Laitch wrote:This is the same problem that you described in this post, correct?

Point-Seven-five wrote:Sounds like you might have a bad engine coolant sensor.  Fairly common problem on these old beasts.  Makes the engine run too rich when it's warmed up.

Should be around 250 ohms between pins 10 and 13 on the big connector on the Jetronic control unit when the engine is at operating temperature.

The cause can be a bad sensor or a dirty/corroded connection.

Laitch wrote:If you have not replaced the fuel filter and and also replaced the internal tank fuel lines with J30R10 or equivalent submersible fuel injection lines, now might be the time to do it. A clogged filter and/or internally collapsing fuel lines can block sufficient fuel to keep the engine running. Fuel lines leaking from their clamped joints with pipes in the tank can also interfere with performance.

Blowing air through a clogged filter will be difficult so that is a test you can try.

letsgetpractical wrote:It will not idle when the bike is warmed up.

Dai wrote:1985... is this an 8-valve or a 16-valve engine? '85 was a changeover year so saying a bike is a 1985 K100RT can be misleading. Also, it may not have started life as an RT (mine didn't). Reason for asking: if it's an 8-valve engine then you could be on to something. If it is 8-valve, look for a purple/white wire (probably look more like shitty purple/light brown) at the rear of the throttle bodies and pull it off the vacuum switch. BMW deprecated the vacuum switch because it caused idling problems - I'm not sure when, so you may not have one. If that's not the problem just tie the wire back - don't bother reconnecting it to the vacuum switch.

Both engines: if you're thinking of the TPS (throttle position switch), that's on the r/h side of the throttle body bank. Loosen the two screws just enough so that you can rotate the TPS housing. You'll need a quiet environment. Rotate the switch away from you (clockwise) very slowly and gently. You're listening for a tiny, almost inaudible click as a microswitch engages. If you don't hear it within about ten degrees of rotation, rotate back towards you (anti-clockwise). When you hear the click, rotate clockwise a tiny amount (you may even hear another click). What you have just done is to effectively turn the TPS sensor off, so it will be removed from the problem-solving process. Downside; if you've gotten used to the ignition system switching off and using the engine for braking when slowing down, it's now going to behave like any non-BMW bike and the engine braking won't be anywhere near as much. You get used to it. Upside: it makes cornering and roundabouts much easier because the ignition system isn't switching on and off with the throttle being opened and closed.

Dai wrote:1985... is this an 8-valve or a 16-valve engine? '85 was a changeover year so saying a bike is a 1985 K100RT can be misleading. Also, it may not have started life as an RT (mine didn't). Reason for asking: if it's an 8-valve engine then you could be on to something. If it is 8-valve, look for a purple/white wire (probably look more like shitty purple/light brown) at the rear of the throttle bodies and pull it off the vacuum switch. BMW deprecated the vacuum switch because it caused idling problems - I'm not sure when, so you may not have one. If that's not the problem just tie the wire back - don't bother reconnecting it to the vacuum switch.

Both engines: if you're thinking of the TPS (throttle position switch), that's on the r/h side of the throttle body bank. Loosen the two screws just enough so that you can rotate the TPS housing. You'll need a quiet environment. Rotate the switch away from you (clockwise) very slowly and gently. You're listening for a tiny, almost inaudible click as a microswitch engages. If you don't hear it within about ten degrees of rotation, rotate back towards you (anti-clockwise). When you hear the click, rotate clockwise a tiny amount (you may even hear another click). What you have just done is to effectively turn the TPS sensor off, so it will be removed from the problem-solving process. Downside; if you've gotten used to the ignition system switching off and using the engine for braking when slowing down, it's now going to behave like any non-BMW bike and the engine braking won't be anywhere near as much. You get used to it. Upside: it makes cornering and roundabouts much easier because the ignition system isn't switching on and off with the throttle being opened and closed.  I looked for the vacuum switch wire and switch and didnt see any.  Is that visible without removing anything?  I believe I have an 8 valve bike but it was sold in Germany and brought over by a US soldier there at the time, so it may be configured differently and not have the vacuum switch?  I rotated the TPS until i heard a click like you said and then tightened it down.  Didnt make any difference.....

Dai wrote:Post a pic of the l/h side of the engine. We're flying in the dark ATM.

daveyson wrote:Because yours is an early '85 model, it should have the vacuum switch, which gets vacuum, as Olaf states, from throttle body one. I'm thinking a vacuum leak here would be less catastrophic than a normal vacuum leak, for example from the fuel pressure regulator. I'm only thinking that because I assume the vacuum switch serves the same purpose as the vacuum timing advance in the carby days. Remember in the carby days the distributor had a mechanical advance and a vacuum advance. The timing was advanced under vacuum to allow for low load situations which mainly come into play at mid throttle ranges, so shouldn't have an impact at idle, unless of course the hose is split or has slipped off its port. Remember this port was typically above the throttle valve, so the vacuum at this point is lower than usual.

The hose is connected to the left of the vacuum plug at throttle body one. The vacuum switch itself you can see from the right side of your brick, between the radiator and the air filter box.

Hopefully your hose isn't kinked and split like mine is.

letsgetpractical wrote:

daveyson wrote:Because yours is an early '85 model, it should have the vacuum switch, which gets vacuum, as Olaf states, from throttle body one. I'm thinking a vacuum leak here would be less catastrophic than a normal vacuum leak, for example from the fuel pressure regulator. I'm only thinking that because I assume the vacuum switch serves the same purpose as the vacuum timing advance in the carby days. Remember in the carby days the distributor had a mechanical advance and a vacuum advance. The timing was advanced under vacuum to allow for low load situations which mainly come into play at mid throttle ranges, so shouldn't have an impact at idle, unless of course the hose is split or has slipped off its port. Remember this port was typically above the throttle valve, so the vacuum at this point is lower than usual.

The hose is connected to the left of the vacuum plug at throttle body one. The vacuum switch itself you can see from the right side of your brick, between the radiator and the air filter box.

Hopefully your hose isn't kinked and split like mine is.

thanks, I will check that out!

daveyson wrote:Good idea, a very quick way to eliminate it as a cause of the problem.

I was mainly just aiming to answer the question as to where the switch is.

Sometimes I think I should test this one day, which I did once with a test light, but I should do a better test to see if it really does advance the timing. I'm pretty sure it will, even with the split in the hose, since I think I remember that the switch did send a signal to the ignition control unit when held at mid rev ranges, but not at idle. I did post the results somewhere but couldn't find it back.

92KK 84WW Olaf wrote:

letsgetpractical wrote:

daveyson wrote:Because yours is an early '85 model, it should have the vacuum switch, which gets vacuum, as Olaf states, from throttle body one. I'm thinking a vacuum leak here would be less catastrophic than a normal vacuum leak, for example from the fuel pressure regulator. I'm only thinking that because I assume the vacuum switch serves the same purpose as the vacuum timing advance in the carby days. Remember in the carby days the distributor had a mechanical advance and a vacuum advance. The timing was advanced under vacuum to allow for low load situations which mainly come into play at mid throttle ranges, so shouldn't have an impact at idle, unless of course the hose is split or has slipped off its port. Remember this port was typically above the throttle valve, so the vacuum at this point is lower than usual.

The hose is connected to the left of the vacuum plug at throttle body one. The vacuum switch itself you can see from the right side of your brick, between the radiator and the air filter box.

Hopefully your hose isn't kinked and split like mine is.

thanks, I will check that out!

As said, it will run fine without it. All I was suggesting is disconnect the hose at the throttle body and put a vacuum cap on instead, that way your vacuum is unaffected. I admit on 2 of my Ks I have kept that vacuum switch connected and doing its thing but they work fine without it.

letsgetpractical wrote:

92KK 84WW Olaf wrote:

letsgetpractical wrote:
thanks, I will check that out!

As said, it will run fine without it. All I was suggesting is disconnect the hose at the throttle body and put a vacuum cap on instead, that way your vacuum is unaffected. I admit on 2 of my Ks I have kept that vacuum switch connected and doing its thing but they work fine without it.

I have looked on my bike and do not see a hose on the left side of the throttle body, which is I assume is where throttle body one is.  When I looked for vacuum switch I saw 2 cut wires, one purple like color which is the wire for the vacuum switch?  Also on throttle body one there is just one capped vacuum port, is there to be another port somewhere for this vacuum switch?

letsgetpractical wrote:Also on the right side of the throttle bodies there is a vacuum hose going out of a nipple on the farthest right throttle body.   Would that be the vacuum hose going to the vacuum switch, if not, where does that go?

92KK 84WW Olaf wrote:If you have the vacuum switch, or had it, there are 2 vacuum points on no 1 throttle body.

1-4 have them in identical positions, on no 4 it goes to the fuel pressure regulator using a pipe instead of having a cap. 1-3 should have vacuum caps. On no 1 the second one was a pipe which went to the vacuum sensor. This vacuum pipe is very prone to cracking and will cause you problems so just use a vacuum cap instead.