'81 CB750KZ Brat

Kezert

New Member
Hello folks,

I'm new on DTT, my name is Max and I live in Holland. This is my second bike. Before this project I owned a Kawasaki VN750 which I restored partially.

Lets get started!

About a year ago I fell in love with the looks of this bike:
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So I figured I just had to have one, in that style, myself. A 1981 model CB750KZ DOHC was bought.
Bike runs good and has quite low mileage so little work has to be done on the engine side of the project.
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Also bought a 1982 DOHC CB750F2 as for spare parts for a (really) small price:
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I have made quite a bit of progress already, so I will be posting some updates every now and then.
 
Started with tearing the 'donor' bike down. This bike is in really bad shape,
I bought it for a small price so it will just be for occasional spare parts like nuts and bolts if I need them.
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Been thinking about where to put stuff like the battery and other electric components.
So I then started with tearing the project bike down. (Unfortunately I don't have many pictures of that)
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Until I was left with an engine, two wheels and a frame (almost).
Lowered the forks a bit (I thought it was around 4cm) using this guide:
http://www.xs650chopper.com/2009/06/mulligan-machine-lower-your-xs650-forks-low-buck-garage-tech/

Heightened the back (about 5cm I think) with some homemade shock risers.
I used the bottom of the CB750F2 shocks (which were already ready for demolition), welded a piece of metal onto it and drilled a hole in it.

Bought a raw aluminium 19 inch fender without bracket so I started making a front fender bracket out of this old front fender from the CB750F2.
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I cut it apart and took out the internal metal bracket. Formed it into shape using a vise and a hammer.
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Then started grinding/cutting/welding the frame. All the unnecessary tabs were grinded off.
Then started welding on the loop and welding in some reinforcement brackets. Drilled some holes in the brackets for seat and rear fender mount.
Also welded a battery tray for the small Lithium battery to fit under the tank.
And got a leather seat made by a local lad. Ended up like this:
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Picture of seat:
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Use that f tank, then you won't have badge brackets to deal with. I'd also use the f swing arm, wheel, and caliper to give you disc rear. It makes no sense not to, also use the f forks. In fact use all the f suspension. It's way better than the k suspension. Although I just noticed that swingarm on the f is pretty rusted. But those are cheap on ebay.
 
generalyen said:
Use that f tank, then you won't have badge brackets to deal with. I'd also use the f swing arm, wheel, and caliper to give you disc rear. It makes no sense not to, also use the f forks. In fact use all the f suspension. It's way better than the k suspension. Although I just noticed that swingarm on the f is pretty rusted. But those are cheap on ebay.
Thnx for the tips Generalyen. Sadly I'm too far into the project to change things now.

Made a mount under the swingarm for the regulator/rectifier.
Just a tab of metal with holes and thread in them.

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Bought a Supertrapp exhaust. I'm using the 4 into 1 exhaust system from the CB750F2.
Need to make some sort of bracket for it. Because the muffler sizes don't match unfortunately.

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Some things sent to powdercoater.

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And some things sand blasted.

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Did a top end revision on the engine. Some pics:

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Got some brackets cut for the bike.

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Painted the engine with engine paint.

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Got some parts back from the powdercoater. Felt like christmas eve.

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Battery tray picture.

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Putted some tires on the wheels (BT45). Also fitted new bearings.

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Putted new bearings (front and back) in the rear fork.

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Time for a little mockup. Fitted new steering bearings, assembled the front fork with new seals and oil.
Engine covers are from the F2 as I don't want to scratch the new ones just yet. Saddle and tank just mounted loose for the picture.

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looks hot but you are going to have some seriouis chain slack issues .you cant just adjust the chain like normal,nope, having the swingarm dropped you have to leave a ton of slack in the chain
 
xb33bsa said:
looks hot but you are going to have some seriouis chain slack issues .you cant just adjust the chain like normal,nope, having the swingarm dropped you have to leave a ton of slack in the chain

Thnx xb33bsa. I don't really understand what you mean though, but I have bought new sprockets + chain so I don't think it will give me a problem. I will just have to cut the chain to the right length I think.

armourbl said:
Nice work. You seem to be moving quick. Have you done one of these before?

ben

Thank you. This is work from the last couple of months, I already did a lot of work but haven't finished the bike yet. And no, this is the first bike I'm making into a caferacer. Just did a ton of research on the webs haha.

SilkySmooth750 said:
Looks great man! I'm following this one for sure!

Thnx man!
 
Badass way to make that tank work!! the line at the bottom turns it into a completely different tank.... Gohddang!

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XB's right on duh chain mang... the counter sprocket is static and the swinger goes up and down yes??

Look here: and see if you might be too lowrider on the swinger.
 

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great diagram and as you can see look at the top one a stright line is between the 3 axis this is what determines how much chain slack you must have and is what the oem spec goes by
because as the wheel travels above or belo this straight line the chain becomes looser because the rear axle is getting closer to the cs sprocket
so if the stock spec is say 1-1/4" slack while on the centerstand this is because when the wheel comes up to the 3 axis staright line it will be near dead tight
dropping the swingarm creates more slack that cannot be adjusted out because you will break something i hope this was a good splainin
dropping the swingarm only 1" could easily create an extra 1-1/2" slack that you cannot do anything about
except a roller tensioner
this all is a very easy thing to actually observe put the bike up on centerstand unhook shocks and move swingarm thru travel
all new motorcycles (evenj street bikes )ever since the early 80's or so have had a gearbox re-design to put the c/s sprocket as close to the swinger pivot as is posasible in order to have as little chain slack variation as is possible and the other bonus has also been the ability to use a longer swingarm without stretching wheelbase win win
this change all started in motocross, during the long travel suspension revelution era 1974-1980
 
The added chain slack and swing angle will also most likely cause the chain to hit the frame coming off the front sprocket and the swing arm entering.
 
Thanks for the support guys. Gonna make new shock risers and maybe some kind of roll tensioner.

Did a lot of work on the bike in the last couple weeks.

Read on the webs that CBR600 coils would be a great upgrade for the bike so I bought some. Coils came off a running bike and measured good on the multimeter.
You can use the original brackets from the CB750, you just have to drill a hole with thread in the frame to make them fit.
Or you could fabricate new brackets but thats a hell of a lot more work.

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Also further assembled the engine.

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Mounted the fenders, lights and the speedo.
Had to bend the headlight & speedo brackets a bit to fit.
Brackets will get a tan soon.

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For the front turn signals I designed a simple bracket and got it cut so that they fit on the lower steering stem.
And I'm mounting the rear turn signals on the round bit of metal behind the rear shocks.
Had to buy some M12x1,25 thread for signals to make them fit, the original thread ends that came with the turn signals were too short.
Got on the lathe and drilled a hole in the thread for the wires.

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Assembled the front brakes with brand new pistons and rubber rings.

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Test fitted the exhaust and some other things.

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Next up is wiring!
 
The bike us looking great, keep up the good work. Let me know if your ass slides around on seat when you really get on the throttle. I've always wondered if that would be an issue on these brat bench seats.
 
Thanks guys. Time for an update.
I honestly forgot to make some quality pictures off the process the last couple of weeks.
But here is what I've been doing the last couple of weeks.
PS; Haha, I'll let you know generalyen.

Bike is completely re-wired now with new Lithium Shorai battery, lights, flasher relay, handlebar controls,
starter button + ignition switch relocated in tank, Acewell speedometer, CBR coils etc. Got some little things left to do.

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I used waterproof connectors and braided line where possible for a nice clean look.

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Painted the exhaust and made a bracket for the Supertrapp muffler.

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Installed the grips, clutch cable and gas cable. Brake system assembeld and bleeded. The choke has a new place on the steering stem plate.

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I wasn't happy with the brown saddle so I turned it into a black leather saddle.
Made a new tank bolt to secure it to the frame. Takes little space in height so the saddle will fit nicely above it.

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Tank has been sprayed. Here is a picture of how far I am now with the bike.

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I need to do some small things like bracket for the brake lines, install buttons in tank, spray the emblems, make a chain guard thing, etc.
I'll keep you updated. Let me know what you think.
 
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