CB900 Scrambler / Racer

So... I'm building a vintage scrambler/adventure/back roads/race bike. The starting point is a 1979 CB750K with a 1981 CB900F motor. I picked it up 4/12/2013 on the local CL for a few hundred bucks because it wasn't running. It has been sitting for the past 10 yrs.


Here's the back story.

I'm a bit of a moto blog junky and check up on bikeexif, pipeburn, thekneeslider ... a few times a week at least. This has led to a burning itch to do a vintage build of my own so it was only a matter of time. I'm a huge fan of the scrambler and brat bikes by the likes of CRD, LHS, Wrench Monkeies etc. so that's my inspiration.



I have the urge to hoon a bike of this design like this:

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(yeah, I know they aren't Hondas. I'll make some Honda videos when it's done )


Here's more why:
I got back into riding in 2011 after a 4-5 year break by getting a street legal DRZ400E. It definitely felt like I was getting back to my roots of riding dirtbikes all the time as a kid. Since then I have ridden all sorts of dual sport oriented rides, lots of technical trail riding and some longer multiday adventure rides in CO and WY.

In 2012 I decided to give racing a try and chose RallyMoto as the way to do it. It's stage rally format like you may have seen with Rally cars. Full speed on closed back country roads! Stages are between 3 and 20 miles long It's a total hoot. http://rallymoto.com/ http://nasarallysport.com

Here's what it usually looks like from the riders seat.
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My first race was on sand in at SandBlast Rally in SC.


The next one was Rally WV at Snowshoe, WV. I upgraded to a KTm 450XC-W mid season in 12.


I'm not one of the fast guys but I have tons of fun doing it. One of the neat things about this type of racing is that there are classes for all sizes of bikes so you have everything from 250 four stroke trail bikes to BMW boxers racing for overall points and against similar sized bikes for weight class points.

For RallyWV 2013 I decided to enter the heavy class (750.1cc+) so I'll be putting some knobby rubber on the CB and hitting the gravel to go at it with my buddies on big bikes.



More to come...

I started this build log over on DTT and I'm going to start posting it in parallel over here. I'll get it all copied over in the next few days so all my ADV buddies can get the action while they are here.

The plan is to break this build up into two stages. The first sage is just getting the bike on the road for as little money as possible. The second stage throughout the summer will be performance upgrades like suspension, brakes and maybe a set of flat slide carbs.

The bike sat for 9 years so a carb rebuild is in order as well as knocking the dust and rust off all sorts of bits. I'm painting the wheels and frame satin black. Some bits will be left silver as accents. the tank will be red on top with black on the lower area below the body line.

Week 1
I started with easy stuff and disassembled from front to back while waiting for the carb kit and new carb boots to arrive.


Needed new fork seals but the wheel bearings and brakes were good.


The caliper was pretty crusty. This is after some work with a wire brush. That pile of debris at the bottom is what came off these things.


I took everything to work and hit it with the bead blaster. Love that thing!
You can tell by the caliper half and lower tree I hadn't done yet just how pristine it makes gritty parts. I spent lots of time taping off areas that I didn't want covered in beads. Also, I ran bolts into any threaded areas to keep the grit out as well.


The tires where hard as a rock so rather than wrestle them off the rim I cut a chunk out wit a band saw then cut the steel belt in the bead with a cutoff wheel.




While waiting for the carb kit to arrive I pulled them off and dropped the bowls to see what I was up against.

Not great but I've seen worse.

I live how your bike has turned out!


Signed-

The Future.

Longest build thread e-v-e-r.

(Note the messed up date imprint on the first image...10/01/2025)

Nothing gets by the discerning ADV crowd! Nothing!

The funny thing is that's actually the Craigslist photo from the PO. I didn't bother taking pics before tearing into it.

Week 2
The carb kit arrived so I set up my work station on a card table in the living room. The carbs on the 900 are Keihin VM43B.



I managed to delete the pics I had of the all those tiny parts everywere but I can say that next time I have a set that needs a full tear down I'll probably send them off. It is good to know how these carbs work in and out though and it should make future tuning an easier process.
Here's what was left behind after the fact.



I ordered the wrong kit from e-bay so I had to wait for a new accelerator pump diaphram and fuel rail o-rings befor final assembly.

In the mean time I got the wheels painted and the new tires mounted.


The color is satin black.

I got brave and started detabbing and did some chopping too. I love running an angle grinder.

As you can see a little here I cut down the countershaft cover a bit too so I can inspect the CS sprocket without removing the cover.



Swingarm and other misc parts went to the bead blast cabinet.

Here's a quick video of blasting the swingarm.
http://www.flickr.com/photos/27840566@N05/8710938197/

That Left Hand Cycles CB750 you posted at the top (the OD green one) is sort of inspiration for my XV920. Right now it's even sitting in OD green (rattle-can paintjob, but oh well it'll get dropped and scratched so I don't care). My build thread is in this same section.

Some of the "for show" stuff on the LHC CB750 are kind of goofy (lack of back brake for one) but I'm digging the minimalist nature of it.

Enjoy the project!

I did the frame and wheels on mine in rattle can. I know I'm going to take it places it doesn't belong and any coating will get damaged. Rattle can is easy to touch up.

Can't imagine no rear brake. The LHS shock mounts are probably ok for pub hopping but mine will be made right for taking this thing off some sweet jumps.

Week 3
Painted some more parts...zzz


One of the guys I work with generously donated the battery to the cause as well as letting me borrow his shiny new sync gauges and tuning fuel can. ;D
The final carb parts arrived which is good because I needed some moivation after a week of degreasing and painting. All the parts went back together without a hitch. I was a little suprised since this was my first time rebuilding a set like this.

I eyeballed a bench sync, crossed my fingers, threw the harness back together and mounted up the carbs.

A little choke and 2 cranks later it fired up. 3 turns of the idle knob and it was purring on its own after a 9 year slumber.

Here's a vid of the second start since I was too excited to bother with a camera on the first start.

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Side note: I changed the oil and checked the valves when I first pulled it in the garge and everything looked great. The oil looked new actually. I believe the previous owners story of only having about 1000 miles on this motor after it was transplanted. His receipts and the looks of the top end seem to jive.

With that ceremonial first start out of the way I pulled the wiring and carbs back off and set about some other details.

I reassmbled the forks and scrounged through the spare parts boxes to get the headlight together. 7 inch light really hangs out there but I think I kind of like it with the vintage dirt bike bars. The bulb for this light is an H4 so I could probably put an HID kit in and put the ballast under the tank without too much trouble. I like extra lumens.


Also masked off all the sensitive parts and put some fresh enamel on this fine running motor.


Ordered a rear hoop from one of the DTT vendors and it arrived so that will go on soon which will allow me to fab the seat pan. This isn't a great picture of it but I hit the exhaust with a scotch brite pad to knock the bling down a little.

Went to Amazon for some wiring wrap in various sizes to replace the miles of decomposing, gooey, rotten electrical tape that are on the wiring harness now. Should make hiding the stock harness a little easier and allow future access for other accessories.


I've got a Trail Tech Vapor Tach/speedo on the way to replace the stock gauges. I'm going to turn a couple of plugs to block off the holes for the tach and speedo cables.

Also have some basalt exhaust wrap coming as well. Mostly for looks.

So... that pretty much has me caught up on the updates. Everything will be real time from here on.

8-May-13

Got frisky last night and decided to weld my rear hoop. I borrowed a HF mig flux welder for this task. Not the best but it'll do.

I've never welded before but I have watched some YouTube training so I figured that should be good enough.

I practiced on some scap sheet metal of teh same thicknes and on some scrap tubing from the frame.T hese will be ground smooth anyway so no need to look pretty.


Big hammer test confirmed they were good enough so I moved on to the real deal. I left the cross brace in place untill this was done just in case the frame might try to pull.

Success!


Cut the cross brace and prettied it up a bit.


I thought I had some good material for the rear seat pan mount but it was thinner than I wanted so I'll go through the drops pile at work to find something better to put in.

At this point I need to get it back on it's own wheels to get an idea of the stance so I got the swingarm and rear wheel back in place.


I will re-use part of the inner fender to keep mud off the battery and carbs.
I picked up some crash bars from ebay to protect the cases in the event of inevitable drop. Didn't realize it untill they arrived but they came from Puerto Rico.

They were pre-crashed so they were only 30 bucks. I pounded them straight then hit then with a band file to smooth the rash. They cleaned up nice so I hit them with primer and paint.

I threw the front fender on last night so I could mark it up and chop it down a bit. Chopped down and painted satin black seems par for the course on this style of build... but then I ended up liking it a little the way it is...

I could cut it down all the way just leaving a hoop as a fork brace... or I could go and do a small high fender with a mud flap like vintage street/trail bikes...

I need to get the tank back on to know for sure.
I'm gonna cut the tabs on the headlight mount and move them down to get the light closer to center on triples now that I can stick metal together.


Seat pan fabrication pics.







I'm out of town until Monday so no progress to be had this weekend...

S&*^)*^& F*(&(*& S*+S)(_)( S !!!!!!!!

Had a nice long weekend with family. Came home to this....


Before I left I had the jack almost all the way down so it would just hold the bike up level so I could get an idea of the ride height and stance. At that level the safety catch doesn't engage and the cheap POS jack bled off over the weekend.

Not too much damage.

A nice ding on the tank but it was still in the primer stage so it could have been worse.....


Scuffed the egg on the stator cover....


Bent the headlight ears but didn't bust the housing... (I might flip and lower these anyway)


I had been wanting to inspect that exhaust anyway...


This bike will be on it's side plenty of times in places it shouldn't even be in the first place....this is just the first trial run

The glass is half full.... LBC

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Made my heart sink...
[/quote]

Exactly!

Well, She's upright and ready to push ahead.


I wasn't super excited to back track to fix the damaged bits so I decided to grab the cut-off wheel and attack the front fender for motivation. Shortened it front and back. I'm pleased with the result.



I went after the fins on the head with a sanding block as well to bring some detail back to the motor. I may knock the paint off the fasteners as well. It's still too monochrome at this point. I'm definitely going to lay some silver metalic paint in the Honda logos on the cases covers to bring those out.


Welded up the rear cross member that will be the rear seat mounting bracket as well.


Pushed most of the tank dents out from the inside then hit with some glaze and primer.




Tried out the paint scheme for the tank badges. I'll shoot them in black then mask off the letter faces and re-do the metallic. More gooder.


I realized last night that some of the detail work is keeping me from getting her on the road so tomorrow night will be focused on that direction.

To Do.
Assemble Controls
Mount Battery
Finish seat mount and foam
Clean and mount wiring harness
Mount new gauge cluster
........
...
.

I'm all caught up on the story here. Everything will be real-time from here on out.

-LBC

Cool...thanks for the update...seeing your hoop and seat mount ALMOST makes me want to do that on the XV...but I just ordered a replacement subframe off ebay for $9 so I think it's gonna just get modified a bit and slapped on. I'm kind of like you...this time of year I'd rather be riding it than working on it!

That jack episode makes me think of my Craftsman lift...I've almost had that same thing happen when it bled down, luckily the bike I was working on at the time (the RT1 in some of my pics) had the kickstand down so basically it just gently lowered it onto it's tires and kickstand! Now I make sure that it's sitting on the catches anytime I lift it...my procedure goes like this...lift the bike, make sure it's past one of the stops, lower it until it's supported fully by the stops, then give it a short pump to relieve a little pressure on the hold. Usually stays put that way and doesn't seem to bleed down.

I'm also thinking about cutting down the front of the fender on mine, but I think I'm going to leave the back a little longer, at least keep SOME of the gravel/rain/mud protection.

I like your scotchbrite idea too...might have to steal that for the pipes and maybe even the front fender on mine.

Keep the faith!

I like the line I cut but I may make a rubber flap for the back of mine if it spits out too much crud.

I stole the idea from an episode of Fast N Loud. Continue the chain!

Didn't feel like working on the bike last night but I did anyway. Left work late and did some work around the house so I didn't make it to the garage until almost 10. I'm due for a good work space clean-up as you can see. Last night felt like I did more searching for tools and parts than actual work but progress was made. When I went out and looked at her this morning I was glad I sucked it up and pressed on.

I turned a plug for the tach cable out of some stainless bar stock after work a few days back. It fit snug with a little work from a round file on the set screw groove. No RTV needed.


Fixed the dent and put down some paint. I had to sand out the glaze I put on yesterday and sand away more of the stock paint arount the damage because the stock paint was lifting from the stress of the dent. Much better the second time around. I'll let it dry for a day or two then block sand it one more time to be sure.


Pulled the exhaust so I can do the wrap. Also want to paint a few spots I couldn't get to with the pipes on the motor.
The nuts for two of exhaust studs were 13mm while the rest were 12mm. Sound familiar? ;D

They aren't as heavy as I thought they would be. If I remember tonight I'll put them on the scale


Painted the end caps on the exhaust cans a few days ago. They turned out well enough.


Drilled the seat pan so I can weld the studs. I plan to run bolts through and weld the heads in place but I needed some different length hardware. I'll pick that up on the way home tonight.


Test fit on the gauge mount is promising. I'll probbaly tuck it in behind the ignition switch a little more on the final version. I'll end up changing it when I do a fork swap down the road so I won't get too fancy. just a piece of aluminum with a z bend.





Also took a stab at laying some accent color on the case logos. I'll probably recruit someone with a steadier hand to do the faces of the letters in black.

Neat Project.
Looking forward to following build.

Thank for tuning in!

Welded the studs for the seat pan last night. My test run on some scrap was all sorts of crooked because I didn't put tension on the bolts. For the actual pan I put the bolts in and tightened them all down before spot welding them in so they would stay straight.


Next I used flap wheel on the angle grinder to smooth the welds and do some final shaping on the pan. I put down two layers of a foam workout pad I picked up at K-mart. I layed them on as rectangles and used a razor blade to trip to rough shape. It has a nice soft neoprene sort of feel so it should be as comfortable as a minimalist seat can be. I used a 3M #90 spray adhesive because that's what I had on the shelf. It says Hi-Strength on the can so it must be good. Seems like it will definitely do the trick.


Had to set the tank on to check out the lines. Getting closer. The flap wheel on the angle grinder will be handy for final shaping of the foam. I may add another layer of foam on the lower section to make the step in the seat less pronounced.


I had the cover off the stock seat and pulled it over the pan a few different ways. The only way to get the shape I'm looking for is to have a cover sewn with seams in the right places to make the shape happen.


Bead blasted the paint off the valve cover bolts and re-installed for a little bit of contrast.

I just stumbled across this thread. looks like you are going to have a very nice CB when you are done. looking forward to the finished project