Still not havin success getting my 86 K100RS project to run right.
To date I've replaced injectors, plugs, reconnected the wires to the temp sensor and bought a new battery. After trying in vain to start it (over and over again), yhe new plugs seemed to help immensely and it was starting from cold at the touch of the button. After warming up though, it would not start uless i unplugged the fuel pump. Today i changed out some very fuel-laden oil and when I tried to start it I had to resort to unplugging the fuel pump right from cold. Even then, it would not idle and revved roughly, coughing and spurting. I checked and cleaned the plugs which were pretty black. I fiddled with the idle air mixes and idle speed and finally got it to idle ok, but still not revving smoothly. I found that it ran better with the fuel regulator vacuum line off and port plugged. I tried hitting the starter button at idle and it stalled, yet at just off idle it would rev when the starter button was pressed. I drew vacuum on the line going to the fuel regulator and its leak free. I even pulled the air box to make sure the line hadn't been pinched the last time I had the airbox off and all was good. After getting flakey "hot" ohm readings, I have a new temp sensor on order but after today I'm not so sure that will do anything for me, especially since it wouldn't even start cold.

This situation is getting very discouraging. I'm about to sell it and buy something with carburettors!

Have you replaced the old Z tube?  Cracks that are almost invisible in that tube will raise hell in the low rpm running of your motor.

A bad temperature sensor or connection will cause it to flood from cold. The temperature sensor should be fitted without Teflon tape or sealant as it requires a good earth through the threads. Both the sensor threads and socket are required to be clean and free of corrosion. 
Regards Martin.

Is there any chance you got a sensor for a 16v engine? look the same but not compatible.

Just to clarify too, its the sensor behind the radiator.

92KK 84WW Olaf wrote:Is there any chance you got a sensor for a 16v engine? look the same but not compatible.

Just to clarify too, its the sensor behind the radiator.

Well, I haven't received the temp sensor yet - in fact, its a week and I haven't even gotten an acknowledgement of the order from Max BMW. 
After I re-connected the wires to the temp sensor plug (P.O. said they were chewed through by a mouse but they looked pretty cleanly cut and stripped) I tested resistance back at pin #10 at the plug for the EFI controller. At 45 deg. F, resistance read around 3 ohms, which I believe is within the normal range. After warming the bike up (coolant measured around 160 F with IR thermometer) it was hard to get a stable reading. Ohms would read 300-500 for a second and then go to 1. Here's a short vid showing how badly it runs even after warmed up. I fiddled with the idle speed and mix and got it to idle at least, but it still won't start unless I unplug the fuel pump and plug it back in quickly once it's running.

Point-Seven-five wrote:Have you replaced the old Z tube?  Cracks that are almost invisible in that tube will raise hell in the low rpm running of your motor.

Could I test for leaks by spraying the tube with carb cleaner to see if it changes the way it's running?

swampyankee wrote:Could I test for leaks by spraying the tube with carb cleaner to see if it changes the way it's running?

Sure. That's one of the time-honored methods published here; propane could also be used.

If you haven't replaced it already, you probably should.  Removing it is the only way to look at it closely, the cracks can be almost invisible with the tube in place.  With it off the bike you can flex it where the clamps are and see if the cracks open up.

That tube wants to crack where the clamps press on it.  Almost every original tube has probably cracked and is leaking by now.  If not, it will in the not so distant future.  Along with the clutch o-ring and vacuum port caps, I replace it as part of prepping a new to me bike. 

The price on those damn things is spiraling and the sooner you replace it, the less it will cost you.  Just a few years ago they were 8 bucks, now they are over 20.  Pretty soon it will cost more than the clutch boot or a tank roundel.

$14 at EME. Beemer Boneyard has them for $9.95 but I ordered from EME before I looked elsewhere. 

So you replaced it already?  Have you checked it for cracks recently?

No no, I just ordered it based on your recommendation

Have you checked the TPS to see if it is working…or calibrated to the correct voltage at idle?

I haven't tried that yet. Trying not to make too many adjustments and then find that everything is out of whack when I finally find that one stupid thing that was the culprit.

I have a Z hose and temp sensor on order. The plan is to replace on at a time and check for improvement after each.

The reading is only resistance with an ohm meter. Engine off.

If you disconnect the sensor from the harness you can test resistance at the pins.
This is how the computer varies the pulse of the injectors. If it’s not changing the fuel mapping will be off. To rich at idle. Then open the butterfly valves, more air but same fuel supply.

If it’s good, check in box.
If not, your approach of component by component is sound.

Had my first rides this weekend. Winter returns next week.

Why won't you pull the Z hose now to check it?  It would be a good data point.

Since I will be taking it off to replace it anyway, I will pull the Z hose and examine it at my next opportunity (probably not til the weekend).

When I replaced my Z hose it made a large improvement on my 85 rs. Settled Dow an smoothed out.

Sorry to hear you still haven't got it running, but the two parts you have on order (temp sensor and z-tube) are the most common parts that create the issues as described.

You'll probably want to read up on the injection system, but here's some basics ( ); The electric set-up is working on the resistance of the various sensors to ground. For this reason it is important to ensure your ground connections are always good, particularly when you start getting issues. There are only limited ground points, so very easy (most important is the ground connection on the frame underneath the tank). However there are some sneaky ones as well: The cooling water temperature sensor on the earlier models (LE-Jetronic) grounds through the cooling water stub on the block and the two wires coming from the sensor are two separate sensor leads, one for the brain and one for the temperature switch of the cooling fan. The other one is the ground running through the starter motor.
The early injection system works on the basis of trying to keep the fuel pressure constant relative to the pressure in the inlet manifold. In order to do this you have the fuel pressure regulator (keeping the pressure at 2.5 Bar IIRC) and the vacuum line, which compensates for the manifold pressure. This is to allow for a constant pressure difference over the injector tips, as the system works with fuel quantity control through the opening time of the injectors. More power demand will result in longer opening of the injectors to get more fuel in the manifold. The base amount of fuel is controlled by some basic sensor info, like the cooling water temperature, the amount of air 'sucked in' as indicated from the air mass sensor and the setting of the TPS. 
The TPS on the earlier versions is a switch, which indicates 'closed throttle' (remember listening for the 'click' on closing the throttle) or 'fully open throttle' to the brain, which, when activated, will result in additional fuel. So for those used to carburetors and think that janking the throttle constantly will improve starting or whatever else; think again....   The TPS has been known to get water in the contact area and thereby giving wrong signals, so always good to check that as well.
I'm possibly teaching you to suck eggs and have all fallen asleep by now, but think it's always important, particularly for those new to these bikes, to restate some basics that possibly help in channeling the fault finding process. As always there is a lot more, like all the switch gear in the speedo and keeping all the electrical cable connections clean and......
Hope this helps a bit. Cheers, Rene

On the contrary Rene, I'm all ears when it comes to a good explanation of the fuel system. Although I've acquired a decent enough understanding of the elements, you clarified how they work together and when they come in to play.

Over the weekend I did replace the Z tube, temp sensor, and the cooling fan which was knackered. I tossed in a new air filter while I was at it. Although the thing started right up, I wasn't getting good throttle response and as soon as it started coming up to temp it started stumbling and stalling. And at that point the only way I could get it restarted was to unplug the fuel pump and reconnect as soon as it fired on residual fuel. I fiddled with it long enough to see that my replacement cooling fan (a SPAL aftermarket unit) was operational. On the suggestion of a fellow forum person, I pressed the starter button while it was idling and the engine smoothed out and the RPMS increased quite a bit, which told me it was running lean. When it was hot enough for the fan to run I checked resistance at pin 10 coming from the temp sensor and got the appropriate ohm reading. That and the fact that the fan is running tells me the temp sensor is good and well-grounded. While I had it running I blocked the air box intake and the engine didn't even flinch. I searched around the airbox and plenum searching for a gaping hole or crack, and used both propane and starter fluid to check for a change in the sputtering idle. The idle raised and smoothed out when I sprayed the plenum boots, particularly #2 and 3. It also changed to a lesser degree when I sprayed around the large connector between the air box and plenum, but that could've just been the fumes from the spray getting to the throttle body boot just below it.

I may have some time tonight to go peel apart the plenum-to-throttle body boots and see if they have perished. But at this point I feel like I'm just continuing to blindly replace parts without ever fully diagnosing the problem. 

I'm getting pretty discouraged about now.

Did you propane around the boots between the TBs and cylinder head? Those usually go before the boots  above the TBs. Being bolted right to the head they are subject to more extreme heat.

duck wrote:Did you propane around the boots between the TBs and cylinder head? Those usually go before the boots  above the TBs. Being bolted right to the head they are subject to more extreme heat.

Yes, but I didn't get any perceptible difference in running. Granted, the idle RPM was very unsteady so it was tough to discern what was a "normal variation" in the RPM. Of course, once I have the TB's off it would be foolish not to at least inspect the Manifold boots you reference.

Don't get discouraged.  You are working through the problem in a systematic way now, and are finding and replacing all the funky parts that are causing your problem.  You are no way the first person down this particular road.

It's good that you have found the leaks at the #2 and #3 boots.  Over the years I have been here those boots have caused quite a few problems.  It's also possible that the lower bushings are leaking where they contact the head.

The lower bushings usually leak because of distortion in the flange that attaches to the cylinder head.  Several owners have corrected that problem with some silicone gasket material.  Put a thin bead of sealant on the flange and install it on the head.  Finger tighten the screws only enough to see the sealant just starting to squeeze out.  Let the sealant cure overnight and then finish tightening the screws the next day.

I managed to get out to the barn to remove plenum and the associated boots. The boot on #1 cylinder was split as I was wrangling the plenum off. I wondered if it had split as I flexed it during removal but upon closer inspection there was dirt in the split area which indicated that it had been split and sucking air and dirt for a while. #2 boot was split to a lesser degree but also showed dirt in the crack. The good news is, # 3 and #4 boots look fine so I only need to replace 2  
I had already sourced them at EME but I'd like to actually call someplace to make sure I get the right clamps. I will give Max BMW a call and see if they can help, unless someone here has a good source.

48mm Suzuki band clamp   Part# 09402-48208

I would go to a Suzuki dealer with one of the boots and ask for that part number clamp.  Check to make sure it fits properly.  Some have mentioned that p/n09402-46208 may fit better.  

Last I heard the clamps cost about $4.75.

There is also a McMaster-Carr clamp p/n 5388K32  They are a lot cheaper, like a pack of 10 for $16.61

Thinking this thing through while looking at ordering boots, clamps, etc. and reading about others' boot replacement stories. I can see how a couple boots from the plenum chamber to the intakes might affect the running of all cylinders since air could not only leaking into the cylinders of the split ones, but also cross over into other cylinders, as well as just throwing off the whole air meter in the airbox. 
But most people describe their bike as just running rougher than normal, yet still being rideable. 
Even replacing the Z hose and temp sensor have not significantly improved things. Could it be that there is more to it than just these leaky boots? I mean, once this thing warms up I can't even get it to start until I unplug the fuel pump. And even then, the hesitation to throttle input is such that it makes this bike unrideable. I've looked at the top half of the airbox, the plenum and the rubber connection between them looking for a gaping hole or at least a bad leak, and have found nothing.

Could these boots hold the key to this bike being finally operational, or am I still missing something??

Personally I replace all of the lower boots "while I'm in there." If the front two are leaking then the rear ones probably don't have a ton of life left in them and you'll have to tear everything apart again to replace them if/when they do fail.

I give them a rub down with 303 before installing them. Can't hurt.

On my latest K75 rehab I just bought three used ones from a low mile bike on Ebay for $20 shipped. Since many Ebay sellers have free returns there was no risk but they were in great condition so I used them without hesitation.

As the saying goes, the cheapest part on a BMW is the owner. That's me.

I WAS joking about not having to replace 2 boots. The "good" ones are as old and as dried up as the bad ones. I doubt they would survive reinstallation.

swampyankee wrote:Could these boots hold the key to this bike being finally operational, or am I still missing something??

Looking at the photos of your boots I am amazed your engine did anything.

At well over $250 to replace the lower bushings, I would go with a little silicone seal on the flange.  Those boots need to be replaced, and for $50 for a set, I would go with new. 

I don't know what to recommend for the clamps.  It depends on what is available and how much you want to spend.  There are several valid options.

I just called EME and ordered the boots and Oetiker clamp bands. I have a set of old end-cutting pliers I believe I can modify (I have access to a full machine shop if my pedestal grinder can't mod them). I also ordered a set of caps for the vacuum tubes on the throttle bodies. I have been using a pieces of tubing with screws stuck in them for 2 of the 3.  

As has been recommended, I'm hoping to re-use the manifold bushings. I used propane as well as ether around the bases and couplings of them and didn't detect any leakage. If needs must, I will reseal them with RTV.

So i installed the abovementioned boots and vacuum caps.with those in place, the bike would not even start, just sputter and pop. After leaving it a week, I started questioning the spark so I pulled plugs and checked and got some strong, some weak and some nonexistent. Hmmm... I had checked primary resistance on one coil and seemed good but I decided to just pull the coils and check secondary resistance. I found a crack in one coil and a bad reading across secondaries. Hmmmm... although I have found and replaced a bunch of faulty intake rubber pieces, could a bad spark be at the root of my severe running issues??

swampyankee wrote: After leaving it a week, i started questioning the spark so I pulled plugs and checked and got some strong some weak and some nonexitant. Hmmm... i had checked primary resistance on one coild and seemed good but I decidede to just ol the coils and check secondary resistance. I found a crack in one coil and a bad reading across secondaries. Hmmmm... although i have found and replaced a bunch of faulty intake rubber pieces, could a bad spark be at the root of my severe running issues??

Absolutely! What color are the towers where the spark plug wires connect to the coils? 

That will tell you if you have the old or new style coils.  An '86 should have the new style, but who can be sure.  The old style coils have been a problem since the bikes first came out.  That's why there is a new style. 

Even the new coils will have problems when the body is cracked.  Coil insulation is important(as well as the plug wire insulation).  As the throttle is opened, more air enters the cylinders and the cylinder pressure rises substantially.  The higher the cylinder pressure is, the the higher the voltage necessary to make a spark.  That higher voltage will leak out if given a chance, weakening the spark.  That is why high compression engines or engines with turbos or superchargers often need ignition upgrades to run properly.

I didn't take any pictures, but they look like the one in the pic below, complete with the "Zyl 1 u 4" (or "2 u 3"). The bike has an 11/85 manufacture date, so it has some earlier features.

So it's probably that a bad coil would be the culprit of most of my running problems? Like flooding easily while trying to start, horribly rough idle if it does start, bad breakup while trying to rev, etc. Granted I did have air leaks via Z tube and split plenum-to-throttle body boots, but even when I replaced the boots and Z tube I wasn't convinced they were the total problem since reading about other folks having the same issues, they created minor running problems, not the major disfunction I'm experiencing.

swampyankee wrote:So it's probably that a bad coil would be the culprit of most of my running problems?

If you had checked and replaced the coils months ago, it is likely that there still would have been significant running problems given the neglected condition of your Brick. A torn z-tube can inhibit starting depending upon the condition of other critical components—spark plugs, plug wires, valve clearances, injector condition, injection timing. Some other sources of running problems are faulty fuel injection control units and faulty ignition control units. 

The interdependence of all these items is the reason why a patient system-by-system check is the way to go with the motorcycles. You're into that now.

heres an odd one , to check the spark coils . fire it up in the dark with coils cover off ... and try to notice if there is any jumping of the spark across the  secondays (high voltage side )

you might be amazed ....   alternatively just test them for about 10,000 ohms ' from spark cable  to spark cable mount 
i have seen a few that kinda worked but spark jump to the laminated core outside the epoxy coil former 

just thought to put it out there 

good luck

I did source a pair of coils and just waiting for them to arrive. Meanwhile, I wanted to verify the resistance of primary and secondary. Primaries on both coils measure 2.1 - 2.4 or so as I recall. Secondary on the intact coil reads 9.8. The cracked coil gets no reading