you need to have the fuel distributor rebuilt/recalibrated by a specialist.

The only mixture adjustment a Mechanic would/should make on an assembled engine is done on a single screw that affects all 8 cylinders equally -- see jpeg:
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The other point of note is that this mixture adjuster only affects the idle mixture as I understand it. Secondly, no two gas analysers ever seem to give the same CO reading so "close enough " is more than likely OK. If the car is idling nice & smoothly then you will be about right - anywhere between 0.6/0.7 & 1.0 on the CO seems to be OK.

 

I don't believe that's true.

The CIS system adjusts fuel delivery with an airflow sensor plate and a control plunger. The plate is attached to a lever, with the control plunger on the other end. As more air comes in through the intake, it pushes the sensor plate down and the attached lever pushes the plunger up. The higher the plunger, the more fuel is delivered through the fuel distributor to the cylinders.

Now, to the topic at hand. The mixture adjustment screw is located just above the connecting lever I mentioned above. As you screw it up and down, it adjusts the position of that lever relative to the sensor plate and plunger. That means that for any given position of the sensor plate (i.e. amount of air coming in), the position of the plunger (amount of fuel delivered) will now be different because you've changed the mechanical relationship between the two. In other words, it effects the entire range of running conditions, not just idle.

 

This is correct.
Any adjustment you make to the screw will affect the entire range of the fuel distributor from idle to fuel throttle.

If you use a gas analyzer and have CATS, the CO reading should be around .9 to 1.1

I use air/fuel ratio instead. I have a LM-1 from Innovate Motorsports. The AFR at 2000 RPM is set at 13.5:1 on my 308 GTSi. This is just on the backside of the power curve. If I try to set it at idle, there is too much outside air, which gets in from the exhaust pipes. At 2000 RPM, there is enough pressure to provide a more accurate reading.

Jeff

 

Can someone confirm that a US spec QV definitely has this metering unit adjustment screw my car has a rubber bung in it but no screw underneath when removed. If so any idea of the part number as mine is missing

 

The adjuster mixture screw does is, set the position of the plate during idle higher or lower. As you pushe it down (CW) it sends more fuel to the system as if it sucking more air. So, at higher RPM, the plate doesn't even touches the adjuster screw, so this is why I think it only affects the idle. But I could be all wrong on this.

 

Yes, it is below the hole and cannot be seen.

 

I, too, have a weird rubber stopper. Clearly a hacked off something-or-other that was used to fill the hole once the "permanent" US spec plug was crudely drilled out. It fits pretty well, but I'd rather tap the hole and have a screw in there.

Beau LeBlanc

 

These two diagrams might help with your mixture questions. Got these from a Bosch technical instruction booklet for K-jetronic.
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Second image
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Charles Probst's book Bosch Fuel Injection and Engine Management: How to Understand, Service and Modify explains in simple terms how the Bosch fuel injection systems work. A good read.

I initially bought the book because I didn't really know what 'CIS' was. I learned that it is basically a mechanically and hydraulically controlled system; one evolutionary step beyond carburetors. Simple and impressively robust.

 

Thanks guys, so no matter which market they should all have this mixture screw which means mine is missing.

Ill get one made as cannot see part number on the ferrari exploded diagram

 

I don't imagine it would run without that screw...

As Brian said, but adding that the mixture screw is about 1.5" - 2" below the hole, not immediately under the seal.

 

Doesn't the access hole have to be resealed for the system to work properly?

Also isn't that when the real exhaust reading should be taken, after resealing?

 

Yes


Yes

 

I'd be very surprised if it was not there. I cannot imagine how it would run. What makes you think it isn't there. Have you stuck a long 3mm allen wrench down the hole in an attempt to find it?

 

You are. The plate never touches the screw. It adjusts the position of the plate relative to the position of the arm allowing the central piston to be higher or lower for any given plate position.

 

Hello! Any ideas what would be easiest and safest way to take the plug out from the metering unit? I mean that plug which is located to mixture setting screw's hole. Understand that in US models it's denied to make any changes to mixture but I'm in europe,.. Reason why I might have set mixtures all over is that I removed all "world saving"-stuff and I think it might run bit lean now,.. Maybe,.. Have to check emissions,.. Mine SL is with K-Jet also and there is just a rubber plug in that same hole,..