Hey gang!
    Thanks to all your help, I have my 1985 K100 RS running incredibly well. It seems pretty much perfect at this point. There's just one thing I'm wondering about:
    When I release the throttle, the transition from engine powering to engine braking seems abrupt and strong. If I let the throttle off as slowly as I can, engine power eases off just as I would expect right up until the end, when it suddenly jumps to very strong engine braking (enough to make the front end dive if I do it in 1st). Is this perhaps an actual problem, or just a case of me needing to get used to how this engine works?
    The transition in the other direction (beginning to apply throttle) is also sharp, but I can manage it if I'm very careful with twisting the throttle.
    Also, the bike idles a little high (1050-1100). My thinking is that that's not causing the problem. If anything, it seems like that might have been an attempt on the part of the previous owner to ease the power-to-braking transition. Am I wrong? Could that be causing an issue?
    Overall, it's not really a big deal. I think mostly I'm noticing this because I'm used to a 76-year-old, softly tuned, 13HP 350cc engine. So the K100 engine feels REALLY strong to me. But now that I've fixed all the real problems, I'm free to obsess about things that probably aren't actually problems.
    Thanks again for all your help!
    -Jon

Sounds like you need to disable the TPS switch.

Yeah, thanks for the tip! I just read a bunch of posts about disabling the TPS. Sounds like that's just what I'm looking for!

I also thought my K idled too high, but read somewhere that it needs to be idling over 1000rpm to supply enough current to the computer. I have heard different since, but I'm sure others will chime in.

Clymer manual says 900-1000rpm. I'll get around to adjusting it one of these days, but it's such a trivial problem that I haven't bothered yet.
  My to-do list started with items like 'Replace Hall Sensor', 'Replace Fuel Pump', 'Replace Final Drive Ring Gear Seal'. Then it progressed to items like 'Upgrade Radiator Fan', 'Install Headlight Relay', and 'Grease Clutch Splines'. Now it's down to things like 'Replace Grips', 'Calibrate Speedometer', and 'New Sidestand bushing'.
  So, yeah, the idle adjustment is coming soon. really.

to adjust the tps   ....undo the two retaining screws (I have changed mine to allen head types rather than the Philips screws far easier to adjust whilst on the bike )
and rotate the tps anti clockwise .... looking from the coil covers
just enough so that you no longer here the click as the throttle is closed .......works for me
but give it a try ..
good luck

The fuel cutoff gven by the TPS would probably feel more abrupt with a high idle. Likely feel a whole lot different with idle and TPS set correctly. 

I know that my RT doesn't feel like a sudden cut, either decelerating or accelerating with a completely functional TPS. The transition is barely noticable.

Al

That's an interesting observation/comment Al. Can't say that I've made that connection myself before, but it certainly sounds like a very plausible explanation.
One thought I've had is that this abrupt change could be due in part to FD spline wear which creates free movement between the input and output splines.

There's already some play in the drive train even with brand new FD splines. I doubt FD spline wear would be noticeable when riding.

Would someone give a brief explanation of the TPS switch please.
Shep

charlie99 wrote:to adjust the tps   ....undo the two retaining screws (I have changed mine to allen head types rather than the Philips screws far easier to adjust whilst on the bike )
and rotate the tps anti clockwise .... looking from the coil covers
just enough so that you no longer here the click as the throttle is closed .......works for me
but give it a try ..
good luck

Spot on Charlie....if you have to have a functioning TPS. Personally I disconnected mine.

KKlompy

Shep wrote:Would someone give a brief explanation of the TPS switch please.
Shep

Throttle Position Sensor. On a K100/75 2V it gives a signal to the Engine Control Unit of the position of the throttle but only closed and wide open where on later models it gives feedback through out the full range of throttle movement.

TPS signal functions:

When the throttle is let closed while travelling, the fuel injection is turned off until the rpm drops below about 2000.  aka closed-throttle overrun, prevents popping of excess fuel in the exhaust.

When the throttle is opened off of the TPS closed position, the ignition timing is advanced, to provide low-end torque and power to get moving.

At WOT, wide open throttle, the fuel injection goes richer and the ignition timing gets advanced for maximum power.

(enough to make the front end dive if I do it in 1st).
Could that be an effect of the shaft drive system?

to me the rt ..lt suffers the worse for the tps function
perhaps it is to do with the riding position and the weight and travel of the front suspension
the k75 front suspension seems to have a much lower travel and different springing and damping characteristics

just some things I have noticed along the path

RedRT wrote:(enough to make the front end dive if I do it in 1st).
Could that be an effect of the shaft drive system?

Best thing to do is simply adjust it so its not functioning. 2min job.

1. Undo the two screws either side. You'll need a short screwdriver or a rachetty phillips head drive thingy.
   
2. Turn TPS counterclockwise till you here the micro switch click.
   
3. Tighten screws.
   

When its working properly you should be able to hear it click juuuust as you open the throttle.

I don't seem to have that 'diving' problem that others have reported. Maybe my front susp. is stiffer. Either way, I prefer the TPS working. Better engine braking.

TPS Report:
    I adjusted the TPS to be inactive at the idle end. I used exactly the technique that Holister described. Super easy job and it made a world of difference, just as others have reported. A MUCH smoother transition both letting the throttle off and bringing it back on. There's still ample engine braking. There's still feedback from the bike when the engine switches from power to braking. But it's a nudge instead of a jolt. It's perfect. And I don't look like a complete lurching idiot trying to go around a low-speed corner anymore, which is nice.
   That's all pretty much what I expected based on reading other threads on this topic. Here's the part I didn't expect: My high-idle problem is gone. The bike is idling perfectly about a needle's width below 1000rpm. I had noticed before that the throttle body control arm was never touching the idle adjustment screw. I figured that the pivot was sticky, or the throttle cable had issues (even though it felt dead smooth), or something like that. It was the TPS. I'm not exactly sure how, but it was preventing the throttle from being able to back off all the way to the idle screw. Now it's fixed. Yay!