85 K100 ...all of a sudden backfiring and stalling when HOT! Still not fixed!

[Edward]

Yes that is correct, the airbox and yes, he fuel pressure regulator is behind it.

[Laitch]

Is something wrong with the fuel pressure regulator, or is it to be part of a feng shui reorganization?

[Atabeach]

Am I mistaken to believe a faulty fuel regulator could cause a rich running condition? The temperature sender , TPS and coils seem to be fine. I’m just going with process of elimination. Does anyone think 13 volts is not enough to run the ignition system. Please advise.

[Laitch]

Am I mistaken to believe a faulty fuel regulator could cause a rich running condition? The temperature sender , TPS and coils seem to be fine. I’m just going with process of elimination. Does anyone think 13 volts is not enough to run the ignition system. Please advise.

I don't believe an alternator outputting 13V while the engine is running is capable of producing engine performance reliably.

Certainly, a fuel pressure regulator be a cause of over-fueling, but so can a clogged fuel return line, a faulty fuel pump or a faulty fuel injection control unit, not to mention the air flow meter which can be tested in place rather than on a bench. Repair using the process of elimination can be too costly for many of us; that's why there's all the testing; however, replacing parts also can eventually improve performance, if the defective part—or parts—eventually are replaced. That seems to be the diagnostic strategy with this Brick. AI can make a great companion for that.

[Atabeach]

Thanks Laitch,  I will take your advice and change the alternator….and move forward from there if there is no improvement….further, I was mistaken. I thought the alternator voltage at 13 would have been more than enough.
This model 1985 has a 33 amp alternator. Would anyone recommend a rebuild kit? Otherwise I am buying used off of Flea bay....

[Laitch]

Thanks Laitch,  I will take your advice and change the alternator….

That isn't my advice.

The battery reading should be ≈13.5V to 14.5V with the engine running. Resting voltage should be 12.6V or above. Why do your battery's figures differ? My advice is to determine the root of each condition one step at a time. Why is your Brick's battery at 12.5V instead of within the optimal range. One case here where the battery wasn't holding a charge was caused by the alternator's cable connection to the battery being loose at the alternator side. Another case involved a bad diode.

All electrical connectors and ground connections of a 40 year-old Brick are first suspects when engine performance is erratic. The components themselves are the next suspects.

[Atabeach]

Laitch:
  The battery and grounds were removed cleaned and reinstalled…reply 1 ( Bike was sitting for a decade and all electrical connections were removed& cleaned.)
The battery…at 12.5 was tested using my battery load tester…showed good…re- assess as needed, 4 months old…

…The bike always seemed to charge and start right up. I never thought to test the charging system, when I did I still thought 13 Volts was sufficient to run…ignition. Guess it may not be.
  So....now, I removed and bench tested alternator ( to clean all brushes & contact areas also )
Turns out she is tired. Ohms across the two brass armature rings should be 2.5….they read 5
 Changing the original 40 year old alternator makes sense…as did the temperature sender ( ohm reading not perfect) and hall sensor….not big ticket items.
The coils and wires checked out ok and were not replaced.
  I am following a logical order here not haphazardly changing stuff.
This is a $750 barn find…so I was expecting some costs….It still has the original fan!
 
Thanks for your …..help. 😳😳

[Atabeach]

So the alternator was changed I am getting a solid 13.7 at around 2500 rpms. Battery was swapped with my other bike...no difference. I changed out the fuel regulator (used) no improvement....The bike always starts right up and idles smoothly at 12/1300 rpm. It backfires when revved or put under load by riding. It smokes at all rpms and all the plugs are still wet with gasoline.

[Edward]

I wonder if your coolant temp sensor is faulty, causing the ecu to think the engine is too cold therefore pumping more fuel than needed? I don't recall but did you adjust your bleed screws to have a in-sinq throttle body?

[Atabeach]

The temperature sensor was changed.  I have not adjusted the screws because the bike was running fine…This happened suddenly. Thanks

[Edward]

tps reset?

[Atabeach]

The throttle position sensor was tested. If I remember correctly it clicks and you essentially test for continuity..Please post the procedure if that is incorrect. Thanks.
 Laitch had mentioned the air flow meter. I don’t know how to test this while component is on bike.
Lastly….mine is an early 85 and has this vacuum switch…( see photo ) ...anyone know whether it may be the problem….or how to test it ?

[Laitch]

so the coils were tested on bike and then removed..bench tested...2.6 ohms
COILS GOOD.

OL reading across both secondary spark plug terminals in BOTH coils.

What's the value at the secondary terminals?

You've only done half the coil test—the input side. OL is not an acceptable value. Either your multimeters have been set too high and are not sensitive enough to make a reading, the test leads are not contacting the terminals, the leads are not well connected to the instruments or the multimeters themselves are not suited for the application.

[Atabeach]

Ok soooo I purchased a new multimeter. Set to 20k ..,My k75 coils test around 10k with this and my old meter…My k100 O/L. Both coils. Across both spark plug towers. Tried it with plug wires on too… the bike starts and runs so I’m doing something wrong. It can’t be O/L.
 

[Laitch]

Congratulations on your new multimeter!

Based on your comparison with the K75, it's possible the coil can only supply enough current to allow the engine to idle. When the demand on the engine goes beyond an idle, high resistance within the coil cannot supply enough energy to meet it.

Do you have a spare coil pack or two?

[Atabeach]

I have 3 coils from a k75. Any ideas 
Laitch...Reasoning 101: this happened suddenly. The bike was running flawlessly....Could BOTH coils get tired at the same time? ...or Could...the $30 multimeter be off?
The coils and wires are 40 years old...changing them is not a BAD idea. TIA

[Scott_]

I've got a couple of sets of coils I recently pull from 2 K1100 tear-downs. I can measure them tomorrow and see what they read, if that would help.

[Atabeach]

Scott that would help. My inkling is what Laitch suggested…they may be tired. They are 40 years old…Email me if you are willing to sell a set..Thanks

[Scott_]

Ok, so I didn't wait till tomorrow... lol
I have a set that is an early '93 and a set that is late '95.
93 set:
13.23 kohms on the plug side 0.7 ohms on the ecu side
12.89 kohms                         0.7 ohms

95 set:
12.85 kohms on the plug side 0.6 ohms on the ecu side
12.87 kohms                         0.6 ohms

I could sell you a set, however looking at your picture I don't think they would be a direct fit for the older k100.  The plug wire set is different style as is the ECU connector is not a spade connection on the  1100's.

[Laitch]

Scott that would help. My inkling is what Laitch suggested…they may be tired. They are 40 years old . . .

According to the photo in your Barn Find thread, the engine case reads 1984—42 years. It's likely the engine was completely assembled that year. The coils in the engines of the early models like that are notorious for becoming unreliable. That's well documented on this site and elsewhere. Later in production—late '85 or in '86—the coils were replaced by coils much more reliable. They could be identified by the orange tops of the secondary terminal towers.

The resistance of the K1100 coils is around 70% less than the primary resistance of the K1002V coils. Figures below are from the LE Jetronic Testing for K Models manual downloadable here.

My understanding is this particular difference can possibly lead to the overheating and failure of the ignition control unit. The failure wouldn't necessarily be instantaneous but definitely could be inconvenient or even dangerous depending upon where it happens. Maybe somebody who has done that swap can chime in. If not, you could be the pioneer!

[Laitch]

They're pigs-in-a-poke but there for the buying. This is what the later generation coils look like via eBay right now.

[Atabeach]

Thanks, Scott… Laitch, I’ll get the orange cap ones on eBay. This will take a week or so …
in the meantime can anyone post the process to check the air flow meter…
 Lastly, there is a fuel vacuum switch near the fuel regulator. It was eliminated in later models. What does it do? Should I leave it be?

[Laitch]

Lastly, there is a fuel vacuum switch near the fuel regulator. It was eliminated in later models. What does it do? Should I leave it be?

Its effect is up for debate, except at BMW. It was removed because it took extra material and labor cost but it effects were debatable, even in Germany. The Removers won the debate. Leave it alone as an original production artifact, or remove the hoses and cap the source points of the hoses then remove the vacuum switch to give to your favorite grandchild to use as a mobile over its crib, or as a crackpipe in its bedroom.

[daveson]

this happened suddenly. The bike was running flawlessly....Could BOTH coils get tired at the same time?

One possibility is that one coil was working well and the other coil was working, but not well. A crack could show an open circuit when tested with a multimeter, but the high current could jump the gap while running, which would be about zilch millimetres. It jumps a big gap at the spark plugs (one wasted) Now if the other coil fails, the problem becomes much worse.

I don't think the spark plug leads have been tested yet, that would be worth a look. Original leads from a K75 should not be used, they're different.

[daveson]

in the meantime can anyone post the process to check the air flow meter…
 Lastly, there is a fuel vacuum switch near the fuel regulator. It was eliminated in later models. What does it do? Should I leave it be?

The air flow meter is dealt with in Bert's guide, which I think you have.

It's not a fuel vacuum switch, it's a vacuum switch. Check anyway the vacuum hose to the fuel pressure regulator, that it's not leaking, or kinked, or blocked, or partially blocked. Do these checks also for the hose to the vacuum switch.

Some people remove it because they notice no improvement in performance with it connected. With that reasoning you could remove a wheel nut and throw it in the bin, there would be no drop in performance but after a few years the disc might develop a warp. It might not warp either, but why bother second guessing the engineer's who designed it that way? It's not meant to improve performance, it's there to improve fuel economy, reduce pollution etc, mainly while cruising. They have put it there for a reason, so I think leave it connected.

At higher vacuum you want to advance the ignition timing because it takes longer for the detonation to complete in these situations. You can use an LED test light to check that it sends a signal to the ignition control unit, from memory I think it only sends a signal at part throttle. You could use a timing gun to check that the computer is altering the timing. I guess no one has done that yet on this forum. You could check the exhaust emissions at cruising, with the vacuum switch connected and compared with it plugged, but I'm guessing it would be a tiny difference.There might even be a drop in revs if it is plugged to remove the signal while cruising.