Another try at adjustable WUR sketch

[Gunter]

Finally, it worked! Here is the inside of a 1980 SC WUR. Does anyone know if the small adjustment screw inside is used for adjustments? I have not done the drilling and tapping in the plug which allows adjusting from the outside, so, picture this WUR without the modification. Do other years have the internal adjustment screw and what is it for?

[Gunter]

Bump!

[wckrause]

My 79 WUR did not have that set screw. Very interesting. The screw would only allow for modification of the cold start pressure. Because as the bi-metallic bar heats up, it's effect on pressure is removed, and control pressure is determined soley by the spring.

The external adjustment screw would work much better,because you could adjust it while the car was running and with a CIS pressure tester connected.

[Gunter]

Has anyone else seen this internal adjustment screw in a WUR? If so, what year? I agree with Bill that the external plug-modification and the fuel pressure gauge is the preferred way, still, there must be a reason for the internal screw. Anyone?

[bigchillcar]

I agree with Bill that only the initial cold-start pressure would be adjusted. This could be advantageous if some other components in your fuel system aren't operating 100%, perhaps like being able to compensate for altitude. Do you have a pic of the entire width of the WUR to attach as well?
Ryan

[Gunter]

Hi 007: There is not much more width on the right side. The whole thing looks like a regular WUR, except for the internal adjustment screw. The '78 - '79 have the same WUR, 1980 SC has its own model and '81 - '83 are again interchangeable. I would like to know if the '81 - '83 also have an internal adjustment. Anyone?

[cmonref]

My Euro '80 SC still has its original WUR, and here's an internal adjustment screw that I found to be quite useful, and the only way I have found to adjust the WARM control pressure.

For an '80 SC, take a look at the end of the WUR that does NOT show on the picture in the original message of this thread. The drawing for an '80 SC is different -- there are TWO springs that influence the cold pressure, and there is a vacuum port at the bottom of those springs. Between the two springs is a diaphram that is the top of the vacuum chamber for the vacuum port. In the center of the diaphram is a hollow machine screw and a nut. Inside the hollow machine screw is an adjusting screw with a 3-4 mm internal hex fitting. This adjusting screw adjusts the compression on the upper spring, thus adjusting the pressure on the operating rod that pushes upward on the fuel diaphram, thus adjusting the WARM pressure of the WUR. To adjust, remove the spring plate temporarily (so the WUR thinks it is WARM), reinstall the WUR, turn on the fuel pump, and check the pressure. Remove, disassemble, adjust, and repeat until you get the pressure where you want it. In about 3 tries you can get the WARM pressure right on the spec. Put the spring plate back in, adjust it for COLD pressure, and you are set.

In this manner, the WARM and COLD pressures are adjusted independently and have no effect on each other. The COLD pressure is controlled by the bending of the horizontal spring plate. When the WUR is "warm", the spring plate no longer holds the spring (at the right) down, so the compression of the vertical spring controls the WARM pressure.

I've never heard anyone else mention this adjustment screw in my 20 years with this car. My SC does not have the adjustment screw in the long spring plate as shown in the picture.

[wckrause]

Brain,

Do you have any pictures? I didn't notice that warm pressure adjustment in my '79 WUR, but maybe I overlooked it, or maybe it's only on the '80 WURs.

[cmonref]

Nope, no pix. It has been years since I "discovered" this when poking around inside my disassembled WUR. And its presence does NOT jump out at you. I stared at it for quite some time before I realized what I was looking at. In fact, I took it apart BEFORE I realized it was an adjustment screw!!

"Gunter" describes his picture in the original message as from an '80 SC. Not really -- that picture is from an earlier WUR. The original one in my Euro '80 SC is like the ones pictured in Pelicans catalog for 78-79, and 80, and in parts diagram 1-7-2. The key difference is the vacuum port under the spring at the right side of the picture in Gunter's message. On Pelicans pictures, you see it externally as a vacuum line connection on the left side of the WUR. If you have that external port, it is possible that you have the same adjustment screw that I found in mine. But I wouldn't guarantee it, because I've never heard it mentioned or drawn anywhere else before in 20 years of watching.

If you're interested, I can scan a picture I have of the cross section of the WUR that shows the vacuum port, and could in more detail describe how I think it works and where the adjustment screw is. But that would have to wait until tomorrow when I can get to a scanner.

Brian Haggard, Chester, VA

[orph]

Because of poor idling following initial start-up, I've been investigating the WUR on my '86 930. Tech article, if I recall correctly, says resistance should be 22-26. When checking mine several times, I always come up with 45-46. Does this mean mine is "bad" and is non-adjustable ?

[Gunter]

Brian, please, post a sketch. My '80 SC is US, not Euro. The WUR on my 3 Liter stock is BOSCH # 0 438 170 009 and it runs beautifully. Now, I obtained a used WUR from an '80 SC with the identical BOSCH number and took it apart. It does not have the "plug modification" as shown in my sketch, but it does have the internal small adjustment screw as shown. It has one round, main spring designed to work against the flat bi-metal plate which, when warmed up, will move the small plunger on the diaphragm and affect fuel pressure. It also has the outside vacuum nipple but I don't have a hose connected. The 1980 SC US stands alone; according to Porsche, and the Pelican catalog, it has its own WUR and cannot be interchanged with other SC-years. Brian mentions two springs but without a sketch I don't understand. Where is this parts diagram 1-7-2? Also, when checking for resistance across the heater element, I get none on both engine- and spare WUR, just closed circuit. Some have mentioned 20 Ohms? Let's get to the bottom of this, please

[Gunter]

This is a sketch from a WUR (What year?) submitted by someone earlier. It shows details of the plug modification but does not show the nipple for the vacuum line or state what year it is.

[Jadams1]

For a reference, I have a 80 Euro SC, the part number of the WUR is 911.606.105.09. It has a vacuum line attached to it.

[1980SC]

There were two main types of WUR's for SC's.

First style was for k-basic and had the lower vacuum chamber, and two springs. (All euros and '78-'79 USA)

The second style was for K-lambda, and did not have the lower vacuum chamber. Only one spring. ('80-'83 USA)

The difference in the USA '80 and '81-'83 is just a slightly different warm-up control pressure curve as shown in the charts in the Bently manual.

All of the SC WUR's have a vacuum port on the top. Only the k-basic WUR has the vacuum port on the side, for full throttle enrichment. K-lambda uses the frequency valve for full throttle enrichment, so it does not need the lower vacuum chamber.

Here is a pic of the insides of the k-basic style WUR. The smaller spring sits inside of the larger spring. This is a USA '79 WUR, but I use it on my '80 Euro. (both k-basic)

The screw which Brian mentioned is on the center section of the lower vacuum chamber which can be seen on the far right.

[Gunter]

Thanks 1980 SC! Great stuff! That makes sense. What resistance do you read across the heater element?

[1980SC]

I just checked, and my two k-basic WUR's read 23 and 25 ohms. By viewing the warmup control pressure graphs in the Bentley manual, you can see that they've changed how quickly the bi-metallic strip heats up, so different years should have slightly different ohm readings.

Rob
1980SC

[Gunter]

1980SC. How do you get the reading? I put my leads on the outside electric connection ( 2 pins) of the WUR and get a closed circuit. How do you obtain approx. 20 Ohms? Where do you put your leads? Cheers.

[cmonref]

Great picture, 1980SC!!

The attached picture is a cross-section of a WUR that is almost identical to my '80 Euro SC. The only difference is that mine has an external tube at #9 that is hooked to a vacuum line. That vacuum line goes to the thermovalve.

In the picture, the WUR is in the "WARM" mode - the bimetal spring has lifted off the top plate (the plate above spring #7). When COLD, the bimetal spring presses DOWN on that plate, compressing the springs -- #7 and the unnumbered INNER spring -- allowing more gas to bypass from #6 to #5, lowering the control pressure, thus enrichening the mixture.

1980SC's picture shows the innards quite well. #10 is a rubber diaphram. It is sandwiched between two washers that have their perimeters turned away from the diaphram as shown in the drawing. The two washers are held together by the part that contacts the lower stops (#11) in the down position and pushes up on the INNER spring. The adjusting screw that I found runs vertically through the center of that part (lower right of 1980SC's picture), and can lengthen or shorten the overall height of that part. This length adjustment results in variable upward pressure on the INNER spring, thus adjusting the pressure on the upper diaphram (#4). The upward pressure on #4 varies the fuel pressure, thus varying the mixture.

[wckrause]

I've seen that diagram before, and I've always been confused by it. It says that the full load diaphram is at the idle position. But for it to be in the full-load position, and to reduce control pressure, then the intake manifold pressure would have to be greater than atmospheric. That can only happen with a turbo.

[cmonref]

There are two opposing forces on the lower diaphram: Upward force from vacuum at #3, and downward force from the inner spring. With high vacuum at idle, the vacuum is stronger than the spring. As engine power goes up and vacuum goes down (intake manifold pressure approaches atmospheric), the vacuum pull UP is dimished and the spring pushes the diaphram DOWN, enrichening the mixture.

What perplexes me is the source and strength of the vacuum signal. The port on the throttle valve is above the throttle plate so the range of the signal is smaller than if the port were below the plate. I then leave it to faith that Porsche and Bosche measured the signal and properly balanced it with the inner spring rate to achieve the desired movement of the lower diaphram.