CB900 Scrambler - Rally Approved!!

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.





In my mind they should be ridden like this:
<a href="http://vimeo.com/3785417">Rumble in the Jungle</a>

Here's 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.
http://www.youtube.com/watch?v=7M1umZweOok

..sometimes it looks like this...
http://youtu.be/_TQMo7IU6-o

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...

Re: CB900 Scrambler - To Ride in Anger

I've seen that scrambler on the blogs it quite a light look to it for a heavy assed CB. I too appreciate the CRD bikes and heres my own interpretation



With all the racing you do, I surprised you have time for custom building. If you need any advise give me a shout.

Re: CB900 Scrambler - To Ride in Anger

notlob said:

If you need any advise give me a shout.

Click to expand...

I definitely will.

I have been through your build thread a few times. I really dig your wiring job. Very clean! I'm not sure if I'll go down that road just yet but it is inspiring.

Your seat turned out great as well. How did you attach your seat cover to your aluminium seat pan?

Re: CB900 Scrambler - To Ride in Anger

The leather is glued to the base. I made the seat pan but got a local guy to me to cover it.

Re: CB900 Scrambler - To Ride in Anger

notlob said:

The leather is glued to the base. I made the seat pan but got a local guy to me to cover it.

Click to expand...

Glue. Brilliant. I never thought of that. I was thinking of drilling holes and riveting it on...gorilla glue should do the trick though.

I found a good shop to make my cover but I need to weld my rear hoop before I fab the seat pan and carve the foam. I'm having the hoop bent on Monday.

Re: CB900 Scrambler - To Ride in Anger

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.

Re: CB900 Scrambler - To Ride in Anger

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/

Re: CB900 Scrambler - To Ride in Anger

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.
http://www.youtube.com/watch?v=O9RM6ULnP7s

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 forum member 2.3efi 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 Tach/speedo on the way to replace the stock gauges. I'm going to turn a couple of plugs 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.

Re: CB900 Scrambler - To Ride in Anger

Looks like you getting a move on. Those wheels look nice. Just a word of warning if your not using the stock airbox be prepared to have a flatspot around 3500 4500 rpm and nothing you do will get rid of it.

Re: CB900 Scrambler - To Ride in Anger

notlob said:

Looks like you getting a move on. Those wheels look nice. Just a word of warning if your not using the stock airbox be prepared to have a flatspot around 3500 4500 rpm and nothing you do will get rid of it.

Click to expand...

The bike came with the pods and no air box. It's jetted for the pods with a dynojet kit and has the different slide springs and the additional hole drilled in the slides. I've read that a common air box is better and I can see why that would be true with the vacuum slides.

Any thought on whether or not oval dual flange filters would be any better? I might see if I can make a couple K&N RC-3510 filters fit. It would be a differnt look anyway.

The other option is to fab a bread box to fit a single oval filter that would feed all 4 carbs. This seems like a better setup to keep debris and water away from the filter(s) too.

Re: CB900 Scrambler - To Ride in Anger

Three things:

This bike is the least scrambler starting point ever. Making it into one, makes you my hero. I'm anxiously watching here.

I really don't advise getting indiv filters...
I have a set of KN RU-2922 filters and the original airbox. If you want either I'd be glad to send them, fit not guaranteed. I've yet to see a well fabbed breadbox but I'm more supportive of that than the above options. Running with air box first to eliminate variables is probably best. I ditched that step too.

Bead blasting is awesome... I really need to invest before I start another bike. Please keep building!

1981 CB750K (getting there now)

Re: CB900 Scrambler - To Ride in Anger

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.

CB900 Scrambler - To Ride in Anger

Looking good man, definitely going to keep an eye on this build.

Re: CB900 Scrambler - To Ride in Anger

[quote author=kraptastic k]
Three things:

This bike is the least scrambler starting point ever. Making it into one, makes you my hero. I'm anxiously watching here.
[/quote]
To be honest I almost talked myself out of racing this thing until I found this thread for a guy building a CB 650 Desert Sled for the Mexican 1000 this year.

http://advrider.com/forums/showthread.php?t=823952

[quote author=kraptastic k]
I really don't advise getting indiv filters...
I have a set of KN RU-2922 filters and the original airbox. If you want either I'd be glad to send them, fit not guaranteed. I've yet to see a well fabbed breadbox but I'm more supportive of that than the above options. Running with air box first to eliminate variables is probably best. I ditched that step too.
[/quote]

I hoped the air box would be in one of my spare parts boxes...no luck. I think carb spacing is the same for 79-8? DOHCs so if you want to donate your airbox to the cause I'd gladly pay shipping and give a report after putting it on the butt dyno against the pods.

My welding skills aren't up to making an airbox from aluminum but I think I could do something like this with composite.
I'd make a properly shaped foam plug, cover it with composite, then disolve the plug with acetone.

http://shop.teamdor.com/

[quote author=kraptastic k]
Bead blasting is awesome... I really need to invest before I start another bike.
[/quote]

YES!
I have my eye on a bench top unit for the garage.
http://www.righttoolusa.com/p/Abrasive-Sandblaster-Cabinet-With-Light-5205066.html

[quote author=DV2000NJ]
Looking good man, definitely going to keep an eye on this build.
[/quote]

Thanks!

Re: CB900 Scrambler - To Ride in Anger

I'd stick with the carb set up it came with. unless you want to fork out for a set of refurbished GSXR carbs or proper flatslide or smoothbores. The added problem with these carbs is that the diaphragm is alloy not rubber like most CVs and therefore more vacuum is need for them to operate correctly, making them more difficult (impossible in most peoples opinion) to tune.

By mentioning stance you reminded me on something else I was going to mention. Your inspiration bike has some long shocks fitted probably from an mx bike. If you look at some of the photos they are mounted upside down and have an eye at both ends which the bottom is fixed to the clevis mount on one side and out of line. Not very good practice and off road could be pretty dangerous. Your unlikely to get clevis shocks long enough to get your stance right so will have to make so extension brackets.

Have you also noted it has no rear brake.

Well done with welding, I wish my 1st time efforts were that good.

Re: CB900 Scrambler - To Ride in Anger

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.

Re: CB900 Scrambler - To Ride in Anger

I've just spent a couple of hours going through the CB650 build ;D
750 is really 'front heavy' by comparison to the 650 but good luck with it, I always like crazy builds

Re: CB900 Scrambler - To Ride in Anger

Worked on the seat pan last night. I've got it shaped and sized but still haven't decided on which way to fasten it. I'd like to do some 1/4 turn dzus fasteners but Ill probably just put studs on the bottom of the pan.



Sent from my EVO using Tapatalk 2

Re: CB900 Scrambler - To Ride in Anger

Seat pan pics.






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

Re: CB900 Scrambler - To Ride in Anger

notlob said:

I'd stick with the carb set up it came with. unless you want to fork out for a set of refurbished GSXR carbs or proper flatslide or smoothbores. The added problem with these carbs is that the diaphragm is alloy not rubber like most CVs and therefore more vacuum is need for them to operate correctly, making them more difficult (impossible in most peoples opinion) to tune.

By mentioning stance you reminded me on something else I was going to mention. Your inspiration bike has some long shocks fitted probably from an mx bike. If you look at some of the photos they are mounted upside down and have an eye at both ends which the bottom is fixed to the clevis mount on one side and out of line. Not very good practice and off road could be pretty dangerous. Your unlikely to get clevis shocks long enough to get your stance right so will have to make so extension brackets.

Have you also noted it has no rear brake.

Well done with welding, I wish my 1st time efforts were that good.

Click to expand...

Thanks for the extra input on the carb setup. I'm gonna run the stockers for now. I doubt they will hold me back. Did you run into any issues making the GSXR carbs work on your bike?

Yeah, good eye on the LHS rear suspension. If I go to a modern eye-to-eye shock I will weld a new shock mount to the swingarm. I can't see the off angle moutning being able to work well without binding. Suspension will probably be at the top of the list of "what next" after I get it on the road. Once I got the seat pan on I could already tell that more clearance on the shocks will be needed. I like your extenders. Those where super clean looking considering you made them with hand tools. Very crafty!

Did you ride the your 750 with stock suspension to have a comparison for the Hagons you put on it?

I think the drum rear on mine will be pretty good for what I want to do. (even though it weights a ton) I use both brakes pretty evenly so I can't imagine totally removing the rear. It does give a clean look though...

I had to weld about a cm at a time then stop and clean it and restart becasue I couldn't see what the hell I was doing.