I am usually pretty good at troubleshooting but this one has me stumped. I am not sure where to look. I'm not even sure if it is an electrical or fuel problem.

First some background;
It is an 85 k100 with about 90k miles. I rode it for most of those miles doing the basic maintenance and it rode well but slowly the problems began to creep up on it. When it hit 90K I debated trading it in versus rebuilding it and chose to rebuild it. I had the head rebuilt, replaced the engine block with a younger used one, replaced the clutch, , cleaned the injectors, changed all the seals and generally cleaned everything up. In doing this most of the problems were cleared up and it now runs great except for the one problem that remained.

There are two symptoms to the problem. I am not sure if they are related but guessing they are. They are exactly the same as before the rebuild.

1.) when you start the bike and it is in neutral, if you give the throttle a quick twist it chokes and sputters. It is not responsive and seems to trip over itself. When it is warmed up it is less so but still evident. If it is very cold it may stall but usually catches itself and then powers.

2.) This one is hard to reproduce. In 4th gear at about 4500 rpm it chokes. It is as if it is not getting gas. It will choke, the rpm will drop and then catch itself and regain power. Sometimes it chokes repeatedly. Sometimes just a few sputters. Sometimes giving it more throttle can seem to blow past it and sometimes not. It does not always occur and is difficult to reproduce. It does seem to generally occur at the same point on my way to work. At about 10 miles in on the freeway on a very slight downgrade. Not sure if that is a helpful clue or not.

When the second problem first occurred before the rebuild I tried to solve it and read that it might be the fuel filter. I replaced the fuel filter and it seemed to help but the problem returned after not too long. Then I replaced the filter again and it did not seem to help. Because the problem is not consistent it is difficult to tell if there was any small improvement but I don't think so.

With the first problem I thought it might be the clutch or splines. On the rebuild I replaced the clutch and the splines looked good. There was no change.

I am hoping it is not a electronic control problem. That could be expensive. Maybe a vaccuum leak but where should I look or begin to trouble shoot it? I replaced that S shaped breather hose that is always cracking. I also red about the connections on the fuel sensor connector causing problems.

Any ideas?

hmm interesting issues

i would be looking at the rubber tb take of boots ...the rubber inlet boots and plenum to tb boots ...

there are many threads on finding the airleaks and replacing them

just my thoughts

Im thinking a lean fuel condition, either caused by a vacuum leak or weak fuel pump. Do you have the option to swap out for another, known fuel pump? I would also spray some carb cleaner around the intake boots. If the rpms increase at all, you've got a leak at that point. If you have one that is cracked (old rubber) it will play havoc with the vacuum.

I don't have a spare fuel pump to try. I did replace my fuel pump about
20,000 miles ago. It was before this problem started. The pump had
stopped working completely. After changing the pump, this problem did not
appear for quite some time.

I tried checking for leaks around the throttle body boots with propane but didn't notice anything. The bike idles great and revs great on a slow roll. It is just the quick twist that produces the stutter and this is mostly just when cold. I had read somewhere once that this was normal for early k bikes but that didn't seem right to me. Also, when in rebuilding the bike I did not replace the boots but did check them very carefully and didn't see any cracks.

It is very possible that the two problems are not related. The more troubling one is the choking at mid rpm in 4th gear. But that is also the hardest to trouble shoot because it is difficult to reproduce.

Another possible clue (or red herring). this bike has always had a light backfire when downshifting or when giving it light throttle when going down a hill. I was told that this was from the luftmeister exhaust. It never bothered me and I never gave it much thought. I mention it now because the mid rpm sputter seems to come about when on a gentle downgrade at steady moderate acceleration.

Is there anything that I could do while driving on the freeway that would help trouble shoot the problem? I have never tried turning the choke on. Would that tell anything. I saw a post on a similar problem that said stuffing a rag in the air intake alleviated the problem. I am not sure how I could do that while driving since it is on the right side. The rag stuffing does nothing for the throttle twist problem.

The choke lever is only a high idle lever, it doesn't really 'choke' the engine (enrich mixture). Everything you've said so far says 'lean mixture' to me, which is either too much air or not enough fuel. Can you do a fuel pump pressure test? And a fuel pump flow test?

I have seen the pressure test mentioned elsewhere. Placing a gauge in the line to the fuel rail. I have not seen a description of a flow test. do you know of a link to one?

Do you think the two problems are related, club_c? In either case, would I need to do the pressure test while driving when the problem show itself?

Yep smells of a lean mix to me. Check the crankcase breather hose for cracks or falling to bits. Try a new filter and check the fuel pump out with a flow and pressure test.

For a flow test just a quick check, these pumps will pump a lot of fuel very quickly. Remove the filter and make sure the hose from the top of the pump is facing into the tank and ign on push the start button, if there is anything less than a real gush of fuel there is probably a flow problem. Then inside the tank place a finger over the end of the hose and press start again , if you hava a real hard time trying to stop the flow then pressure is probably ok but if it stops easelly feed the pump to a dumpster.

I don't have a pressure gauge so I was only able to do Rick's quick test. The pump seemed to pass. Good flow and I wasn't able plug it with my finger.

In looking up the pressure test I came across some notes on the throttle position switch. I tried disconnecting it and that solved my backfire problem but didn't affect the sputter on hard throttle when cold or the cutting out at higher speed. It is going to be raining here for the next 4-5 days so I won't be able to do any more road tests for a while. I will try to see if I can borrow a pressure gauge. If there are any other tests I could try, please let me know.

Thanks for the help.

Another clue that I didn't mention because I didn't think that it was related is that the engine idles too fast sometimes. It sems like it does it moreso when it is hot out but that might just be when it annoys me most. In searching around for my fuel problem I came across a fast idle post where somebody said that it might be the pressure regulator. Does this fit my other symptoms or is there a way to test the pressure regulator?

presure regulators are a dark science ....and not to be entered into lightly ...giggle ....that asside ...usually ...i did say ussually ..they either work or dont properly

having said that i have noticed that the presure regulator really doesnt need any other assistance from vacume tubes that run to it to work ....i guess ..but it is there and for a reason that i dont understand fully i guess ...under max vacuume it additionally lowers the presure of the fuel line ...maybe to improve economy

but we do have a line that picks off the rear most throttle body and feeds vacume to it .....now just supose .that the vacume line that picks off there ...has been contaminated by blowby ...oil and crap leaving the crankcase throught that breather tube and has travelled along the path to he regulator bellows ...could this in fact be a contributing factor ?

i recently claned up the idle bypass adjusters on the throttle boddies and found them on the rear tb to be full of oily gunk ....carbi cleaner soon made the clean up look good ....but could also the gunk have made its way to the regulator along the vacume line ?

and now providing another vacuume leak that has not been on the "to watch list "...in addition to contaminating the belows and springing in the presure regulator ...

but maybe replacing that tube could help ?

just some weird musings im afraid ...

kringb,
If you think it may be running weak, have a look at CF's post:-

http://k100rt.aforumfree.com/t3301-how-to-momentarily-increase-power-on-a-k-bike

At least it would be easy to try when in 4th and spluttering.
Paul.

I would definitely be looking towards the fuel pressure regulator before too much longer - it took me 14 months of on and off fiddling around before I spent £28.00 on a secondhand FPR which resolved all of my issues in one fell swoop. The only reason I left it until last is because it is such a pig to get to. I wish I had tried it first now ! The old 1984 K100 has run without fault since I replaced the unit. I have another issue now but that is almost certainly just an electrical connection problem - common on these bikes. If in doubt try and source a cheap FPR replacement - if it doesn't work at least you will have a spare around for when it does eventually fail - good luck with your efforts

Hi Krambo, I actually have an extra fpr valve that came with an engine I bought that I was planning on trying. In my research I had read the thread with your posts on your fpr problem. I am glad you got yours figured out, but your bike was running much rougher than mine wasn't it? And more consistently rough. Mine runs pretty good with the exception of the intermittent cutting out and the sputtering when cold.

It may turn out to be my fuel filter. When I first had the problem it was much much worse and the fuel filter fixed it for a while. Also, it seemed to occur more often when my tank was low on fuel. I have been reluctant to try another fuel filter because after that first one, the next two didn't have such drastic results and I can't believe they are going bad so fast. My tank is pretty clean. But, I think that I will have to try it just to rule it out. I don't know how to trouble shoot a fuel filter except by replacing it.

If anyone knows of a cheaper non bmw replacement part for the fuel filter, let me know.

I will try some things and post any results I find.

kringb wrote:Hi Krambo, I actually have an extra fpr valve that came with an engine I bought that I was planning on trying. In my research I had read the thread with your posts on your fpr problem. I am glad you got yours figured out, but your bike was running much rougher than mine wasn't it? And more consistently rough. Mine runs pretty good with the exception of the intermittent cutting out and the sputtering when cold.

It may turn out to be my fuel filter. When I first had the problem it was much much worse and the fuel filter fixed it for a while. Also, it seemed to occur more often when my tank was low on fuel. I have been reluctant to try another fuel filter because after that first one, the next two didn't have such drastic results and I can't believe they are going bad so fast. My tank is pretty clean. But, I think that I will have to try it just to rule it out. I don't know how to trouble shoot a fuel filter except by replacing it.

If anyone knows of a cheaper non bmw replacement part for the fuel filter, let me know.

I will try some things and post any results I find.

Glad that you found the account of my own trials and tribulations - it seems from what you are saying that my non-running problems were certainly worse than your symptoms would indicate. I am guessing that any fuel filter that can handle the rated fuel pressure of our bikes would work as a replacement but greater minds than mine will soon be along to comment I feel sure - it will all come good in the end I reckon - these bikes seem to be pretty bombproof considering their elderly heritage and it must be a tribute to German engineering that so many survive to this day and give pleasure to the strange and wonderful crowd that still ride them many miles annually - good luck with the bike

krambo wrote:

kringb wrote:If anyone knows of a cheaper non bmw replacement part for the fuel filter, let me know.

I will try some things and post any results I find.

I am guessing that any fuel filter that can handle the rated fuel pressure of our bikes would work as a replacement

I use a filter which is a Z200 Ryco but any filter made of aluminum and with the 5/16 ends will do the job. Most of the FI systems have similar pressures and flow

+1 rick .....it just makes sence doesnt it ....same fuel flow and presures required to run a car of similar ...to bigger capacity ...i chose one that was rated for the 1.6 -2 litre class of motors ...and it works just fine for the last 15,000 killometers

good luck kringb ...but do the regulator and get it out of the mix of possible issues huh ?

I changed out the pressure regulator but because of the rain and other things I was not able to test drive it until today.

There was no noticeable difference to anything after changing it out. So, at least that can be ruled out. In the test drive I tried to find some other clues and a better description of the problem.

First the updated description:
The description given before of the engine sputter on throttle is still a pretty accurate. As I said, it only occurs when the engine is cold. I am starting to feel like it is not related to the other problem but don't really understand either yet. I made a little video of this that I can post if someone thinks that would be helpful.

The bigger problem is what I had described as the engine choking in 4th gear. that is not really an accurate description. Because it is so intermittent and because I am unable to force the problem to occur it is hard for me to study it but in my long test drive today I tried. I drove for about 2 hours on the freeway and it occurred 4 times.

A better description than "engine choking" is " sudden loss of power". It happens very fast, as you are powering along at a steady speed (apx 4500 to 5000 rpm) there will be a sudden loss of power, you pitch forward as the bike slows, and then just as quickly it catches and power is restored. It is about as fast as a hiccup. Very frustrating. It is so fast and happens when you least expect it. It is hard to gather any data.

I unplugged the throttle position switch and the problem still occurred.

I briefly unplugged the electrical connector to the fuel pump as I was driving and heard the pump stop. There was no lurching or power loss or anything like the problem.

I am not able to push the starter button to enrich the mixture while the problem is occurring but I did test that at idle. It increased the rpms just slightly which is what I believe is supposed to happen.

I was able to get the problem to occur in third gear as well as fourth. With the rpms about the same and steady powering along. I wasn't able to try for long enough in first or second to see if it would occur in those gears.

It happens going up hill, down hill, and flat. Doesn't seem to matter.

It always occurs in the same general rpm range when you are giving moderate throttle and moving along at a steady speed. It may also have occurred under load with heavier throttle but I can't say for sure on that.

One reason that I am vague on when it occurs is because it was happening before I rebuilt the bike. It took me about a year to rebuild it. My memory is fuzzy about things that happened over a year ago. It was not the reason for the rebuild, just a problem that I couldn't diagnose and that I hoped would get taken care of in the process. No such luck, it is exactly the same as before. But again, at least now I can rule out some causes.

Here are the big things that were changed
replaced engine block
cylinder head rebuild
new clutch

----

I will keep watching the problem and list any new clues I can find. I am curious to see if my electrical power is being briefly shut off when it occurs. It sounds like that should be simple to check for but in traffic when the bike is goosing you it is not so easy to do.

Thanks, as always, for all the generous help.

That very sudden loss of power and just as sudden back on seems like an electrical fault to me. I would firstly look at the ignition switch.
Watch when it happend if possible do all the lights go out as well, does the speedo and tach drop to zero, if that happens then it is electrical.
I had the problem with an R80 and found that the ign switch was at fault but it was fixed by having only the one key on the ignition key ring and it was flopping about and putting pressure on the lock assy causing momentary off of the switch. It can cause backfiring but not usually.
An ex-police bike is more likely to have this problem because the ignition switch is used a lot more.

Hi there, mine had similar symptoms two times, the first one, power loss was caused by a broken wire in a connector at one of the HT coils. The second time was a HT coil.
Hope this helps.
Best regards
JAP

Hello kringb - I have just suffered a very similar problem on my 1984/85 K100RT - as it turned out, it was the 4 wire connection that runs up the frame on the right hand side of the bike, behind and below the petrol tank (as you are sitting on the bike). Remove the right hand side cover and it (the four pin connection) should be attached to the frame in that area. It supplies the fuel pump with voltage etc. Take the connection apart and clean (DeOxit) the connector as best as you can, twist the little male spade connectors slightly out of true with some small nosed pliers (carefully) and then reconnect the 4 pin plug. It worked for me and is a well known and documented issue on this forum. Good luck

Krambo: that sounds more like the gear position indicator switch wire connector? For me, because I can shut off the pump and still continue to have power for some amount of time and because my problem is more sudden I think I can rule out the filter and fuel pump. It seems to me the problem is that the fuel injectors are being shut off for some reason. There are a lot of electrical possibilities.

Today I was unable to get the problem to occur so I could not verify if the electrical power is cutting out. That should narrow the possibilities down a bit.

Japuentes: when your coil problems occurred did the bike cut out only at a certain rpm level? And did the bike run fine otherwise?

kringb wrote:Krambo: that sounds more like the gear position indicator switch wire connector? For me, because I can shut off the pump and still continue to have power for some amount of time and because my problem is more sudden I think I can rule out the filter and fuel pump. It seems to me the problem is that the fuel injectors are being shut off for some reason. There are a lot of electrical possibilities.

Today I was unable to get the problem to occur so I could not verify if the electrical power is cutting out. That should narrow the possibilities down a bit.

Japuentes: when your coil problems occurred did the bike cut out only at a certain rpm level? And did the bike run fine otherwise?

The connector of which I speak is nothing to do with the gear position indicator switch as far as I am aware - my problems' symptoms were just the same as if I had accidentally moved the handlebar kill switch to the off position whilst riding - the engine died completely and then (sometimes) picked up again and did another 100 miles as if nothing had happened. It cannot hurt to check out all the electrical connections anyway. Sorry if you think my advice is not worthy of this wonderful forum but I was just trying to help a fellow brick rider

I do appreciate the help, krambo, but I looked and on my bike I only saw the gear position connector. Maybe I am looking in a different place.

My statement about the problem not being fuel pump or filter related was made with the hope that someone could either confirm that or shoot it down. Truthfully. I have no idea what is causing my problem. I am just hoping that it is not the computer.

There are two 4 pin plugs under the right side cover one is the gear position indicator and the other is for thr fuel pump and fuel level light and gauge. Thery can be wrongly connected and often are but nothing comes of it except a bike that won't run till its rectified.
I doubt very much it is the computer at fault far more likely to be a loose connector.

Hi all, I had this problem a while ago as did krambo I think, our findings helped each out then as I recall and since then I have experienced some more.

Just this last weekend my brother in laws RS stopped as we came in to fuel up at cheivley services, checked the fuel and there was at least a gallon in the tank - but the pump was not running. went through the normal checks and found a blown fuse - great , but no the pump still did not run, so we took it out and felt the pump kick when the starter was pushed but it would not run. seized we thought, but how? and how to cure it, as it was 4.30 on fri to late for any delivery trickery.

Well the very nice AA man helped by connecting it to a battery pack (12v no fuse and possibly 100's of amps) and on the third dab it started to run again, sorry this is long winded but my point is this - there is only the mesh pot underneath the pump, the filter is after the pump down to the injectors, since this event the RS has developed the same problem that you describe with yours. Cruising back on the M5 it would feel like all was about to stop but with a wrench off throttle would pick up as quick as it died off. I suspect that something is still momentarily clogging the pump and then clearing itself, we are going to strip it down again now home and test all the plugs and HT lead etc, but just at about 5000 revs is where our trouble was. I am going to run the bike with a spare pump next weekend and see if this changes anything.

I know from my last experiences that the K is very sensitive to any problem thus not giving a smooth engine and can be painstakingly awkward to find the cause, but they usually keep running if not to serious.

I hope this may help you to sort out yours.



All the best IPJ100

Thanks IPJ. Maybe it is the pump. I don't have a spare to try it out so be sure to let me know how yours comes out. The 4-pin fuel pump connector is not under the side cover on my bike. It must be under the tank. I will try krambo's suggestion.

What I can't understand is why I can unplug the fuel pump (beneath the front left tank connector) and the pump stops but the bike will continue running for some bit. To me that means the pressure in the system is enough to continue on without the pump for some short time. If that is so, then a clogged pump would use up the gas in the system and refill it more slowly I would think.

If it is a loose connector somewhere, then why is it only going out in that certain rpm range and never on a bumpy road.

Anyway, be sure to post your results and thanks again.

kringb wrote:Japuentes: when your coil problems occurred did the bike cut out only at a certain rpm level? And did the bike run fine otherwise?

Hi, when the failure was due to the broken wire, yes the fail was found in a narrow rpm band and you could see the rpm needle suddenly falling and recovering.
When the coil failed, the power was lost at high rpms specialy during acceleration, riding at low rpms you could get constant power, rpm needle behavior was not so brusque.
Resistence at the coil´s primary winding was low, both coils were changed.
Hope this helps.
Best regards
JAP

Hi Kringb,

When I had my first fuelling problem last year, it made no sense to me at the time, the bike would start run and rev nicely until i actually tried to pull away on the bike - where upon it would simply cough splutter and embarrass me no end. I put this down to the volume of fuel being available at pressure in the fuel bar, so i suppose that more fuel is required when actually travelling along at 60 odd mph etc at 5k rpm.

My problem last year was that having resprayed the petrol tank some overspray had got in the tank and when i added the fuel it cleaned the inside off the tank and sent the sludge straight into the filter and clogged it up - several times!!!, but i cleaned it with thinners and blew it out backwards several times again, but it was the fact that it would run but not pull the bike along that showed me that you not only need pressure but volume of fuel as well, so maybe a pump with an intermittent jam and the higher fuel usage could give these symptoms.

On another note we took the bike out for a 25 mile run tonight on windy 30mph roads and she did not miss a beat, the sun was out too. good luck with yours and will keep posted on ours

Cheers IPJ

I wonder if there might be a problem with your AMM, where a 'flat' spot suddenly drops the to fueling of the engine.

There are so many possibilities with this problem but I am currently thinking that Krambo might have had the answer.

Yesterday while driving I removed the electrical connector to the fuel pump. As before it continued on for a bit but this time I left it out. When the engine lost power I plugged it in and it quickly surged. The effect is just like my problem. So, my theory about the symptoms of a stopped pump not causing a sudden loss and sudden recovery is wrong.

I originally thought Krambo was smoking dope because the only four pin connector under my right cover is the gear position switch. I think on bikes like mine, with the non-fuel gauge level sender unit, the 4 pin connector is in the left front corner of the tank between the fuel lines. This is the same connector that I suspected and have been pulling in and out to shut off the fuel pump.

But like I said, there are many possibilities for this that would cause the same symptom. The reason I am currently suspecting the connector is because after driving the bike over the weekend and experiencing the problem 4 times I have since then not been able to get it to occur again. I have been driving my ass off and even went out late last night when the traffic is light to drive the freeway for 2 hours. I can't get it to occur and the only change I have made is in pulling the connector in and out doing all these tests. Maybe in doing so I slightly improved the loose connection

If I don't do anything I think that it will happen again as whatever minimal change I made wears off. When it does I will then try tweaking the connectors and see if it is a permanent fix.

I have no idea why it would happen at a certain rpm range. Maybe it is a vibrational frequency thing. Or the pump pulling more electricity? These are bigger questions than me. And I could very well be back on here tomorrow at square one.

kringb wrote:There are so many possibilities with this problem but I am currently thinking that Krambo might have had the answer.

Yesterday while driving I removed the electrical connector to the fuel pump. As before it continued on for a bit but this time I left it out. When the engine lost power I plugged it in and it quickly surged. The effect is just like my problem. So, my theory about the symptoms of a stopped pump not causing a sudden loss and sudden recovery is wrong.

I originally thought Krambo was smoking dope because the only four pin connector under my right cover is the gear position switch. I think on bikes like mine, with the non-fuel gauge level sender unit, the 4 pin connector is in the left front corner of the tank between the fuel lines. This is the same connector that I suspected and have been pulling in and out to shut off the fuel pump.

But like I said, there are many possibilities for this that would cause the same symptom. The reason I am currently suspecting the connector is because after driving the bike over the weekend and experiencing the problem 4 times I have since then not been able to get it to occur again. I have been driving my ass off and even went out late last night when the traffic is light to drive the freeway for 2 hours. I can't get it to occur and the only change I have made is in pulling the connector in and out doing all these tests. Maybe in doing so I slightly improved the loose connection

If I don't do anything I think that it will happen again as whatever minimal change I made wears off. When it does I will then try tweaking the connectors and see if it is a permanent fix.

I have no idea why it would happen at a certain rpm range. Maybe it is a vibrational frequency thing. Or the pump pulling more electricity? These are bigger questions than me. And I could very well be back on here tomorrow at square one.

I still stand by my post re. the four pin connector to the right hand side of your bike - pulling off the under tank connector (also 4 pin - but round pins rather than spade connectors) will also cut power to the fuel pump and display the exact same symptoms as the dodgy 4 pin connector previously mentioned. Maybe some PO has moved the connector to which I refer but as the wiring length is limited, I would guess that you are just not checking the correct connector. It is a wise idea to go through all electrical connections throughout and give them all the DeOxit treatment anyway - it might save you some heartache in the future - I wish you luck in your efforts and hope to hear of positive resolution to this problem before too long.

Your bike is an early model that had the thermistor type of fuel level sender and connection at the front left under the tank.
There are not a lot of these around anymore as they have been replaced by the later type with the connector at the right side cover.
Sorry for the wrong info.

No worries. I didn't know that there weren't many others out there. I'll have to remember that if I ever sell it.

"For Sale: Finest year k100 with the rare thermister type fuel sensor. No low balls offers!"

Rick,
I don't think they are that rare, to change to the later float fuel level device means changing the tank, altering the frame tank fixings and probably a lot more, so I would think that the majority of the 46,568 bikes that were made before the end of '85 still have the thermistor device.
Cheers,
Paul.

Same here, and there are still quite a few of the earlier types around!