Join Date: Sep 2008
Location: Honolulu, HI, USA
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.