I have a 1989 K100RS SE that I'm putting back together to put on the road.  Everything is ready to go, but I'm having problems with the fuel system.

The engine is very hard to start, and when it finally catches it only runs on two then three cylinders for a while until it can catch and run on all four.  It is dumping fuel out of the header joint with the silencer. The spark plugs are very sooty black and damp when I pull them.  Cleaning them will give me one, sometimes two starts before they need to be pulled again.

Obviously, it looks like the engine is running very rich.  Fuel pressure is correct, and the water temperature sensor is good and giving accurate readings. Fuel is fresh, only a week old.

I am checking Bert's troubleshooting information and took resistance readings on the MAF.  The readings at the Jetronic connector(with tank disconnected) are as follows:

Pins 8-5 363 ohms which is correct

Pins 8-9 200 ohms which is correct

Pins 7-5 120 ohms  According to Bert's guide, approximately 80-85 ohms is correct

Pins 7-8 375 ohms  According to the guide this should be about 334 ohms

Has anyone here experienced a similar problem with overly rich mixture and high resistance reading on the MAF wiper output?  If so, is there a repair that can be done, or must the MAF be replaced.  I have searched for an answer, but there doesn't seem to be anything documented beyond a thread that goes into doing an almost complete rebuild.

I've had those symptoms with bad plug caps - arcing onto the cylinder head.
Just something else to check!

Update:  I opened up the MAF sensor, and it appears the wiper is not making good contact with the carbon element. 

I am reading about 45 to 60 ohms between the wiper and the element it rides on.  If I press lightly on the wiper the resistance goes to zero.  The resistance I am reading at the wiper is the difference between my readings and the values I should be seeing according to Bert's troubleshooting guide.

I've tried cleaning with Deoxit, but with no success.  Looks like it's deceased.

chris846 wrote:I've had those symptoms with bad plug caps - arcing onto the cylinder head.
Just something else to check!

I thought the same thing, and checked the spark on each plug individually.  All showed a good consistent spark when cranking. 

Because I had to work the throttle to keep the engine running I wasn't able to get a good look at the wires to see if there was any spark leakage.

If replacing the MAF doesn't correct the problem, a spark check is the next item on the agenda.  That won't happen until next week when the MAF I just bought arrives.

sometimes you can move the pc board slightly , so that the wiper makes better contact with the carbon track .
often just slightly is all thats required .
there are a couple of pcboard mount screws retaining the board to the assembly , if your lucky and with the correct fitting screwdriver , they can be released , allowing some slight movement .

else wise , some copy paper can be slid under the wiper to clean the running contact point , once under the wiper wet the paper with a good contact cleaner , then move the paper to clean off the contact points .

thirdly , the connector to the afm from the external wiring is preasure fit only and can corrode under the contact points to the pc board
 remove the retaining screws holding the plastic assembly to release the contacts , again clean under the contact points
if memory serves me correctly 60-80 ohms sound familiar to a bad connection to the plastic connector from the pc board ...


dont forget that the armature to anvil pivot point can lose tension , also requiring a clean .

one of the "fixes" for lejetronic afm units is to make a woven wire connection between the 2 parts

hope that helps

i have done all the above , more than once , restoring operations to flakey afm units , all now working perfectly

good luck

Charlie, Good points, I've cleaned the element and wiper with Deoxit and alcohol and slid a paper wet with them under the wiper to no avail.  The problem appears to be that there is insufficient force on the wiper to make a good contact. 

Since I have a replacement in the mail, I guess I can do a few experiments.

yes the pressure could be an issue
that's why moving the pc board slightly  so that the wiper is tracking on a different part of the carbon ,may rectify the situation ??

good luck

Well I just spent some time fiddling with the internals and managed to get the readings at vane closed to be 90 and 340 ohms which looks to be in the range of a working unit.  I tried to get some readings at a few intermediate positions, but it seems the resistance changes a fair bit through the travel of the wiper.

I'll try to get it installed tomorrow to see if I have done any good.

Thanks for the tips, Charlie!

the best way to test is by applying 12 volts to the circuit (the correct way around

your looking for about 1.2 1.3 volts or lower on full close of the vane   about 7.6 volts on full open , unfortunately its non linear

something like this

Charlie, thanks for the chart and the voltage readings.  I checked today and the voltage on the wiper measured at 1.26V with the vane closed and 7.6V at full open. 

I double checked the resistance again and surprisingly the readings are now 81 ohms pin 7 to 5 with 335 ohms pin 7 to 8. and the air temperature reads 207 ohms in my freezing garage. According to Bert's data that is perfect.  Looks like jiggering the wiper on the element did the job.

No time today, but I plan on putting it back on the bike this weekend to see how it works now.  One additional thing I need to do is address the rubber air dam above the airbox that has hardened and makes getting the filter out a royal PIA.  I think I'll carve it back and attach some neoprene foam rubber that will make the seal but is compressible enough to allow me to change the air filter.

Thanks for all the help!