Haven't found this particular problem in a quick search through this section, so here we go...

Have a new-to-me 1987 K100RS, 35K miles in extremely pristine condition. The only glitch I can find with it is that when coming to a normal-to-brisk stop, say at an unexpected yellow light, I'll downshift through the gears, and at about 15mph when I pull the clutch in and finish rolling off the throttle, the revs will dip below idle (steady 1000rpm) by a couple of hundred, and either flash the charge light momentarily or actually have the engine die before it can come back up to idle speed. There is a tendency to do this on any load-to-idle situation, but it isn't typically bad enough to cause more than a slight dip below idle except as described above.

Doesn't do this on startup (steady 1000rpm idle hot or cold as mentioned). Doesn't do it on neutral rev/dropback. There doesn't seem to be a negative throttle slack - i.e. if the bike is idling in neutral, and I roll the throttle forcibly closed, the idle doesn't change.

The bike does have a Luftmeister exhaust, but that's the only relevant mod I know of (and that's up for trade in the For Sale section, btw).

Is this a TPS issue, or something else well-known, or does it need more diagnosis?

Tx all!

KeS

Since the TPS on your model isn't terribly fancy in its workings (it's just a simple switch, unlike the later models) simply unplug it to see if anything changes.

I set my TPS low..ie; set correctly by listening to the first click then turn it anti clock to illiminate that first click. You will find it shuts down more positively and you will not lose the top end performance.
As TWB says, pull the plug and see how it behaves.
Early TPSwitches are plentiful and cheap so a replacement here maybe the answer.
Ajays

I'll play with it. It seems to be set correctly now (click as soon as slack is taken out of the throttle), but maybe "correct" isn't what I need. If it's killing the injectors about 2k with the throttle closed, well that explains my problem - on free throttle drop as with the clutch in, it simply drops too fast to avoid dying.

KeS

Kevin, I disconnected the TPS on my 88RT some time back, just pull the plug and leave it. I found it transforms the engine remarkably, removing the overly excessive engine braking effect and making the bike much more pleasant to ride, especially when 2 up into tight corners. Also makes the engine kinder to the splines. There are no adverse effects, just make sure the exhaust is air tight at the header pipe connections as the TPS is there to prevent a backfire under back-off down to 2000 RPM, when the fuel is turned back on.

Will do, I'm going to experiment with it today.

It's kind of odd to think of, since TPS are so sophisticated and critical to injection systems today, that they started as simple mechanical switches.

KeS

Well, I'm damned.

I guess my TPS is working correctly, because when it's disconnected the bike idles down on closed throttle rather than dropping like a brick to 2K rpm.

I like the behavior with the TPS disconnected MUCH better. Am I working around a sub-optimal design here, or am I just masking a different idle drop problem by doing this? And what do people do - just put a swatch of duct tape over the top of the TPS switch plug?

KeS

Kevin
If you want to run disconnected then disconnect it elsewhere for the sake of appearance...ductape indeed..!
If you get a lot of backfire then use my method above.
Ajays

Hi there, check the air/fuel mixture set for idle (AKA CO mixture setting). I had the same problem due to a poor mixture. Here http://skylands.ibmwr.org/tom/tech/co-setting.html is a setting proccedure.
Best regards
JAP

Kevin, no, I don't think it was ever a sub-optimal design, possibly just some safety overkill against a backfire. It has been written that BMW test riders in the early days also disconnected the TPS. I think the TPS is also supposed to richen the mixture at the very top end of the rev range.
To answer your question regarding the idle problem, no, mine wasn't behaving badly prior to disconnection, I merely disconnected it on the advice of others on this forum, to make the bike sweeter ti ride (and it is).

Ajays does seem to be the elegant solution. From experience of both this and carburated bikes with an air cut off valve, if you have an air leak exhaust side, the bike will pop whether or not the system is functioning or blanked off.

I suspect the real reason both systems were used was to allow bikes to pass emissions tests on the overrun, theroetically you should use slightly more fuel with the system disabled - I say theoretically because in terms of fuel economy you won't even notice.

Having it disconnected has actually *reduced* popping a bit, so I imagine I was running a bit lean with the Luftmeister exhaust. But I'll try Ajay's misadjustment next - can't have a BMW with something unplugged on it!

KeS

turning the tps so its out of the effective working position ...is a brilliant idea .....thanks ajs .....

bike feels soo much better and im thinking the drive train components will thank you forever

I doubt it is the TPS, I suggest deal with all the usual suspects, treat all electrical connections with DeoxIT especially taking care of the tank connector and connections to the Hall sensors and engine electronics.

Check the spark plugs have the bobbins on the top and are in good shape, many an owner finds some previous buffoon has not fitted them and the spark has to leak across to the threaded top from the plug connector.

Then check very carefully for air leaks using propane, butane or flammable carb spray. Fix any and all air-leaks you find.

Check and balance the throttle bodies following the procedure exactly rather than just tweaking the screws which seem to say adjust me adjust me.

Finally check and adjust the mixture.

After that lot is done the bike should ride and run like a new one.
Regards,
K-BIKE