Author Topic: Weird Knocking Sound Got Worse  (Read 9359 times)

Offline Big_B

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  • Posts: 76
Weird Knocking Sound Got Worse
« on: September 08, 2012, 02:10:48 AM »
OK this is my 2nd post in regards to my 93 K1100rs knocking. Duck has been very helpful overall with my issues. I kinda have my hands full with changing the clutch and would love to find out why she is knocking before it leaves me stranded somewhere. I'm working with a very limited budget after replacing the fuel pump, battery, tires, drive shaft, final drive bearings and now the clutch.

The "knocking" only occurs at idle, anything off idle and it sounds normal/fine. The knocking seems to be coming from the left side of the motor,not near the alt but I could be wrong. It is surging sometimes at low rpms which makes me think I should try synching the TB's. The new development that has me concerned is that the knocking sound just started to be at exactly twice the frequency. Let me try to explain it better. Typically the knock frequency would happen about 2 times per second....now it's at 4 times per second! In fact, I was sitting at a traffic light and heard it all the sudden start knocking twice as quick.

Another interesting symptom is it doesn't knock when stone cold, but does after running for maybe 1 to 2 minutes. The motor sounds great right off idle. I would be heartbroken if it's the bottom end or a timing chain issue. Anyone have any ideas?
  • North Easton, MA
  • 93 K1100RS
'93 K1100RS 75K miles all stock w/Fox aftermarket shock Corbin seat.

Rick G

  • Guest
Re: Weird Knocking Sound Got Worse
« Reply #1 on: September 08, 2012, 05:59:27 AM »
I would sync the throttle bodies before you go any futher as that is one of the most common causes. Check around the TBs for leaks as well, once again its the very uneven combustion that causes the knocking. Untill you kno3w that it is OK in that department there is little point going futher.
You can have a look at the bottom end of the engine quite easily and considering the very minimal cost  ($0) it would be well worth doing if only for peace of mind.

Offline Gubro30

  • ^ Motobrick Curious
  • Posts: 88
Weird Knocking Sound Got Worse
« Reply #2 on: September 08, 2012, 07:25:01 AM »
Alternator monkey nuts?
1986 K75C with K100Std fairing, Black. 50Kmiles

Rick G

  • Guest
Re: Weird Knocking Sound Got Worse
« Reply #3 on: September 08, 2012, 07:43:51 AM »
Alternator monkey nuts?

The rubber cush drive for the alternator when you see you will understand. 3 pairs each two are joined by a thin rubber bit.

Offline Big_B

  • ^ Motobrick Curious
  • Posts: 76
Re: Weird Knocking Sound Got Worse
« Reply #4 on: September 08, 2012, 08:59:41 AM »
Yes I will get a set of carb sticks going and check then check the alt bearings if the noise persists. I'll post my findings when I figure it out.

Thank you for the replies!
  • North Easton, MA
  • 93 K1100RS
'93 K1100RS 75K miles all stock w/Fox aftermarket shock Corbin seat.

Offline ReneZ

  • ^ Motobrick Curious
  • Posts: 54
Re: Weird Knocking Sound Got Worse
« Reply #5 on: September 08, 2012, 04:35:34 PM »
Sometimes an air leak can sound like that.
Greetings, Rene

BMW K100 - 1984
BMW K1200GT - 2003

Offline Big_B

  • ^ Motobrick Curious
  • Posts: 76
Re: Weird Knocking Sound Got Worse
« Reply #6 on: September 08, 2012, 11:16:15 PM »
Good news! As I was reassembling my bike I found that 2 of the vac caps were gone from my TB's. Thanks to Motobrick for giving me the tech advise to be able to repair my bike myself. All the members here have always given me great information and leads on parts. Awesome website!
  • North Easton, MA
  • 93 K1100RS
'93 K1100RS 75K miles all stock w/Fox aftermarket shock Corbin seat.

Offline strangemason

  • Curious
  • Posts: 2
Re: Weird Knocking Sound Got Worse
« Reply #7 on: September 25, 2012, 06:55:13 AM »
Alternator monkey nuts?
My k75 also rattles (not really a knock) at idle, I isolated the noise to the alternator with a steel tube to the ear and then found that the noise goes when I turn the lights on. I assumed the bearing was breaking down, but monkey nuts will be investigated now thanks to the forum. Also although I was told to let this problem develope by a good BMW mechanic who looked at it many miles ago, I recently bought a k75 for spares and found the alter nator missing deep scars where it should have been and even a broken casing, quite a frightening sight. I intend to investigate straight away before mine goes the same way  :yow
ok I have removed the alternator straight after writing the first part of this message and its not so bad. The alternator came off with ease, there was no obvious play and the monkey nuts (rubber cushions) were worn but not bad. I then looked at the drive still attached to the spline and found it moved with a sideways movement, when I went to take it off (13mm socket) it came undone far too easily for my liking. The splines from the engine look fine as far as I can see but the cush drive does not seem so regular although its a job to tell. I will replace the drive and the monkey nuts and let you all know the result. Any comments/advice is welcome :dunno
just one more thing, just ran the bike without alternator and cush drive and its sweet as a nut, thankfully it means that the drive bearings are good and the problem is in the cushion system, now going to price this from motor works and sherlocks, the best providers in the UK
great news cush rubbers or monkey nuts £11.60 uk, drive £7.80uk, james sherlocks bmw, happy days  :hehehe

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