1986 K75C Turbo

Maintenance & Modifications

Here's a summary of all of the maintenance and modifications done to this K75. This is followed by brief descriptions of them with some pictures. 

Prior History

(From the documentation that came with the bike.)

Steering Recall

This happened shortly after the bike was purchased in 1986.  The steering head bearings were modified and a Fluidbloc damper was added in the steering head.

Modified Valve Cams

Modified valve cams (I assume from Luftmeister) were installed at 20K. These make a noticeable performance difference.  I'm going to try to have them "cloned" by a cam shop so I can install a set on my other K75.

Luftmeister Turbo

The Luftmeister turbo was installed in the spring of 1988 at 23K.Here's some pictures I took of the turbo system prior to digging into working on the K75. The turbo is an IHI RHB5 which was fairly popular back in the 80s and used on custom vehicles like this K75 and but also used quite a bit as a stock turbo on Subarus, Fords, Mazdas, etc.... 

IHI RHB5 Turbo

Both the turbine and impeller blades are in good condition and there is no radial or axial play in the bearings so the turbo is still in good working condition.

CHRA: In between the turbine and compressor is the housing for the shaft between them. Given how fast turbos rotate, (tens of thousands of RPMs) the CHRA (center hub rotating assembly) needs a constant source of pressurized oil to cool and lubricate the bearings.  On this K75 that comes from a fitting installed at the opening in the oil pump for the oil pressure switch and then a braided stainless steel hose goes behind the radiator hose up to the top of the CHRA where it enters through a banjo bolt.

And then returns to the bottom of the oil sump.

Wastegate: In order to keep the turbo from overcharging the engine, the wastegate valve diverts gas away from the exhaust turbine.  It is controlled by a wastegate actuator that monitors the pressure in the compressor housing.

Intercooler: Air heats up and looses density when it is compressed. To offset this the intercooler cools the compressed air down before it enters the engine. On this bike the intercooler is a custom split one that has a space in the middle of it in order to allow airflow to the cooling system's radiator.

Air Intake From Intercooler: This is also custom with a second air filter that allows the engine is to aspirate normally until the turbo kicks in.  When the turbo kicks in at around 6,000 RPM, the air pressure from the turbo closes the valve to that filter and all of the intake air comes through the turbo's intake filter.

Crank Case Breather: To keep the higher intake pressure from blowing back into the engine, the crank case breather outlet on the plenum is blocked and there's a filter on the engine breather.

Engine Balanced

From the factory the engine parts of K bikes were balanced to within 1 gram. At 41K Luftmeister balanced it down to 0.1 grams

Valves Ground

The valves were ground at 41K.

Steering Head Bearings

The steering head bearings were replaced shortly before I got it so I didn't have to worry about those.

Cables

The cables were also all replaced short before I got it.

Cooling Hoses

All of the cooling hoses were replaced at 63K.

Drive Train

New Clutch Nut O-ring

While I had the transmission off I replaced the clutch nut O-ring as preventative maintenance.

Newer Clutch

The clutch that came on it was in good shape but, since I had a better, newer one on hand, I installed that with a near new clutch plate.

Moly Spline Lube

I lubed all of the splines with a 50/50 mix of Honda Moly 60 and Wurth 3000 greases. The Honda grease provides the necessary lubrication and the Wurth 3000 is an incredibly sticky grease that helps keep the moly there longer.

K1100 Transmission & Paralever Final Drive

In order to eliminate the monolever "jacking effect" and provide better rear end handling, I installed a K1100 transmission and paralever final drive. The final drive is a 32:11 which is the shortest (most torque) paralever drive for classic K bikes.

Synthetic Gear Oils w/ Moly

The transmission has Mobil 1 75W-140 gear oil in it and the final Mobil 75W-90.  Both have Guard Dog Moly Treatment in them to make shifting smoother and reduce wear.

Engine/Fuel System

Rotella T6 Synthetic Oil

The engine has Shell Rotella T6 full synthetic oil and a new Bosch oil filter.

Cooling System

When I replaced the coolant, I added some Redline Water Wetter additive as it improves the heat transfer properties of the coolant.

I replaced the old radiator cap with a better one.

Rebuilt Fuel System

I guess the last person who worked on the fuel system didn't want to shell out $400 for a new BMW fuel pump so instead they butchered the fuel system inside of the tank and crammed a generic external pump in under the tank. Normally I would just recycle a tank that was like that but since this tank was in very good cosmetic shape I decided that it was worth the effort to get it back to something approaching a normal K bike fuel system.

Here's the internal metal line that was cut and bent and attached to a cheap plastic fuel filter stuffed in where the fuel pump should be.

As I feared would happen, the aluminum fuel line snapped (where the red arrow is) just behind the weld (black arrow) that holds the line to the bottom of the tank when I tried to gently straighten it.  Using a Dremel cutting wheel I carefully cut the fuel line right in front of the weld (blue arrow) and then gently bent that up so I could install a fuel line on it.

Then I installed all new submersible fuel lines, a new fuel filter and a K bike fuel pump where it is supposed to be.

As part of the fuel system butchery, the power wiring for the fuel pump had been cut and spliced to the external fuel pump. I resoldered and double heat shrank it to get it back the way it should be. Then I put some nylon spiral wrap around the wires.

The gas cap was showing its age so I installed a better one. I moved the lock core over so it would still match the ignition key.

As is typical of lots of old K bikes, the hook for the seat lock was broken. While I was replacing the gas cap, I installed a good one after I'd rekeyed it to work with the ignition key.

All New Fuel Lines

I replaced all of the external fuel lines with new ethanol resistant 225 psi fuel lines.

Newer Throttle Bodies

I had some  newer, lower mile throttle bodies on hand so I cleaned those up and installed them with a brand new fuel pressure regulator.

I had a 24K throttle position switch on hand so I used that. I installed new vacuum caps and a new vacuum hose to the fuel pressure regulator.

Series II Fuel Injectors

I upgraded to freshly reconditioned four hole pintle Series II Bosch fuel injectors that supposedly atomize the fuel better and lead to better combustion.

New Air Filters & Lower Mile MAF

I replaced the mass airflow sensor with a newer, lower mile one.

I replaced both of the air filters with new paper ones since they probably filter better than the old K&Ns that were on this K75 when I got it.

Since I live in Seattle, I covered both air filters with water-resistant pre-filters from Outerwears.

I replaced the crank case breather filter with a new non-oiled washable breather filter from Uni-Filter.

NGK Iridium Spark Plugs

New NGK Iridium spark plugs.

Magnecor Spark Plug Leads

Newer, lower mile K75 ignition coils with new 8.5 mm "competition" spark plug leads from Magnecor.

Chromex Ceramic Engine Covers

I had both engine side covers coated in Chromex ceramic coating, installed nicer cover bolts and replaced all of the old gaskets and bolt grommets with new ones.

Custom Luftmeister Exhaust

The custom Luftmeister turbo headers are in great shape.

The custom porcelain coated one piece Luftmeister turbo exhaust is in great shape too.  It didn't have an exhaust gasket so I got some generic stuff from AutoZone and fabricated one.

L-Jetronic Mud Cover

In the later years of K bike production, BMW cheaped out and stopped including the mud cover for the L-Jetronic connector under the seat. The one for this bike was missing so I replaced it.

Throttle Body Sync

Once I had all of that done, the idle set and the TPS dialed in, I got my CarbTune out and sync'd the throttle bodies.

Instruments/Controls

"C" Handlebars

The bike came with K75C handlebars which provide a moderately upright seating position.  Since I plan to use this as a "city" bike for local rides and running errands, I decided to stick with them

Clutch Perch & Lever

The clutch perch and lever were a bit worn so I replaced them with "like new" ones.

Real Cruise Control w/ BMW OEM Switches

Previously I've installed more than a dozen Audiovox CCS-100 vacuum cruise controls on various K bikes.  For this K75 I decided to have a go at installing an electronic cruise control from Rostra.

I installed the control unit in the tail cowl.

And fabricated a bracket to hold the cruise control throttle cable above the midpoint between the second and third throttle bodies.

Since it's 100% waterproof, more ergonomic and not gaudy like adding an aftermarket control pad would be, I control the cruise control with the OEM windscreen switch from a K1100LT.

One nice feature the Rostra electronic cruise control has that the Audiovox doesn't have is an output to indicate when the cruise control is engaged.  I wired that up to the unused indicator in the instrument cluster next to the low fuel indicator.  To keep it from being too bright at night, I installed an amber LED behind the red lens.

"Euro" Headlight Switch

US bikes have an "always on" headlight while bikes sold in other countries have a headlight switch to control the parking lights and headlights. I installed a "euro" headlight switch on this K75.

Newer Instrument Cluster

When I got this K75, neither of the odometers worked. Instead of trying to fix that I installed a newer, lower mile instrument cluster.

And used one of the later vented backplates that has Gore-tex vents in it to mitigate condensation from forming inside of the instrument cluster.

LED Instrument Cluster

All of the backlighting and indicator filament bulbs have been replaced with LEDs.

12 Hour Clock

I converted the 24 hour clock to a more US-friendly 12 hour clock.

Calibrated Speedometer

From the factory K bikes speedometers usually run an annoying 10% fast. I calibrated the speedometer to be accurate.

Altitude Switch

Instead of having to stop, get off and fiddle with the altitude compensation plug every time I pass through an elevation of 4,000 feet, I wired that up to a dash switch.

LED Backlight Dash Switches

I LED backlit the two center switches to make the switches easier to find at night.

Heated Grips

I installed Symtec heated grips operated by an OEM dash switch.

ProGrip Gel Grips

My favorite grips for k Bikes, ProGrip 699s.

Ceramic Coated Bar Ends

Chromex ceramic coated K1100RS bar ends.

Dual Fiamm Horns

Except for the K75RT, K75s came from the factory with wimpy little disc horns that sound like a scooter.  I cut down a pair of 135 dB Fiamm horns and mounted them behind the headlight. These are the same horns that came on BMW cars of the era so they're nice and loud.

LCD Fuel Gauge

Aside from mounting it on the handlebars, there's no place to mount a round 2" fuel gauge but I found this backlit LCD fuel gauge that fits nicely under the clock on the instrument cluster. 

It has a low oil pressure warning too but, since the K75 already has one of those in the instrument cluster, I wired that up to the altitude switch as a visual indicator of when that's on.

Lighting/Electrical

New AGM Battery

It came with a new "half-size" AGM battery. Since it's less prone to corrosion then the original one, I used a K1100LT battery tray.

Custom Battery Cables

I installed some beefy gauge battery cables that I custom made for this K75.

Low Mile 50A Alternator

I replaced the old 32 amp alternator with a low mile 50 amp alternator.

"New" Starter

Not sure how many miles this starter has on it but not many judging by its appearance.

100/90W Headlight w/ Relays

In order to run a 130/90W Xenon headlight bulb, I installed headlight relays with 14 AWG wiring and a ceramic H4 socket.

Using my multimeter I determined that the low beam is always on, even when the high beam is on. Since I installed headlight relays and a higher wattage bulb, I prefer the high and low beams to operate mutually exclusively so I fabricated what I refer to as a headlight master control relay using an 8 amp DPDT PCB (printed circuit board) relay.

LED Turn Signals

In order to run CREE LED turn signal bulbs, I replaced the flasher relay with a Kisan SignalMinder.

LED Tail & Strobing Brake Lights

In order to run LED tail and brake lights, I made an LED-friendly bulb monitor unit. (BMU)

I added several different additional LEDs for the brake and tail lights and installed a strobe controller.

LED License Plate Lights

Since a white LED bulb looks pink in the tail light lens I use a red one. However, to be legal, there still needs to be white light illuminating the license plate so I got these LED bolts for $3.50 shipped on Fleabay from China. I added some chrome skulls with red LED eyes for the lower bolts.

10W Fork Mounted LEDs

For daytime conspicuity in traffic I installed 10 watt LED flood lights on the forks since a triangle makes it easier for other road users to judge your distance and speed.

Light Bar w/ HID Hella FF50s

I fabricated a light bar out of 1" square aluminum tubing, had it powder coated, mounted it under the fairing and installed some Hella FF50 driving lights that I converted to 35W 4300K HID. These are tied into the high beam so they go off automatically when I turn switch the high beam off for oncoming traffic.

Both sets of aux lights are controlled by this three position dash switch.

Front LED Strips

Later on I decided to add some very bright strip LEDs on each side of the light bar to improve turn signal conspicuity. They're brighter than they look in this picture because my camera was compensating for the brightness of the white LED flood lights on the forks.

Auxiliary Fuse Block & LED Fuses

I don't like inline fuses for electrical add-ons so I mounted an auxiliary fuse block next to the factory fuse block under the left side cover. It's populated with LED fuses that glow if the fuse gets blown. I added some laminated labels in English.

Dash Powerlet Socket

I installed a dash pad with a Powerlet socket to provide power to my electrified tank bag. I can run/charge USB and other devices in the tank bag with this:

Body/Cosmetic

Good General Condition

Aside from a few blems on the side covers, the bike is in very good cosmetic condition.

C Fairing w/ Laminar Lip

Since it's in good shape and to keep the bike light, I decided to keep the C fairing. I added a Laminar Lip to reduce wind blast.

Taller & Wider Mirrors

Old	"New"

To see less of my elbows and more of traffic behind me, I installed some taller and wider handlebar mirrors.

Tank & K75 Knee Pads

The bike was missing the rubber pad on the back of the tank so I cleaned off the leftover adhesive and installed a "new" tank pad with some 3M double sided trim tape.

BMW now only sells tank knee pads for K100 gas tanks. They haven't made/sold them for many years but they used to make tank knee pads specifically for K75 tanks that I think look much better. They're rare and hard to find but by a stroke of luck I was able to find a pair in good condition on Fleabay.

Lengthened Side Stand

A prior owner welded some more metal onto the bottom of the side stand to reduce the excessive side stand lean typical of K bikes.

K1100 Rubber Mounted Peg Plates

I like the looks of the K1100 rubber mounted peg plates and they hide the side transmission mounting bolts. The right one also has a better return spring for the rear brake pedal.

Front & Rear Fender Extenders

I added front and rear fender extenders.

Tail Cowl Protectors

To protect the paint under the passenger grab handles.

New License Plate Frame

I don't like "naked" license plates so I bought and installed a plain black metal license plate frame.

New Roundels

I added a small roundel to the center of the C fairing.

I replaced the flat tank roundels with some 3D ones.

I installed another 3D roundel on the rear fender.

Corbin Seat

I had several seats to choose from. I picked this leather Corbin Dual Tour that's in very good condition. I installed the rear hinge with this easy to use clevis pin. To remove it all I need to do is put my finger through the zip-tie and pull. It's much more convenient than the "disappearing" E-clips that BMW used.

Suspension/Tires/Brakes

"S" Forks w/ Progressive Springs

I replaced the original early forks with some later "S" (sport) forks after I'd installed Progressive springs in them.

Upgraded Front Brakes

I replaced the front master cylinder with a nicer newer one.

Then I got rid of the brake lines that go through the steering head and replaced them with a single braided stainless steel line that goes straight from the master cylinder to the right caliper with a solid metal line from the right caliper to the left caliper.

I installed the brake lever that has a small set screw in it for fine tuning the front brakes.

Rear Disc Brake

As a K75C, this bike came from the factory with a rear drum brake.  The paralever final drive has a disc brake.  I added a rear brake fluid reservoir and installed a vented rear brake rotor that came on 94 and later K1100s. The rear brake has a braided stainless steel line as well.

Works Performance Shock

Newly rebuilt Works Performance shock for the paralever final drive.

3 Spoke Wheels w/ Radial Tires

I upgraded from the old Y-spoke wheels to 3-spoke wheels with radial tires (Bridgestone BT-023 110/80-18 front, Avon AV46 Azaro 140/80-17 rear) and installed new metal valve stems.

Luggage/Tools

City Cases

The bike came with a like new set of city cases.

22 L Top Case

The bike came with an OEM tail rack. I scored a nice 22 liter top case and rekeyed it to match the ignition key.

Complete OEM Tool Kit

This is the tool kit that came with it. The box wrenches are all Craftsman. I replaced it with a complete OEM K bike tool kit.

Spare Clutch & Throttle Cables

Even though it has fairly new cables, I threw a spare set of used clutch and throttle cables into the tail cowl just in case.

Last But Not Least

This is my turbo keychain. It's all metal and spins when you blow into it.