87 k75s throttle lag - K-Bikes.com - Excellence in Motion
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post #1 of 3 (permalink) Old May 30th, 2016, 7:20 am Thread Starter
Join Date: May 2016
Location: Ottawa, Ontario, Canada
Posts: 1
87 k75s throttle lag

Im having some challenges with my 87 k75s, when I snap the throttle quickly at idle it doesn't respond, if i roll back on the throttle quickly but steadily its much better. I have already cleaned up the coils and changed the plug ends,the coil ends and leads for this bike are not easy to come by.
Any suggestions ?
Thank you,
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post #2 of 3 (permalink) Old May 30th, 2016, 11:01 pm
H96669's Avatar
Join Date: Jun 2009
Location: Burton, British Columbia, Canada
Posts: 1,650
Vacuum leak?Sounds like it.I think them have a vacuum activated fuel pressure regulator.Hard to see behind the throttle bodies/under the airbox.But well known to fail them old BMW rubber bits/hoses.

BeemerBoneyard does sell plug wires for the K75.

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post #3 of 3 (permalink) Old Jun 2nd, 2016, 12:06 am
Join Date: Sep 2008
Location: Honolulu, HI, USA
Posts: 79
I think vacuum leak too, is messing up your fuel mix. You didn't say if the hesitation existed when cranking the throttle from other than at idle speed. Rev to 2000 and yank the throttle, is there the same hesitation? Vacuum leaks are most significant at or near idle rpm, so if the hesitation is less from midrange, looking for a leak may be where you're going.

Ljet fuel injection is sensitive to unmetered air such as from a vacuum leak (other injection systems are too, but I'm most familiar with Ljet). Things to check, based on my 12 years experience with my '85 K100, and interpreting to the three cylinder K75:

1. The Z-shaped hose behind the throttle bodies and airbox, that vents the crankcase into the back of the airbox. It gets cracked with age and you may not see it until you remove it. Pay close attention to the size at each end, and the orientation, when reinstalling, and clamp lightly.
2. The nipples over the #1 and #2 cylinder throttle body (TB) vacuum connections. The tap on on the back TB has a hose running to a dark place where your fuel pressure regulator lives. On my K100RS, removing the hose from the back TB and plugging the resulting TB vacuum leak increases fuel pressure and idle speed. The other two TBs should have intact rubber caps on them. They're readily available at my local hardware store in various sizes.
3. Make sure the vacuum hose from TB for cylinder #3 and going to the fuel pressure regulator doesn't have a leak. You have to go in from the right side, remove the air filter and box, so see the FPR. Bit of a pain but needful if you're chasing vacuum leaks.
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