1992 K100 RS "breaks down" "surges" between 4-5,000 rpm only

[Overkill]

I have a 1992 K100 RS. It runs good at an idle and good up to 4,000 RPM. From 4,000 to 5,000 rpm it surges very noticeably. After 5000 it runs smooth again. Here is what I have done so far.

 Replaced all plugs and two wires
 Replaced all vacuum caps
 Checked for air leaks, and checked vacuum hoses
 Synced the throttle bodies
 Good fresh fuel
 Valves were checked a few months and very few miles ago by Max BMW

I bought the bike about a year ago not running with a bad fuel pump. I put in a new fuel pump and filter at the time and it has always done this I think. It is my son's bike and he always complained of a vibration in this 4-5,000 range but nothing serious. I noticed it the other day when I was replacing pugs and two wires because they were vibrating off. It is pretty bad if you hold the throttle at 4-5000 it goes up and down a lot but everywhere else it is smooth.

Thanks for your thoughts!

[johnny]

greetings overkill...

welcome to motobrick.com...

if its me... i would make sure the plugs and plug wires are tight and the air filter is good... then i would replace the fuel filter again... can go with the napa 3032... clean and reset the motronic connector... clean and reset the 4 pin under tank fuel pump connector... do a throttle body synchronization... ride it till it wont move...

j o

[Overkill]

I will try all that, Where is the Motronic connector located? Thanks!

[johnny]

im thinking your moto is a 4 valve model with the motronic under the seat... if so its on top of the battery hold down...



if not its on the left side  beside the fuse block behind the side cover...



j o

[Scott_]

I'm also thinking that his moto is the 4V version.
The K1004V's also have the ignition amplifier that can act up. Check the connections, heat sink compound is known to dry up and not conduct heat away, thus causing problems.
Something else to check is the setting of the TPS sensor.

[johnny]

heat sink compound link...

alternate parts list link...

j o

[Overkill]

That is great information! Thank you so much! I will try it and let you know the results later in the week.

[k98DOG]

Hmmm, my bike is doing the same thing.  It reallllllllllllllly chaps my ass.  We'll look into it...

[Overkill]

Update: Completed the following this morning.

New fuel filter installed and new fresh gas.
Adjusted TPS to .375 volts
Cleaned the motronic connector

None of these things made any difference.

Went to put new heat sink compound on the ignition amplifier and mine is attached to the battery box without screws, it mounted on long studs. I got it off but I was removing the battery box and broke one of the rubber mounting studs so now I have to wait to get a new one to see if the new heat sink compound makes any difference.

I don't really know where to go next.

[Overkill]

Update:
Installed a new ignition module with new heat sink compound and still no change. Does anyone know what the specs are for the coolant temp sensor and the air temp sensor so I can test them?

[Scott_]

Went to put new heat sink compound on the ignition amplifier and mine is attached to the battery box without screws, it mounted on long studs.

So is there anything that the backside of the module is touching? It has to be against something metal to dissipate the heat, just air won't cut it.

[Overkill]

Yes, the battery box is all back together, the new module installed and the new heat sink material applied. No change.

[Scott_]

Does anyone know what the specs are for the coolant temp sensor and the air temp sensor so I can test them?

Well you can read this. It is primarily for the 2V K100 but it does discuss the water/air temperature sensor properties and they should be similar.

[Inge K.]

it does discuss the water/air temperature sensor properties and they should be similar.

About 2V vs. 4V watertemp.sensor it can be mentioned that 2V's got a dual element sensor
which each element is connected to ground....4V's got a single element sensor which isnn't
connected to ground.

On a 2V the resistance being measured between each of the terminals and the housing,
on a 4V you measure between the terminals.

[Overkill]

Latest update:

I put the bike outside running and it died completely. Pushed it inside and determined the (newer) fuel pump was not running. After some testing it seems the fuel pumps is good(runs with power to it) and all fuses are good. It turns out that the fuel pump relay is not tripping.

There is power to the relay from the side stand switch but the computer is not grounding the relay.

I just need to understand why the computer is not grounding the relay. The lights work, the red safety switch seems to work.

Still testing at this point.

[Inge K.]

I just need to understand why the computer is not grounding the relay.

On Jetronic euqipped models it relies on the hall sensor signal, I would expect that Motronic is similar.

[Scott_]

I just need to understand why the computer is not grounding the relay.

On Jetronic euqipped models it relies on the hall sensor signal, I would expect that Motronic is similar.

+1

[Jimbenge]

I worked on a friends k1200GT recently that missed also and it turned out that the spark plug caps were shorting out to the block.  I replaced them with different caps and that fixed it (sort of) I also discovered that this bike had resistor spark plugs in it and that contributed to the shorting out to the block.  I put non resistor spark plugs in and it stopped shorting out.  I have run into this problem twice now.  All my K bikes are older bikes and I run non resistor plugs in all of them and have never had a miss. 

[Scott_]

You need to be careful with plug selections on the K-bikes. Yes there are some aftermarket alternatives, but you still have to be careful.
Some of the older bikes have used a resister cap and standard plugs, where the newer ones are using a standard cap and resister plugs.

Not saying someone hasn't changed what was installed from the factory, but I'd be inclined to find out what is supposed to be installed by design and go from there.

[Inge K.]

Some of the older bikes have used a resister cap and standard plugs, where the newer ones are using a standard cap and resister plugs.

Don't know if it's different.........but on this side of the pond:
2V's have resistor in the caps and the connectors, and using standard plugs.
4V's have different coils and using resistor caps, connectors and plugs.

And I do agree about using parts with resistance that correspond to the ignition system design.

[Overkill]

Update: When I left the bike the fuel pump would not run as the computer would not ground the fuel pump relay, (same wire supplies power to the injectors) tonight it all worked great again with NO PROBLEMS for a while then the breaking down in the 4-5k rpm range and then finally when I rap on the computer it died completely again. I had already cleaned all connections too. It would not die wiggling the wires so I am pretty sure it's the computer, now to find one.....

[Scott_]

Some of the older bikes have used a resister cap and standard plugs, where the newer ones are using a standard cap and resister plugs.

Don't know if it's different.........but on this side of the pond:
2V's have resistor in the caps and the connectors, and using standard plugs.
4V's have different coils and using resistor caps, connectors and plugs.

And I do agree about using parts with resistance that correspond to the ignition system design.

That sounds the same over here.

[Overkill]

Update: Put in the new computer same results. I found that the side stand switch was bad. I was running it on the center stand, with the side stand down but in neutral. When I put the side stand up it would run great at all rpms then with the side stand down it would run like crap but still run.

 I replaced the side stand switch and now it will not run at all with the side stand down even in neutral, even with the clutch in.

So the side stand switch was bad and instead of killing the bike it just made it run like crap. Now the switch is good so the bike wont run ANY time the side stand is down so there must be another problem. I assume clutch or neutral safety switch. The neutral light works great and is always in neutral when it says it is. Not sure where to go from here. Any suggestions?

[Inge K.]

 I replaced the side stand switch and now it will not run at all with the side stand down even in neutral, even with the clutch in.

That's normal, the sidestand switch cuts the signal to the fuel pump relay......neutral light and clutch switch is in the starter Circuit.

[Overkill]

SO to warm up the bike it must be on the center stand? You can not leave it running with the kick stand down ever?