Hi guys and gals, my restored 1983 K100 has sadly started playing up.  I was riding home from work when I noticed that everytime I slowed down for a junction or a roundabout when I went to accellerate away the bike was hesitant and felt like it was missing.  I thought nothing of it until I went to start it today and I went through my normal starting sequance, choke to 100%, always fires after about 3seconds on first press but today it didnt. 

I then tried about 10 times and nothing was happening so I tried without choke and it almost fired each time as engine was started but then stalled, I kept trying without choke and it was almost catching and after about 10attempts it did but I was having to be very gentle with the throttle so I held it running and then started to increase the revs and it sounded fine.  I threw my leg over bike set off and it immediatly felt like it had no power and was what I would describe on an old carb bike as bogging down and almost missfiring so I went round the block and returned it home. 

I have just been out into the garage and it is almost exactly the same, wont fire with choke but trying to fire up without.  I checked my spark plugs and all look the same, I can here the fuel pump running also before and after pressing button so before I look at major surgery has anyone had anything similar?  I did quite a through restoration on this bike even all the injectors were proffessionally overhauled.

Thanks in advance

Check the crankcase breather pipe. It's the double-L shaped pipe between the plenum chamber and the crankcase, just to the left of the coils cover. If that splits (and they frequently do), that will lead to everything you've described.

Also, check these guys are still on the first three throttle bodies and aren't split:

https://www.motorworks.co.uk/vlive/Shop/Parts.php?T=5&NU=15&M=40&Ct=FA&SbCt=BA_15_40_FA_50&spPage=2

(Item FUA60594)


and that the vacuum pipe hasn't come off throttle body no.4 or is split.

Dai wrote:Check the crankcase breather pipe. It's the double-L shaped pipe between the plenum chamber and the crankcase, just to the left of the coils cover. If that splits (and they frequently do), that will lead to everything you've described.

Hi Dai,

Yes thats ok as I replaced all rubber hoses and seals when I rebuilt it, the bike has only been on the road 5 months and has up to this point ran like a dream!!!

Crossed edit

Dai wrote:Check the crankcase breather pipe. It's the double-L shaped pipe between the plenum chamber and the crankcase, just to the left of the coils cover. If that splits (and they frequently do), that will lead to everything you've described.

Also, check these guys are still on the first three throttle bodies and aren't split:

https://www.motorworks.co.uk/vlive/Shop/Parts.php?T=5&NU=15&M=40&Ct=FA&SbCt=BA_15_40_FA_50&spPage=2

(Item FUA60594)


and that the vacuum pipe hasn't come off throttle body no.4 or is split.

 Yes, all brand new rubber caps and also new pipe which is still connected from 4 to I think the pressure regulator??  I am thinking of replacing the fuel filter as I have a new one in my spares cupboard.

Stupid question...
Is the oil filling cap on the crankcase tight?
If it's not tight, it will be the same as a cracked hose.

For everybody's experience: Start your bike and let it idle. Then open the oil cap and you will right away hear your engine running like crap.

wilbo wrote: . . . I can here the fuel pump running also before and after pressing button . . . so before I look at major surgery . . .

The fuel pump doesn't run before the starter button is pressed on a 2V engine. Pressing the button activates it. If the engine doesn't start, the fuel pump will run for a moment after the button is released. I wonder what you're hearing when you turn the ignition to On. Crickets, warblers or the neighbors arguing would be candidates, I think.

Is the fuel filter the original or is replacing it just the start of a parts-changing binge? It might be helpful to check all fuel hose connections within the tank for tightness and the under-tank fuel pump connector for its condition. The fuel pump fuse and its connection might be inspected also.

It's too early for major surgery unless you get paid for it.

I had same symptoms with one and it turned out the fuel filter was leaking at the crimped seam. May want to have a look in tank with pump running or check pressure at fuel rail.

Get back to basics are the spark plugs wet or dry?
Regards Martin.

Just a quick update, literally finished putting the fuel filter in half an hour ago and the bike sprung to life almost instantly, a little bit stutterey at first as the fuel filled filter and onwards no doubt to injectors but then ran superb as it used to.  Well pleased but what I did notice as I have never had the tank open when bike running was the return into the tank seemed to be very cloudy almost white, what was a 100%clear tank became very murky quickly, anyone else seen this?  I may just need to run the fuel through it and put a fresh tank of fuel in.

Also replaced part of rubber hose in tank as it appeared to have a large scallop out of it so could have been leaking pressure.

Thanks for all the suggestions guys as another problem solved and only took 30mins.

Has your tank got the return hose installed?  It's the black one inside the tank that goes from the collector near the cap to the filter surround at the intake to the pump.  If that is installed, you shouldn't see any fuel returning. It's there to prevent fuel starvation through cavitation.

Make sure any hose that you replace in the tank is rated EFI submersible SAE 30 R10. If it is not rated as submersible it will eventually cause you grief by melting or disintegrating.
Regards Martin.

Not this hose, Martin. It's low pressure fuel on its return path back into the tank. The hose to which I'm referring and which BMW sells (16131455350) seems like normal vinyl or rubber. The stuff hardens up over time.

Disaster has struck again!  Set off for work this morning and got about 200 yards from house and bike started to miss slightly, then got about a mile and it started bucking and dropped a cylinder eventually stopping.  Managed to get it started and have discovered that if I can get it to above 3000rpm it runs fine??  the rest of my journey is 8miles and above 50mph it woks no issue but as soon as I slowed for the roundabout at the end of the run and dropped to 2nd gear bike bucking and no power then slowly crept up to 3000rpm and then launches itself and throttle and engine all back to normal??

I will have a look at the return hose in the tank when I get home as suggested above.

Rob I was referring to this part of the post

wilbo wrote:

Also replaced part of rubber hose in tank as it appeared to have a large scallop out of it so could have been leaking pressure.

Regards Martin.

Try disconnecting the throttle switch, it may be cutting fuel a too high of throttle position.

So it looks like I may have got to the bottom of it, I removed my injectors and gave them a clean as best I could even though I had them professionally cleaned last year, I gave them a few taps on my wooden bench, put them back and the bike now appears to be running well.  Before I removed the injectors I had the bike running rough and it appeared when I removed the injector wiring off 3&4 cylinders no change to the engine which was why I decided to pull them, I also pulled them and tried starting it so I could see the spray patterns and 3&4 were weak compared to 1&2.  I only let the bike stand for a month so I am surprised this happened.

Anyway I shall put a fresh tank of fuel in and take it for a long blast to try and get a 100miles or so on it.

Good going.  It might have been debris clogging the filter in the injector and you knocking it dislodged the dirt. Might be worth a fuel filter change in the tank just to be safe. A Napa Gold 3032 filter is only a few dollars; cheap insurance.