'91 RS - Intermittently Cutting Out

[Freelancer]

Hey Fellow Brick Flyers,

Got an issue that just came up and am looking for suggestions as to where to start looking.

The Problem-
  Today on way to get the bike inspected, it started acting up. The symptoms were:

-Tach falls to zero
-Bike acts like it is running on only 2 cylinders.
-Bike would do this for a 1/2 to 1 mile and then the tach would come back on.
-Then would run normal for a mile or so and then repeat.

The Bike:
1991 K100RS 4 valve
Has Motronic 2.1
Over 77,000 miles on Odometer

*Things of note:
 -Bike seemed a little "surgy" today
 -While acting up the turnsignals, headlight, brake light, speedometer and gear indicator all worked normally.
(Not sure about cluster illumination lights because it was to bright. Looked like they were on when passed under shade.)
 -Bike acts like it has mild vaccum leak...when bike is hot and I shut throttle to brake from highway speed/rpm it sometimes tries to stall/die. (Am planning on tackling this soon).
 -Bike has 2,500 miles till next service.
 -Bike lives under bike cover outside in Texas(when not being ridden).

Recent history:
 -New Fuel pump 1,500 miles back (same time as paint job).
 -I rebuilt the starter about 500 miles back. Reassembled correctly, only concern is that I didn't tighten the starter casing screws tight enough. (Did finger tight with screw driver then backed off just a hair)

Mods:
 -LED dash lights and additional LED rear markers, LED headlight and Rigid light bar.(Total amp usage ended up being same to just less than stock.
 -Remus exhaust

My gut first instinct is maybe time to put fresh thermal paste on ignition amplifier but would like a good list of things to check before I start shooting in the dark.


Btw, she passed inspection. :yes :2thumbup:

Input appreciated,
Freelancer

[Scott_]

That would be 1st on my list: heat sink paste.
After that,
fuel pump connector plug,

[Freelancer]

Thanks Scott!

Mulled it overy a bit and this is what I think may be happening.

Losing the tach and what felt like 2 cylinders has me thinking that one of the coil packs was dropping out.

-So the thermal paste is definitely first on the list.
-Next, will be to check the plug into the amplifier.
-Then an inspection of the grounding and power wires on the coil packs.
If these don't work, I'll check the 4 pin fuel tank connector, the Motronic and finally the HES.

Will keep you guys posted on what ends up being the fix.

Quick question. Is Permatex thermal paste the best/only stuff to use? Cost seems right, am just asking 
for alternatives if the local store is out.

Thanks,
Freelancer

[Brad-Man]

The heat sink paste for computers will work just fine and is better than original.

[Freelancer]

Well, applied new paste and the bike ran fine until tonight. She is back to doing the same thing.

Symptoms-
1)Power dropped and could hear that only 2 cylinders were firing. (Sounded like a flat twin crossed with on old VW flat 4.)

2)Tach dropped to 0 but everything else worked.

3)Decided to limp home the last 17 miles... Full power comes back during last mile like nothing had happened.
(Maybe coincidence but power came back just as I hit hi-beam switch.....hit the switch earlier and nothing happened.

4) Tail pipe was dry when I got home. (Though at one point, thought I smelled rich/unburned fuel.)


What I've already done-
A)Cleaned the amplifier and applied new thermal paste.

B)Cleaned the chassis ground.

C) Checked, cleaned the amplifier plug and applied di-electric grease.

D)Checked & cleaned Motronic plug/pins.


Next steps???

a)Check plug on gas tank (Could this make just 2 cylinders or injectors stop firing?)

b)Find way to figure out if bike is losing spark or if 2 injectors are not firing???

c) HES- Could this the be the problem?

d) Could I have gotten water in the tank/ lines? (Seals and Gaskets are all new when bike got paint.


Suggestions???

Thanks,
Freelancer

[rbm]

Have you eliminated the coil primary circuit? Your tach signal comes from there and losing the tach is a sign.  It might be harness or connections at the coil or at the Motronic.

[Scott_]

The tach signal is picked up from the coil pack for cyl 1/4 if that helps you trace it any better. It should be a black/blue wire.
If the coil pack itself is failing open on the primary side, or you are loosing the +12 at the coil, you will loose spark and tach. As RBM is suggesting.
The tach signal is picked up from the switched ground side of the coil.

You could try to swap the physical coils. If you still have the cutting out, but the tach stays reading, then it is the actual coil. If the tach still drops out when cutting out, then it's most likely a wiring issue, or the actual motronic output, or the HES signal controlling the motronic.
You could try the hairdryer test on the HES as you say it starts acting up late in the ride after the engine would be hot.

[Brad-Man]

+1 on what Scott said...

Darn - thought you had it solved when no response for so long...

[Freelancer]

Hey guys, thanks for the input. I have to admit that the girl is annoying me a little on this one. I know the problem and the likely culprits but have not been able to  reproduce the problem while on the center stand.

What I did this weekend was to swap the Motronic with my spare. Turns out my original had a Luftmeister chip in it and the spare has a Harmon chip.
  After the swap it took about 10-15 seconds to settle in then ran as good as if not better than before but there are still a few quirks left over.

 The quirks I mention showed up recently. The most notable is when I rev the bike on the center stand. Occasionally the speedometer will tick just a little as it passes 3k rpm but every now and then it will jump up to 20 mph and then settle back to 0.

Other quirk has to do with led instrument lights. Since I put in led's, I have to rev the bike a little (up to 2 - 2.5k) to get the charge indicator to turn off. This is common with led's in instrument pod. The quirk is that lately I've had to rev up past 3k.

My current plans are to try the hair dryer test next. If it causes the problem to surface then it's a new HES. If not then I'll ride it until the next time it happens and then put in new ignition amplifier. If that doesn't work then new coils.

The way I'm approaching this may seem a little backward but I'm going with the parts I have on hand while I'm sopping for the others.

Note- the quirks could be due to poor grounding/ bike hasn't moved much lately. Actual frame grounds have all been checked and.cleaned.


Again, thanks for the input. Will let you guys know the results.

Later,
Freelancer

[Martin]

Definitely sounds like coil dropping out, get hold of one of those inductive HT testers. Remove your spark plug cover, and go for a ride, when it does it pull over and test. It might help to have someone with you to work the throttle.
Regards Martin

[TimTyler]

How old is your battery?

[rbm]

The quirks I mention showed up recently. The most notable is when I rev the bike on the center stand. Occasionally the speedometer will tick just a little as it passes 3k rpm but every now and then it will jump up to 20 mph and then settle back to 0.

Bad contact somewhere.  Could be a corroded connector or bad wires.  The signal coming from the sensor is very low and is easily affected by poor electrical contact anywhere in the circuit from the sensor itself all the way through the harness up until the circuit card in the instrument cluster.

Other quirk has to do with led instrument lights. Since I put in led's, I have to rev the bike a little (up to 2 - 2.5k) to get the charge indicator to turn off. This is common with led's in instrument pod. The quirk is that lately I've had to rev up past 3k.

Either change the Charge indicator back to an incandescent bulb -- or learn to live with this quirk.  It is a result of choosing to use a LED.

[Freelancer]

Hey Martin, thanks for the input/idea.

@TimTyler - The battery is getting due for a change. It's about 5 hard years in without much battery tender time.

@rbm- Thank you for the quick analysis of the quirks. Will start looking at the the wiring/speedometer circuit diagram. Maybe the wires run parallel to the tach/coil circuit. Would be nice to find one quick wire patch fix for both speedo and coils.



Just a quick update.

Did the heat gun test and no problem on the HES.

On a whim applied heat to coils, bike ran fine.

Applied more heat to the coils while holding bike at
4k rpm. Continued test until I noticed the exhaust glowing red.
Results....Bike ran ok. No cutting out .....but...started to make an occasional noise.

 Sounded almost like a rod slapping bottom but when I listened on right hand side the noise was muted.
Pulled the clutch and noise mostly went away/changed to more of a miss or pre-detonation.
Removed the heat and dropped the revs and it all calmed down.

Am thinking the slapping is either the sprag, clutch or maybe the altenator(???).
Also think miss/pre-detonation may have been the combination of  prolonged heat on the coils and revving the bike while in neutral. No air flowing over the motor makes this a much harsher test than normal street riding.

...But, I could easily be wrong.

On the bright side, was not able to get the coils to drop out under fairly extreme conditions. I'm going to go back and look at the overall picture, the try to forget everything and go for a ride. At some point the answer may come to me.

Ideas and input still much appreciated. Will let you guys know how she runs tomorrow.

Later,
Freelancer

[Scott_]

This recent testing is still with the "spare" motronic?
Sounds like you might have found your issue.

I also agree with RBM, if you want the charging circuit to work as it was before, put the incandescent lamp back in for the charge light.

The speedo wiring(even though it is shielded) runs up the right hand side of the bike near the alternator. Maybe your issues with the alternator charging are influencing your speedometer quirk..... just a thought.

[Freelancer]

Hey Scott,

Yes, the test was done with the spare ecu that has the Harmon eprom.

By "might have found your issue", are you referring to the ECU or the Coils?
I ask because the heat did affect the coil performance but didn't cause one to drop out.

 At first I was going to write it off as expected result when an electric field coil is subjected to extreme heat.
 After today's ride, I'm thinking that a coil swap might still be in order. Never had a coil drop out, bike just seemed a bit bi-polar.
It would run super smooth one minute then a little ragged and surge (y) the next. This tended to happen in slower stop-n-go traffic and would clear up at highway speeds.


As to the altenator charge light. I understand that this is a quirk of having the leds. I brought it up because I'm now having to rev the bike much more than when led was first installed to turn the light off.
I don't mind having to do it, just thought that the need for higher revs might be abother symptom.

As to the speedo sensor wire routing. Yeah, I was aware of the path but the suggestion to inspect the wiring near the altenator is a good one.


As things stand now, I plan to:
1)buy and install another set of coils
2)buy and install new battery
3)install spare ignition amplifier
4)Pull combination switches and give them a good cleaning
5)buy spare HES as a "just in case"
6)while changing battery I may install spare altenator and new "monkey nuts"

And while doing all of this I will go through and inspect whole electrical harness. Honestly, I hope/think that 1-4  will fix the remaining issues.

....Ya know, at some point I might want to look at the load shed relay

Thanks guys, please continue posting ideas/theories as  to what I've missed.

Later,
Freelancer
 

[Laitch]

As always I'm looking for the easy way out. If your bike has a sidestand switch, maybe it's failing. I didn't notice that on your list.

[The Mighty Gryphon]

I have no first hand experience here because I have later model bricks, but I have heard that there are two versions of your coils.  The earlier version was prone to failures and was replaced some time in the late 80's with an improved version.  I think the improved version is orange where the plug wires connect.

[Inge K.]

I have no first hand experience here because I have later model bricks, but I have heard that there are two versions of your coils.

That is correct about the 2V models, Freelancer have a 4V model.

[Scott_]

As always I'm looking for the easy way out. If your bike has a sidestand switch, maybe it's failing. I didn't notice that on your list.

The side stand switch would kill the entire bike, not just 1 coil pack.... just sayin.

[Laitch]

I'm just going by my experience that failing connections will intermittently make engines stutter, Scott. I agree with you that this one wouldn't kill one coil pack if that's what's happening.