adjustabe wur idea
 

i think that the wur works as follows:
the heater heats the bimetal at a constant wattage. when the outside air is cold, as drawn in from the vacuum ports the heater can only expand the bimetal a certain distance, which causes the pressure to be increased which causes a richer fuel supply. as the outside air temperature increases, the differential temperature decreases and the bemetal can expand to close off the pressure and lean out the mixture.

i believe that by adding an adjustable resistor (AR) to the power feed to the wur, 0 to 75 ohm should do it, then you will effectively create an adjustable wur without having to remove and drill the wur. i think that at 0 ohms on the AR the heater will be full onl and cause the bimetal to lean out the mixture unless the outside air is very cold. as the AR is moved to 75 ohms the heater will esssentially be de-energized and cause the outside air to contract the be-metal and enrichen the mixture.

just a thought and worth a try if you are looking for an adjustable wur.

gary




if the above is correct i think that by inserting anadjustable resistor in the power feed to the wur, that you could make an adjustable wur without having to remove or do any work on the wur. because if you reduce the current flowing to the heater the bimetal will close tighter against the pressure port which will enrichen the mixture. by lowering the resistance in line with the heater the heater will increase the temperature and cause the wur to lean out the mixture.

All that would do is slow down the rate at which the WUR leans out the mixture.

Outside air isn't drawn into the WUR, and outside air temperature has nothing to do with the operation of the WUR. Once the bi-metal lever is heated up either by the heating element or through engine heat, it no longer has any effect on control pressure.

what purpose is the heater?
 

i think that the wur is more complex than that. i dont think it relies on force balance to achieve the mixture adjustment. i believe that since the heater is always operational, ( at least on my 1975) then the element is always being heated by this device. that made me think that there is a reason for this heater to be energized when the car is running.

if the engine heat was the method of heating up the bimetal then why would the heater be necessary as the engine heat or lack of heat would perform the same function, i.e. when the engine is cold the bmetal would enrichen the mixture when the engine is warm the bimetal would lean out the mixture.

the wur is more sophisticated than that, and i think that by using the heater a very innovative method of dynamic fuel mixture adjustment is occuring using differential temperature caused by the outside air and by the heating element. if this is not the case then the system would use a simple warm up thermostat to adjust the fuel after the engine warms up without needing the heater or all of the diaghrams and chambers.

that is why the wur is so confusing and can cause so many problems. if the heater is not working you will always be rich, try disconnecting the connection to the the wur and take it for a drive i would bet that no matter how long you drive (maybe in a very warm cllimate) the car will run rich. then reconnect the cable and see how it looks.

full power on the other hand to the wur only allows the wur to lean out the mixture when the outside air temperature overcomes the heater and the bimetal moves to the full lean position.

i may be wrong but i think that the people that designed this device were way ahead of a simple warm-up regulator.

gary

Yes it would, but it would take much longer for the mixture to get leaned out.

There is no outside air flowing through the WUR via the vacuum lines. If it were to do so, it would be called a "vacuum leak" and would need to be fixed. Try blowing into one vacuum line and see if you get air out the other. If you do it's because the internal diaphram is torn, not because it's suppose to work that way.

Your hypothesis about how the WUR works is wrong.

Old Post: Picture of WUR Internal Set Screw
 

Here is a picture of the internal set screw on the bimetal arm inside the WUR, from my 82 SC. The part number is 0 438 140 090, and it has one vacuum connection at the top that goes to the throttle body, but nothing at the base near the main spring.

The set screw and electrical contact are actually on a second, smaller bimetallic arm.

You can also see in the yellow circle that there is an electrical contact switch, which I believe interrupts the heater circuit once the bimetallic arm has bent fully. Perhaps the set screw is to adjust this temperature point and therefore the warm control pressure.

Can anyone explain how this screw would affect the operation of the WUR?

http://forums.pelicanparts.com/uploa...1061953497.jpg

Olivier

I suggest that you put this question, with pic, as a seperate thread on the board if you get no answer. The sc's have different WUR's for '78-'79, 1980 stands alone, and '81-'83 has a different one again. On my '80, the vacuum line isn't even connected to anything and the WUR works fine. I noticed the same adjustment screw on the inside and assume that it is for calibration. Good luck.

Wow!

I have to agree with Bill that the WUR is a simple device. It is just a back pressure device. The bimetal strip counters the spring force when cold and the heater relieves the spring force as it is heated. With a fuel pressure gauge, one can monitor the control pressure from a cold start to see that it gradually rises. It does not take long. The vacuum (‘76-‘77 and ‘78-‘79), depending on how it is applied (top or bottom of spring-diaphragm), will affect the spring force too. On the ‘78-‘79 the vacuum connection is on the side, opposing the spring force to heal reduce control pressure during cold starts. That vacuum source is interrupted via the thermo-valve. The’80 did not have an atmospheric pressure (intake pressure) tap; the 81-83 did have an atmospheric tap on top of the WUR. These don’t really have much accept on the WUR. Atmospheric connection was meant to compensate for changes in atm pressure changes (barometric and elevation changes). I think it would have top be a large change for one to see that the “vent” port does work. Or try blowing into it. There is no air flow through the WUR. Key word: FLOW.

Remember the lambda cars had lean-rich adjustment via the frequency valve.

It is likely that the heating element does short out as the bimetal strip rises per the earlier suggestion related to the little screw on the stripe assembly. Think toaster. Or it just hovers at some threshold between to much heat and just enough heat due to the current flow to it. Resistance on the heater is what creates the heat right? So a higher resistance may increase the ramp rate from “cold start pressure” to warm control pressure. Zero resistance is bad and so is too high of a resistance.

It’s really not that precise, but it works and a couple of pounds delta in the control or cold pressure will not make the CIS misbehave.

When warm the control pressure is controlled solely by the spring. That’s it.

You want variable control pressure? How about a PID loop with a digitally controlled back pressure regulator with logic or human interface. Not really practical.

Just get your cold pressure within spec (adjustable WUR as others have done makes it easy) and your warm pressure in the range as suggested by the readily available P-T charts for your CIS engine. If your spring or diaphragm are shot, get a new WUR or rebuild it, or look me up and I’ll build you a PID Loop controlled setup. Price discounts for Pelicans.

Souk -

Could you explain a little more on ways to make warm control pressure adjustable on the go? I occasionally change my warm control pressure, but it requires disassembling the WUR as Brian Haggard mentions on the first page of this thread.

http://forums.pelicanparts.com/uploa...1062004991.jpg

I'd love to be able to make adjustments on the fly, but I haven't been able to think of a way to do that??

Thanks,
-Rob
1980sc

The small set screw inside the WUR changes the sensitivity of the second (smaller) bimetal strip in opening the set of contacts visible in the photo I posted. At the plug connection on the outside of the WUR, I get 25 ohms with these internal contacts closed and 10 ohms with them open. It seems that this is the way the WUR regulates the warm control pressure, sort of like a simple thermostat to reduce the amount of resistance through the larger bimetal strip as everything warms up.
Actually, now that I think about it again, it is probably to alter the speed that the WUR moves the system from cold to warm control pressure. The small bimetal strip is not heated, so it is only affected by the ambient temperature inside of the WUR, which would be mostly dependent on the outside air and engine temperature. A closed internal contact during warmer conditions would heat the larger bimetal strip more quickly, so the car would not run rich for as long. In colder conditions, the large bimetal strip will get less resistance and the car will take longer to go from the enriched to the lean state for warm running.

Olivier

Methinks you are right about the resistance controling the duration of the warmup cycle. But it does not control the fuel pressure once the cycle is complete. At the end of the warmup cycle, the bimetal strip has bent upwards far enough that it no longer has contact with the spring mechanism, so it no longer has an effect on the pressure. Beyond the warmup cycle, the bimetal strip probably still needs a little heat to keep it bent upward and "out of the way".

I agree...I think the only way to affect warm control pressure would be to change the spring or "adjust" its shape, which would be hard to do accurately and hard to reverse. I set my set screw so that I would get the slow warm up cycle below about 50 degrees, but I will probably have to tweek it more when it gets colder. It is easy to check the switching point based on ambient temperature just by checking the resistance at the plug connection.

Agree. Messing with spring rates is not a particularly wise thing to do. It is hard to argue that Porsche/Bosch did not do it right. Once the "cold" and "warm" pressures are set, things run pretty well and I don't think we can improve on it.

More great posts and technical discussions from the past.

What is the effect of venting the vacuum to atmosphere disabling the diaphraghm ?

http://forums.pelicanparts.com/attac...sketch-wur.jpg