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1985 K100 questions...I know zilch and need a bit of input.


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Hi All,

I'm heading to look at an 85 K100 RT. The price is decent, and with 38,000mi it's worth looking at. The only problem is that I don't know what I'll be looking at or for. I'm a decent wrench, but I don't know anything about K bikes. I've looked it up and read a bit but I need some input from those who know...you.


All info would be appreciated as far as what to look for. The only known issue I'm aware of is a big "dip" in the idle when it starts cold. I'm also told that when revved hard in neutral and you let go of the throttle it does the same thing. It dips like it's threatening to shut down, but doesn't. To my non K knowledgable self, I'm thinking a vacum leak, but if there are any known issues please enlighten me.

The idea behind this purchase is to find a bike my girlfriend (experienced Japanese sport bike rider) can ride with me on the longer, touring journeys. Funds are limited, and this seems like a good, cost effective option.

Any thoughts?

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I hope she has long legs. The 85 K's are tall in the saddle. Oh -yes and the 85 K seat is hard as a rock, and unique to that year.


Service records on the spline lub are a must. Other known issues that don't necessarily impact the quality of the ride- the speedo and tach are subject to failtue, the trip meter as well. The early warning lights for low gasoline - yellow one (7 liters) usually stays on even when full, red one (4 liters) works as it is supposed to.


Other than those at 38K it is just getting broken in.

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Dennis Andress

The 85s were kind of one off; in 86 the tank, frame (to accommodate the different tank mounts), seat, side covers, and tail piece were all changed.


Look for:

  • Oil in, or dripping out of the swing arm. This happens when the trany seal leaks and can damage the rubber isolator in the driveshaft.
  • The instrument housing didn't vent very well and would hold moisture causing corrosion on internal electrical connectors. This made the speedo and tach intermitent.
  • There's a crocked little hose between the engine block and the airbox. It's on the left side of the engine, just aft of the throttle bodies. This is the crankcase vent. It deteriorates and sucks air.



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I can't comment on the throttle issue. Certainly never experienced it on mine.


Also sent you a PM with a question.

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Dennis Andress

All K up to 1991 or maybe later for some models had a flapper vane in the air intake as an air mass sensor. This thing caused considerable lag when the throttle was opened suddenly. It was real apparent when done in neutral.



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Thanks. I was reading some about that thing earlier. Is there a way to check it that you know of? Short of replacing it that is.

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Dennis Andress

About all you can do to check it is to push on the vane with your finger to make sure it moves freely. It's in the top half of the airbox. Gaining access wasn't too had, take off the tank, losen an air tube clamp, and wiggle the top of the airbox out -- careful of the wire bundle running into the airbox top and connected to the sensor.


But, the problem is not that the vane gets sticky, it's that the mass of the vane makes for a sluggish design. They all have this hesitation, and they did it from the day they left the factory.

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hmmmmmmm good to know. So it impedes airflow? Nobody has come up with a new improved thingamawidget to solve the issue? Still, if that's the problem (seems like a good place to start) factory spec is fine with me. Thanks

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Dennis Andress

Impedes is a good word.


The Throttle Position Switch is on the aft end of the throttle bodies. I think it has two switches, one is closed when the throttles are closed, the other is closed when they are wide open. When adjusted correctly you should be able to hear a soft "click" from it when the throttle is slightly cracked open. It should click again when it's closed. The fast idle "choke" lever is a handy way to check this.


The fuel injectors are shut off when the throttle is closed above 1700 RPM. You can feel this by closing the throttle in third gear at somewhere around 3000 RPM. There should be considerable engine braking until 1700 RPM at which point the injectors will come back on and you should feel a little kick of power. Can you guess what controls this? The switch in the TPS.




When you take the tank off there is often a bit of residual pressure in the fuel lines. This means you'll get sprayed with gas when you remove the hose. The #1 throttle body should have a vacuum hose connected to it which routes to the fuel pressure regulator. Sucking on this hose will open the regulator and relieve the pressure.



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All good info. Thank you. Well, she bought it (she didn;t want to but the evil little voice that she lives with sat on her shoulder and said get, do it, you know you wanna and she did!)so we loaded it up.


The test ride was fine, no problems. Cold motor cranked right up and purred, that dip was there, but it was slight and I felt like we could overcome it with what I've learned here from you.


We rode through a wicked rain storm on the way home with it in the back of the truck and when we unloaded it and went to crank it, it protested. A battery charger fixed that and it fired right back up afterward...except for once.


At this point she was glaring at the little wicked voice (uh, me) and saying not much.


At low idle I'm seeing 12v a rev will run it up to 13.7 or so. I'm thinkin this isn't enough? and might even explain the slight miss at low rpm?


Anyway, she paid very little for it and I don;t think she was hurt yet. Neither does she.

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Dennis Andress

Welcome to my world. My wife and I both ride K12RSs. Buying parts means getting two of everything.


12 volts at idle is pretty good. I would take it to mean the battery is in good health. 13.7 is a good charging voltage; I wouldn't want to see anything higher. If it became hard to start after getting wet I'd look for corroded electrical connectors and battery terminals. The early Bosch injection systems respond very well to simply opening every electrical connector and spraying each one with contact cleaner.


The headlight should go out while the starter is running. There is a relay that controls this. It has 12 volts on one side and a ground on the other. The ground goes through the starter motor. It's kind of weird but the windings of the motor are ~ 0 ohms when it's not running but an open when it is. Worn starter brushes cause the headlight to flicker and/or not come back on. New brushes are absurdly cheap...


You might want to replace the fuel filter. It's in the tank. You gain access to it by taking the gas cap off.


If the bike's been sitting for awhile the fuel injectors may be gummed up. 1/2 a bottle of good injector cleaner and a good ride will help.

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Thanks. I'll be checking all of these things in the next few days.


I'm aaaallllll for her having a touring bike in addition to her sport bike. It keeps her off my back seat on the long ones :) ahhhh

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My K will turn over 178,000k this week. (Original owner, bought new in '86. Original color was the Maddison Silver, repainted at 100k.) Yes, you will run into some "issues", yet with the relatively low mileage, should be a fine ride for many years/miles. How much denaro did this "new" machine cost?

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