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Author Topic: 92 K75S suddenly runs terrible  (Read 3810 times)

Offline Andy FitzGibbon

  • ^ Proficient Motobricker
  • Posts: 123
92 K75S suddenly runs terrible
« on: July 02, 2018, 01:35:45 PM »
A buddy and I were out for a ride on Saturday... me on my '85 K100RS and he on his '92 K75S. Hot day, temps near 90 degrees. After an hour or so of riding, and while headed down straight road at 55 mph, his bike suddenly started running terrible- unable to idle, hunting/missing at part throttle, and enough backfiring through the intake to blow the vacuum caps off the throttle body.  We drained the gas and replaced it with some known good fuel (guy on the side of the road was running it in his lawn tractor), which made no difference.

Ideas? After swapping the fuel, we gave up and headed to my house for my trailer. I'm thinking maybe crank or throttle position sensor, or maybe a timing chain guide failed and allowed a bunch of slack in the chain. Having an '85, I'm not familiar at all with the Motronic FI system, and I've been traveling the last couple days and unable to work on the bike or educate myself. 

Thanks,
Andy
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  • Montrose, WV
  • 1985 K100RS, 1972 R75/5, 2012 Suzuki DR650

Offline wally.fisher

  • ^ Motobrick Curious
  • Posts: 94
Re: 92 K75S suddenly runs terrible
« Reply #1 on: July 02, 2018, 02:02:06 PM »
Inspect and clean every electrical connector you can find, most importantly the big connector under the seat as well as those under the tank. Be  methodical. Do one at a time and if possible start the bike after each connector then you will know what has worked.

Sent from my ONEPLUS A5000 using Tapatalk

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Offline Andy FitzGibbon

  • ^ Proficient Motobricker
  • Posts: 123
Re: 92 K75S suddenly runs terrible
« Reply #2 on: July 02, 2018, 02:02:32 PM »
We checked the 4 pin, tightened up the pins slightly. It seems to be fine. The pump is working for sure (pumps fuel when disconnected from the rail) though it's possible it's not functioning 100%.

Andy
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  • Montrose, WV
  • 1985 K100RS, 1972 R75/5, 2012 Suzuki DR650

Offline Martin

  • ^ Quintessential Motobricker
  • Posts: 4475
Re: 92 K75S suddenly runs terrible
« Reply #3 on: July 02, 2018, 05:36:28 PM »
Check the fuel return line from the FPR to the tank. Due to mine being non OEM it collapsed where the line enters the tank on a hot day. It then ran very badly until I rectified the restriction. The OEM hose has a preformed curve and a lot of bricks by now are running straight hose.  Depending on where you live and the quality of the hose and other variables is probably fine most of the time. The collapsing can be hard to detect as when you pull the tank back it can correct itself.
Regards Martin.
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  • North Lakes Queensland Australia
  • 1992 K75s Hybrid, Lefaux, Vespa V twin.

Offline The Mighty Gryphon

  • Administrator
  • ^ Quintessential Motobricker
  • Posts: 6843
Re: 92 K75S suddenly runs terrible
« Reply #4 on: July 03, 2018, 07:59:41 AM »
First off, the K75 has a Jetronic like your K100.  How many miles on the K75?  What is the service history?

If the bike is backfiring into the intake, the first place I would look is at the intake valves and the timing chain.  I doubt that the crank position or throttle position sensors would have any effect on the problems you are seeing.
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  • In my garage in Marilla, NY
  • '91K100RS White/Blue
Current:
'91 K100RS16V "Moby Brick Too"

Past:
'94 K75RT "Ilsa, She Wolf of the SS"
'92 K100RS16V "Moby Brick" (RIP, deceased in a vehicular assault)
'94 K75S Special Edition Dakar Yellow "Cheetos"
'89 K100RS Special Edition "Special Ed"

Offline Laitch

  • Faster than a speeding pullet
  • Administrator
  • ^ Quintessential Motobricker
  • Posts: 11299
Re: 92 K75S suddenly runs terrible
« Reply #5 on: July 03, 2018, 08:10:29 AM »
Check the little z-tube connecting the crankcase to the air box for a rip in it.
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  • Along the Ridley in Vermont.
  • 1995 K75 89,000 miles
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