WUR / idle mixture question
 

Cars going in for its MOT this Saturday (yearly government inspection in the UK for cars over 3 years old).
Last year the idle mixture was over 3% but they let it pass because I couldn't get to the mixture screw due to a custom intake bonnet that was installed.

I've now changed the bonnet and can get to the mixture screw fine.

It is rich as it surges on cold startup and a little on warm even.
When they lean the idle my warm and boost AFR will be off. I've leaned it a bit and it will certainly make a difference if they lean it even more.

My WUR has a hole drilled in the bottom to allow (I was told) warm and boost adjustment.
I've since read its mainly for warm but it will effect boost a little.

On the WUR which way do I turn it to lean or richen?

I'm hoping setting the idle mixture to 3% will only make a small adjustment to the AFR (warm and boost) range which I can dial back with just the above WUR adjustment.

Any thoughts?

Cheers

Turning that screw under the bottom plug clockwise will lean your mixture. (Raises your WCP)

You'll probably find that the general consensus regarding adjusting the idle mixture screw is that it will have only a minor impact on mixture beyond idle. Once you get your foot into the throttle, then things like your WUR adjustment and system and control fuel pressures will have a much larger impact on the mixture.

Apparently I'm missing something. Unless they do something to your car when "they lean it even more" that prevents you from enriching your mixture, why not let them do their thing, followed by you doing yours?

I recently failed an emissions test when I didn't lean the idle mixture to the satisfaction of the sniffer device. I knew the 13.5 or so my car likes would be too rich so I had leaned the mixture until the readings were in the mid 15s. Wasn't enough so I drove across the street and leaned the mixture until it was reading 17.1 at idle. Went back across the street and passed. After passing, I crossed the street one more time, set the AFRs back into the 13s and motored happily home.

Of course, I might be mistakenly assuming you have an AFR meter installed. Even without one you might be able to adjust it sufficiently by sound.

Good luck.

Tweeking the screw in the bottom of the WUR will change your CPs (mixture) across the range - it is changing the tension in the spring holding the bleed diaphragm - which controls mixture/WUR pressures. As posted earlier , screwing it in (clockwise) hardens the pressure up = leaner mix.
But the idle screw (3mm allen screw as opposed to 4mm in bottom of WUR) goes anti clockwise to lean. 1/2 a turn is a big (prob too big) adjustment.
You can set this to the 'happy' place by ear. Basically tweek it one way til the engine note just starts to go off. Note the position of the allen key - eg 3 0'clock. Then tweek opp direction til engine note just goes off. Note position and return to mid point. You will be very close to where you need to be. As Mark says this has minimal effect beyond low throttle openings.
Alan

Thanks for the info, just what I needed to know.

Will let you know how I get on, cheers :)

The CO% readings that some regulators use for pass/fail for your car have nothing to do with where your engine wants to be. As an engine ages it seems to want more fuel and is not happy at some arbitrary CO reading. If the two happen to coincide, well, happy days. But if they don't, to get the best out of your engine, you need to find where it is happiest. And if you pay careful attention to the engine note, you can find it fairly easily. Practise back and forth a few times with the 3mm allen key and you will be able to get quite precise settings. Try not to depress the allen screw when tweeking the key - ie use a light touch. Don't push down on the key, just rotate.
Regards
Alan

I should clarify - as I'm getting confused between my SC and 930. The 930 has a spring loaded adjuster screw. You have to depress the spring bit til the adjuster engages with the screw. Don't press down any more than needed to engage. (The SC adjuster is in direct contact all the time and depressing it deflects the airplate).
Alan

Update:

MOT went fine, no issues at all, CO was at 2.7% so no need to adjust it.
Its probably leaner this time round as I did lean it a bit a while ago! Guess work.

Right, tonight I did some fiddling.
Car started and surged for a bit as usual from cold.
Went for a run and warmed it all up.
Got back and marked with paint and a pin the starting position on the WUR body and 4mm adjuster so I can also go back to stock.

I richened the WUR with the engine off just a little, tiny tweak and the car then has problems idling, its just kept dying. So leaned the idle a little and it then started and idled OK.
Went for a drive and AFR was about 12.5ish and steady to about 5.5K then leans right out to like 14!!!!

So richened the WUR again, started OK, went for a run, had 11.2ish on boost and it went like a train, turned around and got 12ish and the way back and again lean over 5.5K

I've now turned the WUR about 10degree or say 4mm or so to richen but its holding at 12ish AFR very steady until 5.5K then high 13's

I had to stop there for the night, but it will be interesting to see how it starts and idles from cold now too.

I remember reading quite a few of you have lean issues at higher RPM. I'll have to look into that, my car obviously has an SC euro fuel distributor, so not sure if its at its limit about 5.5K or maybe its the standard SC fuel pump!

I'll try to richen the WUR more tomorrow and see if I can get it down to 11'ish as that was sweet. Maybe that will help with the lean out over 5.5K

Sounds like your boost dump may not be working. You need some CIS pressure gauges to check that. But a simple way may be to hook pressure into the WUR boost port (same as boost pressure) while idling and see if goes pig rich.
Alan

I've got a manual hand pressure pump I used to test the MSD boost retard so I could give that a go, cheers.

At 0.5 bar the WUR pressure should drop by about 0.5 bar from memory. But without the gauges, you just want to see the AFR drop a lot.

Identical problems to mine :

http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/839036-go-guy-modified-fuel-head-cis-advice.html

I'll try to adjust the WUR first tomorrow and if that fails get the preassure pump out.