1986 BMW K100 "U"

Introduction: This is a 68K K100 I picked up at the end of June, 2013 for $750.  It was originally a K100RS but the previous owner took it for a low speed low side on some sand and rashed up the right side of the fairing.  He was selling it for a low price because he could only get it to run for a few seconds at a time by spraying starter fluid into the air intake. 

While he was helping load it onto my trailer, he heard a clanging in the tank.  I thought to myself "It can't be THAT easy to fix." but, sure enough, when I got it home I pulled off the gas cap and discovered that someone had left off one of the hose clamps for the fuel filter and the hose had blown off.  Five minutes and a hose clamp later and it fired right up!

Here are the pictures from the Craig's List ad:

Since the upper and right side fairings were rashed and I already have a couple of RS-faired Ks, I decided to install a K75C fairing that I had leftover from one of the iterations of my K75 Frankenbrick. Not the best fairing for touring but I wanted something small and light as I intended to have this be a "city" bike for running errands and such. I decided to name it the K100U.  Not sure exactly what the U stands for: utility, ugly, unconventional?  It really doesn't matter to me.

I'd originally planned on servicing it, riding it for a bit and then flipping it but, as I worked on it, it kind of grew on me so I decided to keep it for a while.  Aside from the bodywork, I also did a couple of things to make it a better rider.  Namely installing 3 spoke wheels with good radial tires and swapping in a 32/10 K75 final drive so it's faster, having about 13.5% more torque than the 31/11 final drive it had originally.

What it looks like now:

Modifications & Maintenance

The summary table below lists the modifications and maintenance I have done to this bike (that I can remember, I did a lot to this bike) and is followed by a description of each.

Engine/Fuel System
  Engine Flush
  Synthetic Oil & Bosch Oil Filter
  Adjusted Valves
  Aftermarket Exhaust
  Fuel and Air Filters
  Redline Water Wetter
  Crank Case Breather Hose
  NGK Iridium Spark Plugs
  Magnecor Competition Spark Plug Leads
  Newer Ignition Coils
  New Fuel Lines
  Reconditioned Fuel Injectors
  New Vacuum Caps & Throttle Body Sync
  L-Jetronic Mud Cover

Controls
  K75C Handlebars
  Firmed Up Handlebar Clamps
  Manic Salamander Ceramic Coated Bar Ends

  ProGrip Gel Grips

  Heated Grips

  LED Lit Instrument Cluster

  12 Hour Clock
  Calibrated Speedometer
  Fuel Gauge

  Altitude Switch
  LED Backlight Dash Switches
  Flip-a-Lever Throttle Lock
  Real Cruise Control
  Dual Fiamm Horns

Drive Train
  New Clutch Nut O-ring
  Clutch Plate
  K75 32/10 Final Drive
  Synthetic Gear Oils w/ Moly Additive

Suspension/Tires/Brakes
  "S" Forks w/ Progressive Springs
  Fork Boots
  Ohlins Shock
  Stainless/Metal Front Brake Lines
  Sintered Brake Pads
  Vented Rear Rotor
  3-spoke Wheels w/ Radial Tires

Lighting/Electrical
  Lightweight AGM Battery

  Upgraded Battery Ground Cable

  15K 50A Alternator
  8K Starter
  90/100W Headlight
  LED Brake/Tail/Turn Signals
  Upgraded LED Tail Lighting & Brake Light Strobe
  LED License Plate Lights
  Auxiliary Fuse Block
  Three Powerlet Sockets
  10W Fork Mounted LED Aux. Lights
  PIAA 1100 Driving Lights

Body/Cosmetic
  K75C Headlight/Fairing w/ Laminar Lip
  K75S Belly Pan

  Standard K100 Radiator Shroud
  Chromex Ceramic Engine Covers
  Knee Pads

  K1100 Peg Plates
  Front & Rear Fender Extenders

  Tail Cowl Protectors

  New License Plate Frame

  Added Layer Of Insulation

Luggage/Seat
  Key-Matched Gen 2 OEM Side Cases
  EMGO Top Case on Givi Rack
  Leather Corbin Dual Tour Seat

Engine & Fuel System

Engine Flush: One of the issues that crops up with old K bikes is that oil sludge builds up in the one-way gearing for the starter (a.k.a. starter sprag) which can cause the gears not to engage so that the starter spins without turning the motor.  As preventative maintenance to reduce the chance of this occurring and to get the engine internals clean of any built up sludge since it had been sitting for three years, I ran some flushing oil in the motor prior to changing the oil.

Synthetic Oil & Bosch Oil Filter: I installed a new Bosch premium oil filter with Shell Rotella T6 5-40W full synthetic motor oil.

Valves: I checked the intake and exhaust valve clearances. Two of the eight were slightly out of spec so I reshimmed those.

Exhaust: It came with an aftermarket stainless exhaust made by Motad in The UK.  I think the stock K100 exhausts are rather ugly and this one sounds good (and is pretty quiet) so I decided to keep that on the bike.

Fuel and Air Filters: I replaced both the fuel and air filters with new ones.

Red Line Water Wetter: The coolant has Red Line Water Wetter additive in it as that improves the heat transfer properties of the coolant.

Crank Case Breather Hose: The rubber of the crank case breather hose was old and cracked so I replaced it with a new one.

NGK Spark Plugs: I installed new NGK DR7EIX iridium spark plugs gapped at 0.024".

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Spark Plug Leads: I replaced the spark plug leads with new 8.5mm Magnecor KV85 Competition spark plug leads.

Ignition Coils: I replaced the ignition coils with some newer, lower mile ones  that I scored on eBay for cheap. (from a 1991 K100LT with 40K miles)

New Fuel Lines: I replaced all of the 27 year old fuel lines, both the external ones (with 5/16" FI-rated hose instead of the stock 8mm) and the ones internal to the gas tank with the brand new submersible fuel lines from BMW.  I also replaced the anti-cavitation line from the bottom of the fuel pump.

"The Cup:" Replaced the two drain lines on the right rear of the fuel tank with "The Cup" (a.k.a. air accumulator) to a single drain line that comes out at the rear of the right peg plate.

Reconditioned Fuel Injectors: The bike ran fine with its original fuel injectors however, as preventative maintenance, I had them professionally refurb'd to optimize fuel system performance.

New Vacuum Caps & Throttle Body Sync: I replaced the throttle body vacuum caps with brand new ones from BMW.  While I was at it, I also replaced the vacuum hose from the throttle bodies to the fuel pressure regulator with brand new vacuum hose.

Once I'd replaced the spark plugs and other fuel system components with new parts, I sync'd the throttle bodies with my CarbTune for optimum performance and smoothness.

L-Jetronic Mud Guard: In later years BMW cheaped out and did not include a mud cover for the fuel injection computer connection.

Controls

K75C Handlebars: Although I generally prefer S/RS handlebars for spirited riding, I installed C bars on this bike since I'll mostly be using for local riding and errands.

Firmed Up Handlebar Clamps: K100 handlebar clamps are rubber mounted in the upper fork tree.  These handlebars felt a little mushy so I firmed them up by adding 1/8" thick bronze thrust washers under the top and bottom washers.

Ceramic Coated 18 oz. Manic Salamander Bar Ends: Once I went through the whole bike, I got it running almost as smoothly as a K75 so it doesn't really need bar ends but I had these lying around and like the looks of therm.

ProGrip 699 Gel Grips: I've never liked the hard rubber BMW grips so I cut them off and replaced them with these that I run on all of my other K bikes too.

Heated Grips: Before putting the grips on I installed some Symtec heated grip elements controlled by a BMW OEM dash switch.

LED Instrument Cluster: All of the old filament bulbs in the instrument cluster have been replaced with LEDs. I usually do green LED backlighting since that seems to work best in these clusters but decided to backlight this bike's cluster with violet/purple LEDs. I'd include a picture but the LED lighting doesn't come out very well on my camera.

12 Hour Clock: K bikes came with a "Euro" 24-hour LCD clock in the instrument cluster.  Being American, I've never liked this.  The clock in this bike's instrument cluster has been modified to function as a 12-hour clock.

Calibrated Speedometer: For whatever reason, most K bike speedometers run about 10% fast from the factory.  I recalibrated this bike's speedometer to make it accurate.

Fuel Gauge: I installed a VDO fuel gauge on the handlebars - with violet LED backlighting to match the instrument cluster backlighting.

Altitude Switch: The US models of K bikes have an "altitude plug" to insert in the wiring harness when riding above an elevation of 4,000 feet.  The plug tells the fuel injection control unit to adjust the mixture to compensate for the thinner air found at higher altitudes.  Since it seems kind of silly to stop and get off of the bike to fiddle with the altitude plug every time you pass through 4,000 feet, I installed a simple BMW OEM on/off switch in the dash pad so all I need to do is turn it on or off when I pass through 4,000 feet.

LED Backlight Dash Switches: To make it easier to find the dash switches at night in the dark, I put LED backlighting in the two center switches.

Flip-a-Lever Throttle Lock: I installed a Flip-a-Lever throttle lock in the throttle perch.

Real Cruise Control: I have this on all of my other K bikes and, having been spoiled by it, installed an Audiovox CCS-100 Cruise control on this bike.  It's controlled by an OEM windscreen switch from a K1100LT which eliminates the need for an ugly aftermarket control panel, is waterproof and has good ergos.

Audiovox CCS-100	Throttle Connection
Windscreen Switch for Cruise Control

Dual Fiamm Horns: Standard K100s, K75s and K75Cs came from the factory with little wimpy scooter horns while the other models came with 135 dB  Fiamm air horns which were also used in BMW cars of this era so the horns on this bike can actually get other drivers' attention.  I had to cut them down slightly to get both of them to fit inside of the C fairing.

Drive Train

New Clutch Nut O-ring: Given their age, inevitably these bikes have sat for some time during their existence.  This often causes the clutch nut O-ring to dry out, crack and start leaking engine oil.  That was the case with this bike so I replaced the clutch nut O-ring with a new one.

Good Clutch Plate: While I had the clutch apart I measured the clutch plate.  It has more than half of its life left at a thickness of 5.08 mm. (New is about 5.5 mm and the minimum specification is 4.5mm.)

K75 32/10 Final Drive: In order to make the bike a bit faster and give it some more pull coming out of turns, I replaced the original 31/11 final drive with a nice 32/10 K75 final drive with a speedo sensor that I scored on eBay for <$30 shipped. The K75 final drive "shortens" the final drive gearing from 2.82 to 3.2, yielding roughly 13.5% more torque. I also got my hands on a very nice 20 spline drive shaft.

And I swapped in a black swing arm to match the K75 final drive.

Moly Spline Lube:  I lubed the clutch and drive shaft splines with a 50/50 mix of Wurth SIG-300 and Honda Moly 60.

Synthetic Gear Oils w/ Moly: The final drive has fully synthetic 90W gear oil in it and the transmission has fully synthetic 140/75 gear oil in it. Both have been treated with Guard Dog moly additive to minimize wear and make the bike shift as smoothly as possible.

Suspension/Tires/Brakes

S Forks With Progressive Springs:  Being an 86, this bike came with the early K100 forks that don't even have a fork brace.  For better handling, I installed a pair of the newer "S" (sport) forks with some Progressive springs I had lying around.  I disassembled them and got them nice and clean inside before swapping in the Progressive springs and installing the forks on the bike.

Fork Boots: I also installed some new fork boots when I installed the forks.

Ohlins Shock: I had a spare one lying around.

Braided Stainless Steel Front Brake Line: I got rid of the old brake lines that run through the center of the steering head and split off to the calipers. To optimize braking, I installed a new braided stainless steel brake line from the master cylinder to the right caliper and then a solid metal line that runs under the fender from the right caliper to the left caliper.

Sintered Brake Pads: All of the calipers came with sintered pads that have lots of life left so I left those installed.

Vented Rear Rotor: When I installed the K75 final drive I also installed a vented rear rotor from a 94+ K1100.

3 Spoke Wheels w/ Radial Tires: I swapped in some 3 spoke wheels so I could run some good radials.  It's put a 110/80-18 Bridgestone BT-023 on the front and a 140/80-17 Avon Azaro AV46 on the rear. I've been running this combination on my K75 for the past year or so and really like the handling.

Lighting & Electrical

Big Crank ETX15L AGM Battery: This saves about 4.5 pounds over an Odyssey PC680 and about 12 pounds over a full-sized battery. It's not as tall as a regular battery though so to deal with that I flipped over the battery hold down bar and put a piece of 1/8" thick rubber between it and the top of the battery. The hold down bolts don't have enough threads but that was easy to cure by placing four M6 washers under each bolt head.

The bike came with a brand new full-sized WestCo battery which I will put in my K1100LT since the Panasonic battery in that bike is about eight years old now.

Upgraded Battery Ground Cable: The early K bikes came from the factory with a rather wimpy battery ground cable that was later upgraded to a much beefier wiring gauge.  I replaced the ground cable on this bike with one of the newer ones.

15K 50A Alternator: I replaced the stock 32 Amp alternator with a later 50 Amp alternator from a K bike with only 15,000 miles on it in order to have adequate power for the upgraded headlight and driving lights.

8K Starter: Though the old one was working fine, I replaced the original starter motor with a nice shiny one from an 8,000 mile K bike.

90/100W Headlight: I added headlight relays, upgraded the headlight wiring to 14 AWG with a ceramic H4 socket and installed a "superwhite" 100/90W xenon headlight bulb. Adding headlight relays also improves the longevity of the headlight switch since all it has to do now is carry the minimal amperage required to trigger the headlight relays.

LED Brake/Tail/Turn Signals: The brake and tail light bulbs have been converted from old filament bulbs to multi-SMD LED bulbs.  Since the OEM Bulb Monitor Unit in the relay box does not work with LEDs, I took one apart and rewired it to make it LED-friendly.

Not only does this mean that the bulbs will probably last forever but it also means that the brake light will come on faster than a filament bulb would.

I replaced the original flasher relay with a Kisan SignalMinder in order to run SMD LED turn signals as well.  I wrapped it in electrical tape to keep its housing from accidently shorting.

Upgraded LED Tail Lighting & Brake Light Strobe: In order to reduce the chance of getting rear-ended, I added additional LEDs to the sides of the tail light lens and top case base plate and wired in a brake light strobe.

LED License Plate Lights: Since I used a red SMD tail light bulb and you are legally required to have white rear license plate lighting, I added white SMD LED license plate bolts to properly illuminate the license plate.

And added some Zombie Response Team lower license plate bolts just for kicks.

I also attached the rear fender extender with some nice bright 5mm red LEDs.

Auxiliary Fuse Block: I don't like in-line fuses so for the additional electrical items I added I installed an additional fuse block next to the factory fuse block under the left side cover.

Both the factory and aux fuse have covers with waterproof labels in English. I also replaced the OEM fuses with newer, more reliable ATC fuses.

Three Powerlet Sockets: This bike has three Powerlet outlets. One in the dash pad for powering a tank bag and two above the ignition coils.

So that I could have two outlets down by the coils and make them more accessible, I got rid of the original K100 coil cover and replaced it with the top and bottom brackets of the K1100 setup.  I also added a Chromex ceramic coated plate over the coils.

10W Fork Mounted LED Aux. Lights: To make the bike more conspicuous in traffic and reduce the chances of someone pulling out in front of me, I added a pair of 10W LED aux lights.  Studies have shown that making a triangle between your headlight and aux lights improves your safety by making it easier for other road users to judge your distance and speed which is why I mounted them on the forks.

PIAA 1100 Driving Lights: To improve night lighting I added a pair of PIAA 1100 55W driving lights under the C fairing. These are tied into the high beam wiring so they come on/go off automatically with the high beam.

Body & Cosmetics

K75C Fairing and Headlight: The RS fairing that came on the bike was slightly rashed from a low speed low side so I replaced it with a smaller and lighter C fairing that I had left over from a previous iteration of my K75 Frankenbrick.  It's got a Laminar Lip to move the airflow up a bit.

K75S Belly Pan: I also has this Line-X'd K75 belly pan with a stainless steel perforated grill left over so I installed that as well. (I've never seen a K75 belly pan on a K100 so this may be a first.)

Standard K100 Radiator Shroud: The radiator shroud comes from a standard K100. I had it Line-X'd and added a perforated stainless steel grill so that it would match the belly pan.

Ceramic Coated Engine Covers: I had the valve and crank covers coated in Chromex ceramic coating.

While I was at it I also replaced the cover gaskets, installed some nicer cover bolts with new rubber bushings.

Knee Pads: This bike has BMW K bike tank knee pads which facilitate grabbing the tank with your knees in turns.

K1100 Rubber Mounted Peg Plates: Because I prefer the look of them I replaced both peg plates with K1100 peg plates.  An added benefit of this is that their mounting points are rubberized to reduce vibrations from being transmitted to the pegs.

Front & Rear Fender Extenders: I added front and rear fender extenders.

Tail Cowl Protectors: The passenger grab handles both have plastic protectors under them to keep the paint under the grab handles from getting scratched. The original black right side tail cowl handle had some road rash so I replaced the tail cowl grab handles with some nice silver ones that I had lying around.

New License Plate Frame: For some reason, I don't like bikes without license plates frames so I purchased and installed a brand new black metal license plate frame for this bike.

Added Layer Of Insulation:  In order to reduce the typical K bike thigh heat, I added a layer of double-sided foil bubble insulation under the tank, rider's portion of the seat and side covers.

Luggage & Seat

Key-Matched OEM Side Cases: The bike came with left and right Generation 2 BMW "Integral" System Cases.  The locks on them are key-matched to work with the bike's ignition key.

EMGO Travel Trunk Top Case: I had one of these lying around.  It's more convenient than the OEM 22L top case and will hold a full-face helmet so it's good for running errands.  I mounted it on a Givi E181 tail rack that's made to fit K bike tail cowls.

Corbin Dual Tour Seat: Being a seat whore, I have several spare ones on hand.  I chose to use this VGC leather Corbin Dual Tour Saddle for this bike.

BMW K100RS Factory Specifications