A WUR tuning mod

[Alan L]

I could never get my WUR to function properly over the whole fuel range. If I got the low end OK, it always dumped too much fuel on boost. If I dialled the boost fuel back I ended up too lean at the low end - coughing and bucking.
The problem is you only have a couple of parameters to fiddle with. The Control pressure basically sets the range and is supposed to behave properly via internal springs and a boost diaphragm/spring enrichment process.
I spent so much time trying to get this to work properly for the track, I finally gave up and went digi WUR.
But it has always bugged me and I suddenly had an idea a few days ago that I figured would fix the problem. It has definitely improved the situation to the point the WUR is functioning well.
I have basically installed another tuning point which is capable of independantly adjusting the size of the boost fuel dump. So you can set the WUR CPs up to give you an ideal pre boost fuel range and use this mod to look after the boost end.
Cost about $5 (and quite a bit of time in the shed).
Basically I removed the sealing plug in the base of the WUR and replaced it with a thicker plug which allows me to thread a screw/locknut into which projects up into the WUR just below the centre point of the boost diaphragm. By setting the range the diaphragm is allowed to travel you can independantly control the CP drop.
I dialled mine back to 4 psi compared to the factory spec of around 15psi.
At the factory numbers I was running AFrs in the 9-10 range if I had my low end fuel range rich enough to not have any low throttle hesitation.
The dimensions of how to set this up are very tight - you dont have a lot of room to play with. Basically I am letting my diaphragm open about 1mm, so my screw length needs to be just enough for this range. Any longer and I end up with a lot of screw sticking beyond the base of the WUR. This is not fatal - you could probably shim the WUR off the base plate to allow room.
But I machined the head of the screw down to a bare minimum and machined the lock nut down to about half the normal length. My system is near flush with the base. I machines a slight recess into the mounting plate to accomodate any small outwards adjustment on the screw. I also machined the diameter of the screw head down so it was just inside that of the lock nut - so I can insert a ring spanner on to the lock nut.
The depth you set the brass plug is also important for this. The plug is a 20 mm Frost plug - which is about 1 mm thick. I used a 6x1mm screw. Tapped a thread into the Frost plug to lock the nut against. I chose 6 mm because the diameter is just enough to go thru the hole with your Allen key to tweek the adjuster screw for your CP settings. You can see this screw in one of the pics. I have provided a dissasembled pic as well so you can see how the boost diaphragm functions and what the mod is doing to control this function.
The system works and seems to allow quite fine tuning. About 1/4 turn inwards raised my boost AFRs from around 11.5 to 12. A 1/4 turn is 0.25 mm internal adjustment.
Hopefully others might find this a useful trick. You just have to spend a bit of time stuffing around getting the clearances and screw length right.
The spring in the pic is not a standard spring - you wont find a red spring in your WUR like this one. I previously played with spring rates to try and control this problem. Still fine tweeking it now same way in the low- mid boost range.
Alan

[RarlyL8]

Adjustable WUR's have 4 points of adjustment; cold, warm, threshold, and boost.
It looks like you have taken care of boost but not threshold which will give you even more control.

[Alan L]

Brian,
What is the threshold adjustment you mention?
It is the boost dump that was causing me so much grief.
Alan

[T77911S]

what is the threshold adjustment

[JFairman]

Boost threshold adjustment in the bottom of the control pressure regulator is not there from Bosch. It is something a very talented mechanical engineer that owns a silver 930 origonally designed for his own car to make it run better. He's a really cool guy that lives and surfs in southern California.

His name is Brian Leask, you may have heard of him on the forum.

The threshold adjustment is three 3mm allen head adjustment screws that adjust the static hight and tension of the large diameter spring inside the the control pressure regulator independent of the other 2 spring tension adjustments on the bottom of the CPR.
The boost threshold, warm steady cruise, and boost control pressure adjustments are all seperate and they all affect each other a little and finding the best running balance for your car takes some road testing and playing around with them and a good accurate wide band AFR gauge on board.

The cold start control pressure adjustment on top of the WUR/CPR repositions the hight of the flat bimetallic spring with heater element wrapped around it and it seems to work more independently from the other 3 CPR adjustments on the bottom.

For someone that is having a hard time getting the control pressure adjustments working right across the entire driving range of these cars like the origonal poster it's probably from the CO adjustment on top of the air flow meter housing being adjusted one way or the other too many times and it's now in the wrong base start point default position and they are now inadvertantly trying to compensate for that incorrect mechanical airflow meter linkage to fuel head control plunger adjustment with the hydraulic fuel control pressure regulator adjustments.

Also, the at rest hight of the air flow sensor plate in the metering cone may be wrong and out of adjustment.
There is a Bosch PDF showing the correct at rest hight for the air flow meter sensor plate in the metering cone and how to adjust it by carefully tapping on the little adjustment pin in the housing next to the fuel inlet line to the fuel head with a small drift punch.
Tap it in one way too far though and you have to take the entire housing apart to tap it back the other way.

[fredmeister]

Alan,
Thats a great write up. Good job.

Were you able to find a replacement green diapragm in the second picture above in the event yours was worn? I suspect after 30 years this might show some damage from aging and it would be nice to know where to order a replacement.

Does anyone know if this is the heavy diapragm modification that Leask refers to on his website?

[Alan L]

Fredmeister - I live at the bottom of the world, where 930 parts are like the proverbial rocking horse sh.t. My source of parts is mainly thru Pelican - either the F/S column ar our host, or where the good folk here can direct me for suppliers.
As far as I know, no one has found a source for replacement diaphragms, altho I do recall a thread about rebuilt WURs where someone must have access to new bits - but not for supply. Thats as I recall it.
Having said that, the diaphragms seem very robust. I recently pulled a friends '77 WUR apart and the diaphragm was still intact - altho not as worked as the 930. But they do seem well designed for longevity. When they do fail I guess another WUR is in order - since the Digi WUR seems to have fallen over - I sent my last one back about a year ago - at their behest to get it rectified/working. Heard nothing since.
Regards
Alan