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1Back to top Go down   1991 K100 LT Intermittent rev limit Empty 1991 K100 LT Intermittent rev limit Sun Jan 02, 2022 7:50 am

slowpoke

slowpoke
active member
active member
The Bike after 10 miles running will not rev past around 5000 revs. Switch the bike off and it runs perfectly again then after 10 miles the problem returns. Switch off and bike runs again.
Any ideas? thanks


__________________________________________________
George 1991 K100 LT
    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
Does the engine break up, or does it go flat while otherwise running okay?  Does it shut down?  Does it limit at the same rpm every time?  Does it matter which gear you are running in?  If you try to hold at the limit does the rpm slowly drop?

Check out these threads.

https://www.k100-forum.com/t1590-won-t-rev-past-5000

https://www.k100-forum.com/t9254-speed-limit

I'm thinking your fuel filter is plugged, shutting down the engine relieves the back pressure allowing the element to relax.  After a bit of running, it compresses and blocks flow again. 

I would change it, it's not an expensive part, and if you can't recall the last time it was changed, it may be time.

The other possibility is an electronic problem in the ignition or engine control modules.  That's a pretty deep hole to go into.  I'd check the fuel delivery system before going there.


__________________________________________________
Present:
1994 K75RT
1991 K100RS


Past:
1988 K100RS SE
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

3Back to top Go down   1991 K100 LT Intermittent rev limit Empty 1991 K100 LT Intermittent rev limit Sun Jan 02, 2022 9:36 am

slowpoke

slowpoke
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active member
Thanks I had come to the same conclusion I have one on order will try that first many thanks


__________________________________________________
George 1991 K100 LT
    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
Let us know how you make out.  Along with the answers to the questions above about symptoms, it will add a lot to the knowledge base of the forum.


__________________________________________________
Present:
1994 K75RT
1991 K100RS


Past:
1988 K100RS SE
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

slowpoke

slowpoke
active member
active member
Sure will do that when I've fixed it .Quick answers to questions:
1 don't quite understand that one but there is no power loss
2 it doesn't shut down
3 Yes limits at around the same revs
4 doesn't matter what gear
5 no. Held at full throttle it stays at around 5200 backing off a couple of 100 and back up again.
Switch it off on the key and immediately back on again and it will rev as normal then it will run as normal for 5 - 10 miles and then limit again. This first started around 40 miles into the ride. I will do a little work on it when the filter and service kit arrives and let the forum know the results.
thanks again
George


__________________________________________________
George 1991 K100 LT
    

6Back to top Go down   1991 K100 LT Intermittent rev limit Empty Rev limit Sun Jan 02, 2022 3:03 pm

daveyson

daveyson
Life time member
Life time member
A couple of quick checks while you're waiting for the new filter.

Does the problem disappear after lifting the tank filler cap? If so the problem might be a blocked filter or a blocked tank vent, especially if the tank was full before your last trip.

If the LT is like the earlier models, you will have a vacuum hose on throttle body four from the fuel pressure regulator. Remove it to make sure it is bone dry, any sign of fuel means you want another regulator.

Edit: With a blocked vent it might take ten minutes for the vacuum in the tank to build to a point that it causes problems.



Last edited by daveyson on Sun Jan 02, 2022 6:16 pm; edited 1 time in total


__________________________________________________
11/1985 bmw k100rt (late model)  Vin. 0090567
 ~120,000 km
    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
slowpoke wrote:1 don't quite understand that one but there is no power loss
When you hit the 5000 rpm mark does the engine begin to stumble, or does it run well, just without enough power to increase the revs?  It sounds like it has good power and runs normally up to the 5000 rpm point, is that correct?


__________________________________________________
Present:
1994 K75RT
1991 K100RS


Past:
1988 K100RS SE
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

slowpoke

slowpoke
active member
active member
thanks for those ideas 'daveyson' will try all these things one at a time.
The bike revs as normal up to around 500 then hits a brick wall if I keep it just under it will sit at 70 mph all day.


__________________________________________________
George 1991 K100 LT
    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
slowpoke wrote:The bike revs as normal up to around 500 then hits a brick wall if I keep it just under it will sit at 70 mph all day.
While changing the fuel filter and checking the tank vents won't hurt, I suspect that the problem lies elsewhere.  Fuel starvation will affect acceleration, and you say the bike runs normally up to 5000 rpm.  I would also expect a fuel problem to affect cruising speed consistency and cause some stumbling, breaking up and even dying at high speed. 

The consistency of the symptoms is making me think the problem is in the electronics, either the ignition or engine control units. 

Unlike the Motronic, the Jetronic controls on the K100 are analog computing devices and operate on voltage levels and pulse widths as opposed to counting digital pulses.  Unfortunately, there is little information to allow troubleshooting of internal operating issues.  I suspect that somewhere inside the units there is a circuit issue that is causing the limiter to think engine speed is higher than it actually is.  The only way I know of to determine the cause of the problem would be to see if replacing the electronics fixes the problem.

If you want to get a better understanding of the Jetronic operating principles you can read the attached document from Bosch.  Of special interest are pages 11, 15 & 16.

http://jenniskens.livedsl.nl/technical/Tips/Files/Bosch_Technical_Instruction_L-Jetronic.pdf


__________________________________________________
Present:
1994 K75RT
1991 K100RS


Past:
1988 K100RS SE
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

10Back to top Go down   1991 K100 LT Intermittent rev limit Empty Rev limit. Tue Jan 04, 2022 7:40 pm

daveyson

daveyson
Life time member
Life time member
That's convincing, so from Bert's guide I think the timing is six degrees at idle and is reset back to six degrees at 8,777rpm. If with a timing light at the hall sensors, the timing is seen to reset to 6 degrees (or drops) at say 5,000rpm, would that point to the problem at the ignition control unit, and that the fuel injection control unit is probably OK? 

The timing light could possibly also show that the spark is good. I think on you tube  there was a timing light video by "michaelovich"


__________________________________________________
11/1985 bmw k100rt (late model)  Vin. 0090567
 ~120,000 km
    

11Back to top Go down   1991 K100 LT Intermittent rev limit Empty 1991 K100 LT Intermittent rev limit Thu Jan 13, 2022 1:56 pm

slowpoke

slowpoke
active member
active member
Ok update
I have done a few things at the same time but here as follows :
Drained the tank cleaned the last dregs with a vac as there were bits in the sump of the fuel pump.
cleaned the fuel pump filter 
fitted new filter in the tank
new air filter
four new plugs
new oil and filter.
done just short of 200 miles today and the bike ran like a German! watch
so fingers crossed I think I've solved the problem.
thanks for everyone's help.


__________________________________________________
George 1991 K100 LT
    

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