Hey all thanks for taking a look to help a noob like me out.
My k100 idles normal at start, but progressively idles higher and higher, to what i dont know as i currently only leave for short rides to shops but the opportunity to commute is fast approaching.
my inexperienced gut is telling me its the thermostat, dno what exactly but as it gets warmer the revs get higher. Unfortunatly i dont have an rpm gauge on the bike so i can only measure by ear. I cant ignore however this all started at the MOT when the tech pointed out that the throttle revved up when handlebars at full turn, she then yanked the throttle cable to demonstrate which is when i first heard it idle at such an intense anger (since tried to unyank to no avail) however since it starts normal and progressively gets higher i think this might be a red herring.
I know i need to replace thermostat and the throttle cable, but i cant afford to buy both parts... There are other things that need replacement but they more obvious. All in good time but the most pressing matters first.
I have checked my coolant, seems to be fine, i cant fill it, and thanks to advice from this forum it was flushed Oct/Nov ish, the MoT was in Jan so it is persistent (if this helps).
Im sorry to ramble on just trying to give the chaps who know more information to work with. Thank you for your time and help
P.s. If requested i may upload video to youtube of the idle differences however having never uploaded video, or anything more than a pic to facebook this endeavour may be the straw that breaks my sanity...

Has the engine always done this, or is it som ething that has just started recently? Is this a new bike to you? If the engine starts readily and then doesn't act like it's running rich(stumbling, sluggish) then I would say the temperature sensor is probably okay.

It could be a bad throttle cable. First, does the throttle snap closed when you let it go from wide open? Is there a throttle lock screw on the bottom of the right side switch assembly. Make sure it isn't holding the throttle open. Is there any looseness in the throttle grip after you open it wide open and then close it manually? That would indicate that the cable might be dirty and hanging in the cover. A pinched or kinked throttle cable can also hang up.

Second, is the linkage at the throttle bodies hanging up? If you push the throttle bodies open will they snap closed from their springs, or do they hang up? A hose clamp, wire, or cable tie can get in the way of the throttle linkage preventing it from closing completely.

Last, check for air leaks. The crankcase breather Z tube may be cracked and causing a lean idle that causes the engine to run faster at idle, and stumble badly or stall when the throttle is opened. Another air leak is around the boots above and below the throttle assemblies and where they attach to the cylinder head.

In addition to what .75 said, the TPS switch on the end of the throttle bodies can also cause hang ups if it is maladjusted.  To test, loosen the screws holding the TPS  and see if any hang up in the way the throttle snaps back to close disappears.  If it does disappear, carefully adjust the TPS switch so that it has proper operation but is centred on the shaft.

If the idle rises by itself after 30 seconds to a minute from a cold engine start (but you feel that there is no problem with the throttle operation), then it might be the fuel mixture is upset (too lean) because of an air leak.  The cold start circuit in the ECU enrichens the mixture for a minute after the engine starts from a cold condition but reduces the fueling once the engine is warm.  Maybe the ECU is trying to deliver the correct amount of fuel but an air leak in the intake system is leaning out the mixture.  Check for leaks in the usual places as pointed out by .75 - Z-tube, intake bushes, air accumulator, MAF, etc.

Welshy88 wrote: I cant ignore however this all started at the MOT when the tech pointed out that the throttle revved up when handlebars at full turn, she then yanked the throttle cable to demonstrate which is when i first heard it idle at such an intense anger (since tried to unyank to no avail) however since it starts normal and progressively gets higher i think this might be a red herring.

The cable needs to be checked that it's in the correct seated position at both ends, an incorrectly seated cable will make it idle a lot higher. The throttle should always snap back to fully closed make sure the grips aren't fouling and the throttle gears are adequately lubricated with light grease. If it was revving up on full lock, the cable needs to be checked for correct routing, the revs should not rise when on full lock. Once done the idle needs to be set with the bike at the correct running temperature. When cold the starting revs can be raised by advancing the choke to the first notch, once warm the choke can be pushed back and it should idle at the correct RPM.
Regards Martin.

My k100 idles normal at start, but progressively idles higher and higher . . . it gets warmer the revs get higher. 
I have checked my coolant, seems to be fine, i cant fill it, 

When you've completed all the task previously mentioned, check and measure the increased starting speed clearances according to the instructions below. The MOT inspector might have affected those clearances during the yanking tantrum.




What does "i cant fill it" mean—the coolant reservoir is filled to the brim or you cannot open it to fill it to the correct level?

Before you replace the thermostat, drop it into a pot of cold water that has a thermometer in it, bring the water to boil and as it heats note the temperature that opens the thermostat. Never mind your inexperienced gut.

Have you ever heard the fan run?

Ok i will try an answer questions as accurately as possible:
The throttle twister springs back fine, all the way back when you let go, i dont know what a throttle body is, i fear its internal? Or the TPS i dont know wht that is abbreviated for.
My air intake tube was loose so i will check youtube for hw to find leaks.
Its difficult to measure the exact time it takes to run up to high revs and my neighbours complain if i let it sit running, but about a minute into riding i notice the rise in idle. The revs are like its new idle limit, the throttle on my end will be completely closed (all the way back).
Does the thermostat require power? I can pop it in a pot n boil it if its function happens without power, i dont have garage or many tools, my neighbours dont like me running the bike im sure theyd have a fit if i started a campfire on the side of the road lol. The coolant is filled to the brim and i checked yesters, all coolant pipes where warm to touch after quick ride.
I have never heard the fan, i got a loud exhaust, my next trip to shops i will leave it run long enough to get off n look at the fan.
Hmm its probably throttle cable, my brain is started to frazzle but i dont know where the cable connects to the metal bit it controls, i fear i need to take things off to get access. Wait is the throttle body these things that move near the fuel injectors (only know what these are thanks to AA testing them a while back) they move up n down fine (kinda rotational up n down) but return flat upon letting go, have not mesed with such screws unless its entirety unavoidable.
I have had the bike over a year, this problem started since the yanking at the MoT in Jan, 2 months ago... Wasnt really a problem in the cold though but now its getting warm nd i am gna be opening up soon.
I want to thank you all for your replies and instruction, its deeply appreciated. I had planned to make a sig while writing op to explain myself without having to type it each time but here we go....
I am Dan, Son of the Welsh, aka the noob, aka the guy who bought a custom, yes bought not built a custom , aka the guy trying to pick up more than basic maintenance for the first time with a bmw K100 (oh and my idea of basic is probably what you guys just call cleaning), aka the killer of bikes, aka the nutter.
I been riding on the road since i was 17 and offroad since i was 5, the beemer is my first litre bike, i love to ride, i have never sold a bike (well not a working one) i have a strange pact with my bikes, i will kill them or they will kill me, and some have made a damn good attempt on the latter but i think i have found a stalemate here.
Again thank you all for your advice and expertise, for me i must scower youtube, i dont think lubing a replacement cable in vasoline then duct taping it to one end of the original n pulling the other is the appropriate method for this one. I will continue to check back here, i am a frequent lurker but only speak if i know something or when my query is not already addresse and i really dont know much of anything yet.
Stay safe n take care

It sounds like you have a cafe bike, is that correct?  The problem with helping on these things is that the builder is really the only living person who actually knows how everything works on it.

First off, can you post any clear photos of the bike, especially of the left side above the valve cover where the throttle bodies are located.  The right side showing what ever the builder did to the air intake might be useful as well.  Can you take any pictures of where the throttle cable is routed?

As far as your question about the TPS, that is the abbreviation for Throttle Position Switch or Sensor(depends on what engine management system your bike has.  The TPS is an approximate rectangular black plastic thingie with a cable attached to it, located at the rear end of the shaft of the last throttle body.  Depending on your bike, it has either a couple of switches that are activated when the throttles are wide open or fully closed, or a variable resistor that sends a signal telling the engine control unit(ECU) how wide the throttle is open so the ECU knows how much fuel to inject.

I'm kind of thinking you have a modified exhaust and possibly a different air inlet configuration(depending on how ambitious the builder was),  and that the builder probably didn't retune the bike for these changes.  What you may have is just a normal response to the changes in the flow going into and out of the engine, possibly amplified by a worn throttle cable. It's not that unusual for the idle to rise slightly a minute or two after a cold engine starts, and then stay there for the rest of the ride.

Forget your thermostat hypothesis, that's not the point.
First, fit a genuine exhaust. Yes it's ugly but you won't have any issues with it as it is designed to match the engine specs.
Then, check the valves clearance. You can't tune an engine if that is not properly adjusted.
Then check there' s no air leak in the manifold air intake. These gum pipes are 40 years old and became solid or cracked. The best way to check it is to dismantle the air box and throttle bodies from the engine.
That will allow you to accurately synchronize the throttle bodies on the desk, using 4 calibrated rods.
Then, it should be very simple to synchronize and set idle only adjusting the bypass air screws.
Most of - all- bikes looking like yours were "made" from a cheap poorly maintened bike by people who have no idea how a K100 works, and basic operations are missing.