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1992 K100 RS "breaks down / surges" at 4k - 5k RPM..

Overkill

New member
I have a 1992 K100 RS. It runs good at an idle and good up to 3,000 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 (the wires were vibrating off over time)
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 am thinking it might be a weak coil, does that sound right?

Thanks for you thoughts!
 
Yet you didn't change the fuel filter as suggested to you in your earlier post........... :scratch
Just keep flailing, it's good for the economy!
 
I forgot to mention that the fuel pump and filter is new about a year ago when I bought the bike. I bought it not running with a bad fuel pump and it has done this since I got it. I would think if it was a restricted fuel filter it wouldn't smooth back out at 5,000 rpm and higher where it is requiring even more fuel. But I don't know.
 
Yet you didn't change the fuel filter as suggested to you in your earlier post........... :scratch
Just keep flailing, it's good for the economy!


I have not posted "earlier" on this bike so nothing has been suggested to me prior to your post.
 
I don't think that a coil would be responsible for bad running within a very specific RPM - either they're bad all the time, intermittently at any RPM, or at high RPM. Possibly a "resonance" vibration causing an intermittent? Check the other plug wires for arcing in a dark garage; you can also put an inductive timing light on each wire to watch the action.
WAG: the computer (fuel map) or a sensor providing input to it (TPS, airflow)?
 
I don't think that a coil would be responsible for bad running within a very specific RPM - either they're bad all the time, intermittently at any RPM, or at high RPM. Possibly a "resonance" vibration causing an intermittent? Check the other plug wires for arcing in a dark garage; you can also put an inductive timing light on each wire to watch the action.
WAG: the computer (fuel map) or a sensor providing input to it (TPS, airflow)?

Good point, you are probably right. This may have to be a dealer resolved issue.
 
Yet you didn't change the fuel filter as suggested to you in your earlier post........... :scratch
Just keep flailing, it's good for the economy!

I have not posted "earlier" on this bike so nothing has been suggested to me prior to your post.

Sorry, I read BCKriders post that you replied to, similar problem, and somehow your name stuck in my head as the original poster. :dunno
 
Another thought - I'm not familiar with the K bikes' computers, but for the Oilhead brain, there is a procedure to "initially teach" the computer the relationship between throttle opening and TPS voltage (turn the key on but don't start the engine, twist the throttle slowly to full open and closed a couple of times, turn key off).
Maybe this could apply here? Maybe a software glitch created a "dead spot" in the map?
 
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.
 
Last edited:
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?
 
In your original post #1, you state that the problem happens - repeatedly - within a very specific set of RPM.
If this is true, then you need to specifically look at what happens within that RPM range "Only".
Things like the ignition module or fuel delivery are probably not part of this equation.
The only sensor - I think... - that could be a part of this might be the air flow.
Otherwise I still lean toward the computer or something related closely to it; or less likely but possible, a resonance vibration causing something else to go intermittent.
 
For my friend it was the injector connectors

A friend of mine developed what sounds like the identical problem with his '85 K100RS. It turned out to be a loose electrical connection on one of the fuel injectors. The male/female interface had loosed up, but after cleaning and tightening, his bike ran perfectly again. Hope this helps you.
 
A friend of mine developed what sounds like the identical problem with his '85 K100RS. It turned out to be a loose electrical connection on one of the fuel injectors. The male/female interface had loosed up, but after cleaning and tightening, his bike ran perfectly again. Hope this helps you.

Thank you! I will check that out!!
 
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.....
 
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.....

I'm assuming that you have cleaned all the connector surface - both male and female - with a good contact cleaner and some kind of slightly abrasive probe?
 
I have cleaned both ends of the plug and on the computer with no luck. We started it up the other night and it was perfect but before long it began to break down again and then when we wiggled all of the wires there was no change but a rap on the computer itself made the bike die.... so hopefully we have found it. I ordered a used computer so I should know soon, probably next weekend by the time it gets here. I am a little worried that it might be the hall sensor on the front of the motor telling the computer to shut down the fuel and spark but only time will tell for me now. Thanks for all of your suggestions!
 
I have the same issue between 4k and 5k on a 91 K1. Were you ever able to track down the issue and fix it?
 
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