excerpt from Playing with your K-Jetronic Warm Up Regulator, or WUR
by John Pitman

Testing.
To test the pressures in the system you NEED a gauge set that can be connected in the line from the distributor to the WUR, with a tap in the leg connected to the WUR. This consists of a T fitting with a gauge (0-100psi) on the stem, a hose than can fit to the WUR inlet pipe on one side, an open/closed tap on the other, then a hose that will connect to the WUR inlet.
You will also need the following pressure data to evaluate your readings:
928 System pressure 65-75 psi
Control pressure at 50F 17psi
75F 30psi
100F 44psi
Warm Control 41-46psi
If you need more data, get Ben Watson, How to Tune and Modify Bosch Fuel Injection, from Amazon or nearest available source.
You could do some empirical adjustments based on observations without a gauge (eg, sooty exhaust when hot = rich mixture=low control pressure), but there could be other causes, such as maladjustment of the mixture screw on the distributor, and you could just make things worse.
The actual tests you need to do will vary with the problem you are chasing, but this is what I did based on Watson.
Connect your gauge in the inlet line, with tap on the WUR side of the gauge. Start engine, preferably from cold. Note the initial gauge reading, and then wait until it stabilizes at the warm setting. If you now close the valve, you will see the System pressure – if you leave the valve closed, Control will equal System, and the mixture will go lean and probably stall it, so open the valve again once you have a reading. Bad system pressures are probably produced in the main regulator in the distributor – talk to JK here, as it remains (thankfully) outside my knowledge. Also check the pump and filter for delivery rate etc.
Now, if you are VERY lucky (and chasing a problem), you have a low control pressure, rather than high. Note from above that to increase control, we need to push the regulator INWARDS in the WUR body – with a driver or a brass drift, give the centre of the regulator a smart rap, and watch the pressure – it should rise a bit. Take this slowly, as overdoing it will mean remove, dismantle, tap the regulator outwards, assemble, refit, and retest. As you lift the control reading, the mixture should lean out and you should notice an improvement in the idle, exhaust note change, and soot should disappear from the exhaust.
If, like me, your control is high (more likely), you need to find out if it is due to incorrect calibration of the regulator, or a blockage in the system somewhere. Watson recommends locating blockages by – loosen the nearest connector in the direction of flow of fuel, and see if the pressure drops; if it does, the problem is further along; if it does NOT, the problem is one step back up the flow. That is, if you open the return line from WUR to distributor, and the control does not drop (mine didn’t), the problem is in the WUR (mine was)! If it hadn’t, I would then have opened the connection of the return to distributor, then on to the line to the tank, then hose at the tank entry, then pipe in the tank. Blocked pipes should clear with some solvent like Carb/Brake cleaner, and application of compressed air – WITH CARE!
Remember to take all appropriate precautions when opening connections with high pressure fuel inside!
Okay, we have a high Control pressure, and WUR is the cause. Dismantle as above, and clear any blockage found. I have seen reports that the little metal gauze filters were deleted in late models, so if you have trouble clearing the restriction and believe it is caused by the filter, use your best judgment here. I found that replacing the O-ring in the regulator (old one was well flattened) produced a 4psi drop in control pressure, so make some allowance for that.
Jay Kempf suggested that the regulator should be tapped out of the die cast casing about 1mm (.040”) to correct a high control condition, as it is then easy to correct a low control pressure from outside without removal or disassembly again. Measure the height of the iron part above the die cast case first, and then tap from inside until it is 1mm higher.
Support the body on a firm piece of wood or soft jaw vise to do this, with the iron part protruding. Jay also pointed out that since the base cold start settings are controlled by the position of the bimetallic post in the WUR body, this post might need attention. If your car does start cold, and can be warmed up, leave it alone. If you think it needs
adjustment, as with the main regulator body, tap the post outwards from the inside, so that adjustment consists of tapping it inwards from outside later. This will reduce cold start pressure reduction, so if the engine is COLD, or ambient temperature is low, starting may be a problem. In order to remove any cold start aspects from interfering with warm running pressure tests, Jay suggests leaving the whole bimetallic kit out, or at least fitting it so it has no effect – don’t tighten the post nut. This will allow you to resolve hot running pressures separately, and do the cold start part later. Mine had no cold start problems, so I replaced it as I found it.
Now it’s time to refit to engine, with gauge connected as before and test it. If the engine will start you are nearly there. If not, jumper the pump, and see what pressure you get, taking ambient temperature into consideration.
You should be able to get the Control pressure somewhere near spec, by tapping the regulator inwards, if you tapped it far enough out to get low Control. Once running and warmed, tap regulator in until you get into specified range (40-46 psi in Watson). You may find that it will start, but run a bit rough initially, as mine did, but I put this down to a cold WUR on a warm engine – the mixture is rich, but the extra air valve is closed by engine heat – it got so I could remove, dismantle, reassemble and refit before the engine cooled down much after a few practice runs!
Once you have the Control pressure in range, your idle should be nice and steady, and throttle response good – go for a test drive, leaving the gauge in place, and get it warmed up thoroughly. Mine seemed ok after first try to clear the WUR, but after 10-20 minutes driving to soak it with heat, the Control pressure went high again, so I had to go in again.
Also don’t forget that if the CO adjuster on the distributor has been played with, you may have a separate problem there.
If you have a cold start problem, you will need to allow it to cool right off – overnight, depending on ambient temperature. With the bimetallic secured properly, jumper the pump and check the cold Control pressure. If higher than specification, tap the post inwards CAREFULLY until it comes into range. Try to start the car, after connecting the electrical lead to the WUR. If you do this in warm weather, and get acceptable cold starts, you may need to adjust the post more as the temperature drops. You should recheck the warm Control pressure as well – if it is now lower than before, you have probably gone too far, or there is another problem inside the WUR.
Other things being equal, you should now be home free.
By ‘Other things being equal’, I mean WUR problems may not be all you have to
contend with, but if you get the hot and cold control pressures right, at least you can eliminate it from consideration. YMMV
Good luck.