Author Topic: K75 rattle  (Read 265 times)

Offline andrej_gr

  • Motobrick Curious
  • Posts: 12
K75 rattle
« on: July 28, 2019, 12:59:49 AM »
I've got this periodical rattle sound on my 1995 K75. Seems to be coming from alternator area. Pulled clutch doesn't change anything. Can't hear it on higher rpm, but that might be due to engine sound becoming louder. Ideas?

  • Sydney, Australia
  • K75

Offline Martin

  • ^ Quintessential Motobricker
  • Posts: 3805
Re: K75 rattle
« Reply #1 on: July 28, 2019, 01:17:22 AM »
More than likely the monkey nuts in the alternator. Use a stethoscope or alternatively a long screwdriver to confirm the source of the noise. Place the blade end of the screwdriver onto the engine and an ear on the handle then move it around until the noise is at it's loudest.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline The Mighty Gryphon

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  • Posts: 4541
Re: K75 rattle
« Reply #2 on: July 28, 2019, 11:59:30 AM »
Rattle between 1200 and 2000 rpm is loose fit between output gear and balance shaft.  Nothing you can do about it.     
  • In my garage in Marilla, NY
  • '94 K75RT Mystic Red, '91K100RS White/Blue
Current:
'94 K75RT "Ilsa, She Wolf of the SS"
'91 K100RS "Moby Brick Too

Offline Laitch

  • ^ Quintessential Motobricker
  • Posts: 6737
Re: K75 rattle
« Reply #3 on: July 28, 2019, 12:57:19 PM »
I've got this periodical rattle sound on my 1995 K75.  Ideas?
Sonic Defenders.
  • Along the Ridley in Vermont.
  • 1995 K75T 72,000 miles
I wept because I had no radials until I met a man who had no splines.
http://www.motobrick.com/index.php/topic,296.msg53303.html#msg53303

Offline mlytle

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  • Posts: 1039
Re: K75 rattle
« Reply #4 on: July 28, 2019, 03:01:17 PM »
what gryph said.....if it goes away at idle and above 2k rpm.

one of my K75S's does it, the other does not.


  • alexandria, va
  • 92 K75s, 94 K75s, 96 K1100RS SE (custodian), 09 K1300s
Marshall
Project Thread "K75s Midlife Refresh"
http://www.motobrick.com/index.php/topic,7810.0.html

Offline stokester

  • ^ SuperNatural Motobricker
  • Posts: 553
Re: K75 rattle
« Reply #5 on: July 28, 2019, 04:50:59 PM »
what gryph said.....if it goes away at idle and above 2k rpm.

one of my K75S's does it, the other does not.
Ditto:
My '93 has no rattle, the '94 rattles like hell at low RPM until warmup.
  • Yorktown, Virginia
  • '94 K75S - '93 K75S - '91 R100RT - '78 R100S

Offline The Mighty Gryphon

  • ^ Quintessential Motobricker
  • Posts: 4541
Re: K75 rattle
« Reply #6 on: July 28, 2019, 09:28:47 PM »
I've had some luck with carefully adjusting the valves to the same clearance, cleaning the injectors with Techron, and adjusting the throttle body balance.  Makes it better, but it's still there.

I did have some temporary relief for a year or so by tightening the clutch nut on the output shaft after cleaning all the oil off the face where the balancer contacts it.  The trouble is that you need to take the clutch apart to get at it, so it's not something you want to do every year or so. 

The sloppy fit between the parts seems to have been built in.  When I had my engine apart I inspected the parts and the fit very carefully, and the gap is not the result of any wear, the parts were machined to fit like that.  I thought about tightening the fit by welding some additional metal, but held off due to not knowing how I would be affecting the hardness of the parts.

It's just something you have to live with.  It's a harmless part of the K75 engine's charm.
  • In my garage in Marilla, NY
  • '94 K75RT Mystic Red, '91K100RS White/Blue
Current:
'94 K75RT "Ilsa, She Wolf of the SS"
'91 K100RS "Moby Brick Too

Offline Laitch

  • ^ Quintessential Motobricker
  • Posts: 6737
Re: K75 rattle
« Reply #7 on: July 28, 2019, 09:36:09 PM »
It sounds like a cat purring in its sleep. Don't disturb it.
  • Along the Ridley in Vermont.
  • 1995 K75T 72,000 miles
I wept because I had no radials until I met a man who had no splines.
http://www.motobrick.com/index.php/topic,296.msg53303.html#msg53303

Offline andrej_gr

  • Motobrick Curious
  • Posts: 12
Re: K75 rattle
« Reply #8 on: July 28, 2019, 11:07:20 PM »
So as per Martin's advice took off the alternator... Bingo!
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  • Sydney, Australia
  • K75

Offline Laitch

  • ^ Quintessential Motobricker
  • Posts: 6737
Re: K75 rattle
« Reply #9 on: July 28, 2019, 11:15:31 PM »
:cheers:
  • Along the Ridley in Vermont.
  • 1995 K75T 72,000 miles
I wept because I had no radials until I met a man who had no splines.
http://www.motobrick.com/index.php/topic,296.msg53303.html#msg53303

Offline Martin

  • ^ Quintessential Motobricker
  • Posts: 3805
Re: K75 rattle
« Reply #10 on: July 29, 2019, 12:29:35 AM »
Good result.
Regards Martin.
  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

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