Phatslax

Hi All

I have been getting to grips with my WUR and pressure adjustments; I have been using the common approach which is to do the warm pressure first (with the bimetallic bit out of the way) - by tapping the large fuel cell down after letting the WUR heat soak with the 12VC connection on....

....and then adjust the cold pressure by tapping the smaller post down once its cooled down again. (Assuming both have been reset to above their normal operating range)

I have the WUR with the two vacuum connections (one on the lower left side which uses post throttle vacuum to lean out the mix at idle, and one on the top that goes to the non vacuum side.

So far so good and I can get the control pressures near enough without the car running - car runs very smoothly, but gets overly lean (and demonstrates high control pressures) as it warms up at idle.

The amount of effect the vacuum produces is large >1bar control pressure when warm.

My conjecture is that there appears to be a third adjustment that is possible and that I need to make - this is the adjustment that can be made internally via the Allen key that varies the loading on the central spring which pushes the rod into the the fuel cell.

Consensus over the internet seems to be that this simply is another way to adjust warm control pressure, however this adjustment only effects the inner spring, which in turn is modulated by the vacuum cell underneath.

Therefore this is also an adjustment for the effect of the vacuum on the control pressure; i.e. as the spring tension is reduced by winding out the adjustment screw the vacuum will have less effect as the outer spring starts to take more of the load and the inner spring takes less, so the inner spring effect due to vacuum changes is relatively less.

This leads me to an additional step where the vacuum adjustment must be done first by balancing the relative positions of the springs such that when vacuum is applied it gives the necessary offset to the control pressure change as per the specs. (note I have rebuilt my WUR and did not notice how that adjustment was prior to rebuild, therefore its very likely to wrong in mine - normally I guess this is not an issue)

Then as per normal process the warm pressure is set by tapping the fuel cell down; this loads the springs uniformly and therefore (assuming a linear relationship) the vacuum offset should not change too much)

Then the cold pressure.

I have not seen this on any of the Forums, so could be flawed in my thinking. What does this forum think?

The effect would be to control the swings between leaning out / enrichment, at Idle -> WOT. Could it also effect hot starts?

Cymadesign

Take a look at this thread ( https://rennlist.com/forums/928-foru...-a-friend.html ) where @Chopperharris and I discussed this adjustment. Following our last exchange I rebuilt my WUR again and this time followed the instructions from K-Jet Specialists to reset it. The result was tremendous. Control pressures are within spec with and without vacuum and the throttle response is great. I have a wideband O2 sensor and fuel pressure sensor (measuring warm control pressure) and can see both respond to changes in vacuum.
Here's the link to K-Jet Specialists. https://k-jet.biz/rebuild-manuals/

Phatslax

Thanks. How did I miss that!

Chimes perfectly with my experience. Today I did iterative enleanment steps with the inner spring perch adjustment and have now got approx 0.5 bar delta from the vacuum add whereas it was >1 bar previous. Could probably do with another tweak but have the leak you mention in your thread.

delta is the same at cold and warm (engine off, I suspect that AAV screws around with it when cold)

control pressure range and vacuum add is now very close to the specs in the Bosch manual, but overall a bit low in terms of offset.

getting close I hope…

Chopperharris

Hi, sounds like you’re pretty much there already but here are my finding with that pesky WUR.

Firstly when you rebuild it with a new diaphragm make sure it hold vacuum from both ports. If using one of the rebuild kits I found a small amount of Hondabond (or similar) was needed around the small diaphragm screw holes and the center cup for the smaller spring.


For adjustments - in order of how I do it:

Cold pressure is done via the small post (definitely make this adjustable by following one the many guides). Moving the post down reduces Cold Control and vice versa.

Warm pressure is done via moving the large valve piece up and down - tapping this down increases warm pressure and vice versa. To decrease you have to dissemble the WUR to tap the piece outwards.
Alternatively, you can make the warm control adjustable by drilling a hole in the brass (?) cup in the base which allows access to the small spring base which is adjustable with an Allen key. Turning this CW will reduce Warm control, as it reduced the pressure on the valve, and vice versa. Downside i found with this is can cause a vac leak by damaging the thin diaphragm so I try to avoid using this method.

Finally the vacuum vs no vacuum (side port on type 86 Euro S WUR) pressure differential (should be 0.6 bar) is controlled by the height of the brass cup mentioned above. It simply dictates how much that center spring perch, which is mounted in the diaphragm, can flex downwards. If you tap this inwards the differential reduces and vice versa.
The WUR is designed to have vacuum here at idle and ‘normal’ revs which is what the cars is tuned too A/F ratio wise. At WOT there is no vacuum so the diaphragm drops to the level dictated by the cup and who you get your enrichment.

If you have drilled this cup to make warm adjustable then make sure the inside of the cup is clear and there are no obstructions to the perch from the drilled/flayed metal. I also make sure that it’s is sealed via small rubber bung.

What I’m still not sure of is if there is some granularity of enrichment in between ‘normal’ throttle and WOT. I rigged up a gauge inline to the port on the side of the WUR and watched the behaviour whilst revving the engine. It was fairly binary so not sure there is micro adjustment of control. Also did something similar watching the control pressure on the gauges and again there was not enough variance to point to it working this way. Also, maybe the height adjust of just the small spring give changes the progressiveness of the overall spring set up leading to more granularity of enrichment with more revs…..? I suppose it would be boring if there wasn’t some level of mystery involved 🙃

Cheers, Ben

gbgastowers

I wouldn’t keep messing about with this. Just make sure it’s the correct part # from the factory and send it off to K-Jet Specialists in Australia and they will send you a rebuilt one that is calibrated for the part #. Then you are good to go with the WUR for as long as the car is run regularly. Sorry but this will give you the best results and peace of mind over that particular part. Same for the fuel distributor. If you replace the injectors don’t do cheaper Mercedes unless you can’t find original long ones designed for your engine. Gunar

Chopperharris

Yeah agree for the FD definitely - wasted a lot of time playing with that as I remember you did to from an old post! I think my WUR is good now so I’ll leave as is - if I spot a good used one I may get this sent of so can compare.
Why do you say that about the injectors, I thought most thought they were ok?

Cheers

gbgastowers

I think they made them an inch longer for improved efficiency and performance. Just a guess

Phatslax

Thanks for the responses;

After this round of intervention the engine runs well and sounds great. Can’t road test it because the drivers seat is out and my new high torque starter has crapped out. Hot starting is still a sweaty moment even when the starter works.

im running a Mercedes adjustable FD and the original WUR with cheapo brass short injectors. The original definitely needs some pro set up.

another slightly philosophical question; should the WUR be ‘tuned’ to the FD? Or the engine? I suspect in my world where everything js a bit worn it’s makes not much material difference.and the specs are pretty close anyway

Phatslax

this is the interesting bit. In mine the brass bit appears to have no contact with the inner spring perch. This travel is limited by the assembly that has the lower (large) and upper (circular) diaphragm with vacuum contained between - so my conjecture is that the Allen key can not only adjust the warm pressure but also the extent of vacuum travel. BUT also alters the combined spring rate of the 2 springs that then affects the resistance to the heated bi-metallic strip and thus the amount of deflection. But this should only be during warm up so is not so critical.

So.. my theory is that you adjust for vacuum travel with the allen key. This sets that range over which the vacuum is effective relative to the larger spring. Once set then the warm pressure can be set via the fuel cell / valve, because at this point the combined spring rates are set and linear as they are compressed- so the vacuum delta should remain the same.

then adjust cold pressure etc…

But mines a 4.7 euro S too, so will look more closely when I next disassemble…

gbgastowers

Not tuned to that MB fuel distributor. I messed about with one that came on my car for years and finally got the correct one that the car left the factory with. This cured all my problems along with in spec, correct part # WUR and injectors

Chopperharris

Hi,

I’d check again next time you have it apart. I agree the smaller spring position in relation to the larger spring will have some effect but that brass cup is the lower stop which you can see in the helpful diagram in this post:

https://rennlist.com/forums/928-foru...tructions.html

Cheers

Phatslax

Thanks for the link - I will look closer. I'm sure it'll come apart again pretty soon.

So in that event the tuning procedure can be made even more complicated!

I guess the spring perch sits at a certain level and is raised as the vacuum increases, via the side vacuum hose. So in this case the brass hat can be used to adjust the resting position and therefore the maximum enrichment point...However...It could also be set lower so that the perch can respond to another vacuum signal...

I am not sure if the perch can go the other way via the other port which may or may not produce vacuum when the throttle is open? (does it use a Venturi effect attached? like the brake boost line? Its connected back to a port on the throttle body but not sure if this is just make sure filtered air is used.

Chopperharris

TBH I’m not sure it’s that complex. Keeping it simple the WUR is there to allow enrichment at cold start up and at WOT. It does this my lowering the control pressure so the Fuel Disti allows slightly more fuel to flow to the injectors for the same air intake. The two mechanisms for doing this are the heat element activated lever (for cold) and the vac mech we’ve been discussing.

From the factory i suspect they are set up via the Small post height (for cold) and then via Allen screw in the base (for warm). Doing it that way means adjusting warm does not affect the prior cold adjustment. The brass hat/cup is then added to act as resting point for the diaphragm and spring perch at full deflection (no vacuum) resulting in the 0.6 bar drop. Whilst there probably is some granularity between say 1/2 throttle and WOT I’m not sure it really makes much of a difference to the normal running of the car from a AFR perceptive. The WOT enrichment is most likely there to give slightly more power (when you obviously want it by mashing the pedal!) and to protect the engine from any possible pinging etc.

Good vid by this chap on YouTube on the working including cut away devices for illustration. Not sure I agree vibrating the engine will move the post and the valve out of position though!


Cheers
Ben