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Old 01-30-2010, 06:28 PM   #1
duck OP
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Location: Seattle (Berkeley with rain)
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Building the K75F

I've had this idea for a while now and have been slowly collecting parts for it for the past year or so. Tomorrow morning, whenever I get my lazy ass out of bed, I'll get started on building the K75F. What's a K75 "F" you ask? It's a K75 Frankenbike.

It will be a Frankenbike in two ways. First off, it will be built with parts from doG knows how many different bikes. I'll be using parts from bikes I've parted out, parts I've gotten from others I know who part out Ks and good deals I've scored on eBay where stuff had a low starting price and no other bidders. I think that my best score so far has been set of very nice K1100 forks that I scored for $10.49 because only one other bidder bid the starting price of $9.99.

Secondly, it will have a complete K1100 front end. What really inspired me to build this bike was to have a K75 that had decent front brakes. The stock front brakes for K75s and K100s are OK but the brakes on the four valve Ks are massively better with four pot calipers and a 20mm master cylinder. Since I'm used to riding my K1100s I've gotten spoiled and this has almost gotten me in trouble a few times riding some of the K75s I've owned.

My last project bike was a K75S with a patalever rear end. I may do that to this bike eventually but I'm now of the opinion that upgrading the front end is more important. (The closer I get to death, the less I want to die I guess so I'm thinking better brakes are what matter more. )

Anyhow, here's the heart of the beast and my starting point. It's a K75 engine I bought from some local guy on Criag's List with some other K parts who had it as a spare engine for a K75 he'd recently sold. I think he said it had something like 70k on it. That doesn't matter though. It's a K75 engine and it turns. And it's a brick. That's all I really need.

I'll be chronicling the building of the bike in this thread.

duck screwed with this post 02-11-2010 at 12:23 PM
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Old 01-30-2010, 06:33 PM   #2
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In. Ya got me curious here.
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Old 01-30-2010, 07:03 PM   #3
bill pierce
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In.. this is my favorite kinda thread
BMW R1100s,destroyed by deer strike.
Yamaha FZR 1000 (sold)
Honda Cr 500 sold
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Old 01-30-2010, 07:03 PM   #4
Joined: Feb 2002
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I have a spare K75 motor.....I'd love to put it in my wifes VW microbus.
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Old 01-30-2010, 07:29 PM   #5
duck OP
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Originally Posted by PirateJohn
In. Ya got me curious here.
C'mon in. The water's fine. I figure this is a much more constructive use of my time than engaging in verbal discourse/diarrhea with the Stalinists and Maoists of your ilk over in CSM.
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Old 01-30-2010, 07:30 PM   #6
duck OP
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Originally Posted by jdiaz
I have a spare K75 motor.....I'd love to put it in my wifes VW microbus.
I think the whopping 75hp would shatter the rest of the drive train.
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Old 02-02-2010, 08:28 AM   #7
duck OP
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I didn't get much work in on this on Sunday. I spent most of the day on a non-moto project trying to cut my fingers off with a table saw. Fortunately, I didn't succeed in removing any digits and that project's done with so I was able to get back to the K75F project on Monday.

The first order of business was to get the top of it cleaned up a bit. I'm not anal retentive about it being perfect because it will be hidden but I did want to get some of the built up crap out of there and give it a bit of a scrubbing.

A bit cleaner now:

I also wanted to remove the throttlebodies to inspect the rubber boots that hold them to the head. Those can get old and crack. These seem to be in pretty good shape so I cleaned them up with some Armor All and reinstalled them.

Here's some TBs I scored on eBay for cheap. They're not as new as they look but someone ran them through a parts washer or whatever so they're pretty clean. And nobody's messed with the factory tuned little screws with blue paint on them so they should work fine. The brass air screws looked a little dingy though so I pulled them out and polished them with my Dremel. They have lots of shiny goodness now.

I also replaced the little rubber caps on the vacuum tubes since those were old and would be cracking sooner rather than later.

The chances of the fuel injectors being worth a toot after sitting for so many years is probably between zero and none. Here's the old ones and a set I had reconditioned for this project last month.

Then I installed the fuel pressure regulator on the back of the TBs. I replaced the vacuum hose with a new one. I was going to get a new one from BMW but they cost like $20. I just used generic vacuum hose and put the BMW springy thing around it. I'll spend the $20 on beer and bacon instead.

Then I installed the FIs and the fuel rail. Did I mention that I'm fond of stainless steel?

I looted the Hall Effect Sensor off of this engine and sold it as a part a while back. But I have this known good one from a 52k bike I parted out several years ago. In she goes:

Then I installed the throttle position switch.
Here's a little K bike secret: The Ks up through 1990 had an idiot light in the instrument cluster to remind you if the choke was on. They stopped that in 1991 but everything is there to make it work in the wiring harness and instrument cluster. All you have to do is add one of the BMW piston switches (like the ones for the front brake switch and clutch switch) at the back of where the choke cable connects. There's a little spacer you need too. See upper right in the picture below.

I found these spark plugs leads in my stash. They look pretty clean so I doubt they have many miles on them. I was going to replace the plugs too but they looked pretty nice so I didn't bother.

Then I installed the air plenum on the top of the TBs. BMW uses those dumbass Oetiker clamps but I use these thin stainless hose clamps from McMaster-Carr that work just as well and are much cheaper.

The crank case breather hose is another part that gets old. Figured it was best to install a brand new one.

One of the reasons that the tank gets so hot on K bikes is that the return hose from the fuel pressure regulator is routed right behind the radiator and this takes heated gas back to the tank. I insulated the entirety of that return line. Since eventually the fuel rail and injectors will be covered (more on that later) I also insulated the other fuel line and the fuel rail.

Then I installed the radiator hoses. That one that goes through the hole in the crank case cover is really a mofo to get through the hole.

I have my old Pentium II laptop in my basement workroom. It's good for Internet radio but I've also got the BMW parts catalog loaded on it. It came in handy so I didn't have to guess which size hose clamps (BMW has a million sizes of them) to use for the radiator hoses.

Before putting the frame on, I installed the fuel injection wiring harness. It's much easier to hook up the temp sensor and mass airflow sensor before the frame is attached. The best FI wiring harness I have has 100k on it but I test rode the donor bike before I parted it out so I'm sure it works. It also looked good when I cleaned it up.

I scored the mass air flow sensor on eBay for $9.99. It’s only got 18k on it. The air filter is an almost new one that came out of one of my parts bikes. It's probably only got another 50-60k in it. LOL

Now it was time to put the frame on. Life is easier if you lay the wiring harness on top of the engine first. This is a nice main wiring harness from a 92 with only 24k. Since the bike will have ABS, it's an ABS wiring harness. (More on the ABS later.)

When I had some spare time over Christmas, I took the BMW wiring diagram and re-labeled it in English. Makes life very easy.

OK, time to drop the frame on. What works for me is to get the two front bolts on and get them fairly tight. Then I lift up the back of the frame to get the wiring harness and relay box in their proper places.

I scored the frame from a salvager for $150 with a clean title. It's from a 1987 K75C and in good shape. It's only got a few paint nicks here and there so I didn't bother having it repainted. Maybe someday...

Time to populate the relay box. Since I plan to be running LED turn signals, brake and tail lights, I installed a Kisan SignalMinder for the flasher relay and jumped the connector to get rid of the bulb monitor unit.

See that piece for red tape? I left the bellhousing frame bolt fairly loose and that's my reminder to torque it later. I want to do that after the tranny is installed so as not to introduce any stresses in the frame. It probably doesn't matter but I figure it can't hurt.

Up to this point I'd done everything in the right order until I realized, DOH!, I forgot to install the metal brakes lines for the front ABS before I put the frame and wiring harness on. Fortunately all I had to do was lift the rear of the frame off of the engine and I was able to squeeze them in.

Then I installed the fan on the radiator.

And mounted the radiator to the engine and frame.

Now it was time for the engine to shed it's earthly bonds and get set up to get some legs:

And so it slept on Monday night, comforted by dreams of the twisties and canyons ahead in it's once bleak, now bright, future.

You'll notice the front covers are less than perfect and the valve and crank covers, though pretty decent, aren't perfect either. I plan to take care of that stuff when I reach the cosmetic phase of this project. I just want to get it running and riding initially.

duck screwed with this post 02-02-2010 at 09:06 AM
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Old 02-02-2010, 08:46 AM   #8
Metal Doc
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I love these type of threads
"A slip of the foot you may soon recover, but a slip of the tongue you may never get over. " - Benjamin Franklin
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Old 02-02-2010, 08:56 AM   #9
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I'll spend the $20 on beer and bacon instead.
That's all I need to hear.
You're my kind of builder.
So there I was walking into a biker bar in Chattanooga with liberty spikes on my head and a drag queen beside me... - bcurvin
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Old 02-02-2010, 10:22 AM   #10
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Very cool project.
JK Rubi
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Old 02-02-2010, 10:25 AM   #11
"Cool" Aid!
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Old 02-02-2010, 11:10 AM   #12
Avoiding the Skid-Demon
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Proprietor of The Tidewater Forge Hot iron is my passion. Fire is my mistress. Let's dance.
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Old 02-02-2010, 11:34 AM   #13
BMW uber alles!
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Radiator hose install

+1 on the Wow. I'm in, too.

Btw, I just reinstalled my radiator hose through the crankcase "tunnel." Only took me about an hour of cursing, screaming and general hair pulling.

In case anyone else here will need to be doing the same feat anytime soon:

The trick seems to be is pushing the bottom end of the hose down through the top of the tunnel. I used plenty of lube (Back to Black worked well) and scuffed the inside of the tunnel with a red scuff pad beforehand. After jamming it in, twisting and pulling it back out, relubing and trying again, it finally went all the in on the sixth or seventh try.
2003 Honda Ruckus "Rucca"

ex: '94 K75S "Berlina" '92 R100GSPD
'85 K100RS '73 R75/5 '65 R60/2
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Old 02-02-2010, 01:34 PM   #14
Streamin' E
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Liking it!
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Old 02-02-2010, 02:38 PM   #15
duck OP
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Originally Posted by Zagando
Btw, I just reinstalled my radiator hose through the crankcase "tunnel."
Took me about 15 minutes. Armor All was my lube. I suspect some spray silicone might be better but I had the Armor All at hand and was too lazy to go to the workshop and get my can of silicone. I'll try silicone when I take the cover off to have it redone later.

I pushed in one side of the hose so it was doubled over then used a ratchet extension to keep it that way while in the "tunnel." I came up from the bottom because there's less of a bend to deal with in the upper section of the hose.
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