cb400 rat/brat/cafe

cwatkins

Not fu%&!n 'round since 93
Picked up a cb400 for a good price last summer, full carb clean got it running nice and strong. I've finally got the time and money to make a build happen so I'd figured it was time to get crackin and get a thread goin! I'm leaning more towards a bratstyle bike but the tail end is... difficult. I'm considering doing a new subframe but I have no welding ability. This weekend I'll be tuning and timing the engine then all my jets will be here by next weekend so I can ditch the air box
 

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Re: 78 cb400 first build

podsjetsandmuffler.jpg


Got jets, pods and mufflers in today! If I have time between class and work it's goin on the bike tomorrow, a few inspiration pics, these bikes have the look I want

This one's probably my favorite. This shape (with better tires) is exactly what I'm looking for.
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or just a slight frame modification
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Rear shock relocation
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or no frame modification
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Re: 78 cb400 first build

Got air cutoff diaphragms today, carbs are fully assembled and ready for the bike. I'm starting jetting at 78/112.

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78 cb400 first build

Most frame tubing on these bikes is 1" OD
 
Awesome, would 17 gauge be heavy enough or should I look thicker since the rear shock is putting stress on the frame?
 
IMHO 17 is too thin for most any frame work. That's .058"
I use .095 or .120
 
Good to know, I've never done any sort of frame modification before. I can do all of the force calculations but idk about materials!

This is the modification I'm looking to do on the frame. cut at the yellow lines and add new frame in the red. I'd really like to try for the look of the first bike in my inspirations, just a little more complete and refined.

framemod.png
 
Should be doable.
Are you a competent welder? Make sure that you slug the frame where you put new tube.
Your upward tubes need to be notched to fit the horizontal tube.
And your shock mounts are crucial your welds need to be especially good there.
 
I've done a tiny bit of stick welding... I definitely don't feel comfortable welding a frame. I'd design and bend the pipe then have a shop or a friend weld it up for me. Is there anything I should keep in mind when bending as far as structural integrity goes? If anyone knows good welders in Salt Lake info would be more than welcome.

This is all down the road of course, just trying to plan the build and nail down a ball park price range. Immediate plans are
1. Timing
2. Valve Clearances
3. 12" Emgo shorty exhaust
4. Carb Tuning
5. Bleed Front Brake
6. New Rubber
7. Fork Rebuild
 
Sonreir said:
Moved to Specials.
Thanks, Still not sure what direction the tail end's goin haha, I might just work with the frame instead of fighting it. I'll find another bike if I want a true cafe.

Had a few hours to tear into the bike, checked valve clearances and timing. Managed to finagle the carbs, pods, and Emgo Shorty Exhaust on. Started fourth kick then first kick from then on. It sounded damn good with shorties! Really inconsistent idle right now, revs up and then goes really low then back up.... Could that be due to incorrect jetting or do I have another issue?

I think I'm gonna make a sort of velocity stack using a 2" Fernco rubber PVC joint and add some of the foam/screen in there to give a more consistent idle unless anyone has another suggestion...
 
UPDATE: So My battery is completely dead, so once it's charged I'll see if that stabilizes the idle. The gas was pretty bad too... The tank looked clean through the hole but there's muck in the bottom. I've got another tank that's for sure clean, It's gonna be a bitch to fit but it looks much better on the frame.

As far as filters go, I found this bad boy at autozone. This setup should give more of a laminar flow through the carburetor, a sort of mix between pods and velocity stacks.

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Engineering student?

As far as the frame modification... 1/8th wall DOM tube... 1" OD. Dont forget to slug the frame where you cut it.
 
vbtravisd said:
Engineering student?

As far as the frame modification... 1/8th wall DOM tube... 1" OD. Dont forget to slug the frame where you cut it.

Right on the money haha, Mechanical all the way. Thanks for the advice, I didn't know about slugging the frame.

I keep going back and forth on the tail end, SimJen did a sweet build, his tail section looks super clean and all he did was cut behind the shock mounts. I think I might work with the frame instead of fighting it, if I really want a different look I'll jump up to a bigger bike with a proper frame. These bikes were more meant to be cruisers anyways. This build thread is a mess of indecision.

Here's a picture of his build:
 

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Force calcs and laminar flow clued me in. Im a civil... But yeah, slug the frame with a tight fitting piece of tube, rosette welds on each end. If you just butt weld those together it will fail eventually.

Thats a good looking bike, not crazy on the frame modification either.
 
Battery's all charged up, my tank has shit and surface rust all over the inside. What's the cheapest easiest method of cleaning the tank? any suggestions? As far as the build goes I've finally found the look I want. I was browsing and I found silverstonepgt's streetfighter. This tail end is BEAUTIFUL and I've been in love with this look for a long time. I wasn't sure it could be done with these bikes but hope is restored....
 

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cheapest... Distilled vinegar. Fill the tank up, let it sit for a day... shake the piss out of it and pour it out. There are a few threads on here about it.
 
awesome thanks!! I'll give that a try tomorrow. I have a cb750 tank and it's a pain in the ass to fit properly.
 
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