Fuel mixture on 5KTQ?

[quk@sievers.com (Phil Payne)]

In message <1997Mar20.102200.1772.545961@pu-smtp.radio.bbc.co.uk> paul.heneghan@bbc.co.uk (Paul Heneghan) writes:

> If you 
> remove the cap, and insert a long 3mm Allen key, you can adjust the position 
> of the fuel control arm relative to the air flow measuring arm.  This 
> adjusts the mixture.  DON'T REV THE ENGINE when the Allen driver is inserted 
>  - I think it can put strain on the bearings (I've done it once or twice by 
> mistake, and haven't noticed any ill effects, but all the manuals carry this 
> warning).

I had great difficulty finding a long enough 3mm Allen key.  Finally a tool 
shop owner asked: "What do you want it for?"
 
I said: "Adjusting Bosch fuel injection" and he just reached over his head 
without looking and banged it on the counter.

The precise technique differs by model.  Most instructions say to switch off 
_all_ elecrics, though I think the WR ur-quattro says to turn on the 
headlights.  All of them say that measurements should be suspended if the 
cooling fan is running.  The book says to rev the engine (tool out!) and then 
let it settle - always adjust by backing off anti-clockwise and turning 
clockwise to the desired position.  The adjustment is quite sensitive.

> If there is an air leak (split hoses, hard injector o-rings etc.), then 
> extra unmeasured air gets sucked into the system and makes the mixture too 
> lean.  Most mechanics don't bother to find out why the mixture is now wrong, 
> they just enrich the mixture by raising the fuel control arm relative to the 
> air flow measuring arm (from memory, I think the adjusting 3mm Allen screw 
> is turned clockwise to do this).  This means the mixture is correct at idle, 
> but is now wrong at other speeds/engine loads.

Absolutely.  Disconnect and clamp the breathers before adjusting, then re-
attach and check the measurements.  If there's much difference - investigate.

You're essentially matching two curves - air volume against fuel volume.  If 
there's too much unmeasured air leaking in, it can be compensated for by 
shifting the fuel curve until it intersects again - but the top of the curve 
will have moved more.

--
 Phil Payne
 phil@sievers.com
 Committee Member, UK Audi [ur-]quattro Owners Club