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CB400F Cafe Build

This is a discussion on CB400F Cafe Build within the Project Builds forums, part of the Caferacer.net Forums category; Hey guys, Grant here again So I've decided to start a project build thread for my recently purchased'75 CB400F (thanks to everyone for helping me ...

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  1. #1
    Senior Member braveg's Avatar
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    CB400F Cafe Build

    Hey guys, Grant here again

    So I've decided to start a project build thread for my recently purchased'75 CB400F (thanks to everyone for helping me figure out just what type of CB it was) which I plan on turning into a cafe racer (I also got a spare CB400F bike in pieces which I will either try rebuild in time or use as spares). My experience with riding is pretty good, but actually working on and fixing bikes is very limited, but I have a growing workshop and a fervent desire to learn, so hopefully that will see me through to the end of this build.

    My intentions with the bike - get it running as smoothly as a 37 year old bike can run and then modify it to the look I want. In all likelihood it will be a daily run bike rather than a racer, but I sure wouldn't mind getting some real speed out of it and doing as many track days as my wife will allow...

    As for budget, not really an issue, can pretty much spend whatever I need on it, you only live once right?

    What has been done so far:

    Before buying the bike I naturally took it for a good few rides, it felt great but did not idle. The previous owner blamed it on the carbs since it had been ridden mostly on dirt roads.

    When I got the bike it wouldn't start, so I tackled the cleaning of the carbs first and nearly bust a gut trying to get the damn air box back on (any tips on a good way to do this without wanting to kill yourself?) and then charged up the battery.

    Still wouldn't start.

    So before I changed plugs, I decided to call the previous owner and ask if there were any "quirks" that I should know about. It was then that he decided to tell me that he had rewired the ignition switch to only start when on the "off" position. *sigh, coulda told me that before I pushed that bitch all over the suburb…*

    Anyhoo, I set it to "off" and she started right up, although she doesn't idle for shit, so I will carry on just getting the basic maintenance checklist sorted out before trying to troubleshoot too much i.e. new plugs, new oil, checking the points and fiddling around with the carbs.

    So that's it for now, would love to get your sage advice as I go through this project, will try not to ask too many stupid questions

    Tx G

    PS: for those of you that haven't seen it yet in my intro, here are some pics of the bike in its current state, more to follow in various states of undress as the days roll on...


    Be yourself, unless you suck...

  2. #2
    Senior Member kenessex's Avatar
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    Grant,
    I raced one of those for several years and put over 50,000 street miles on it. I bought it new in 1975. You can't get much more power out of one, so don't bother trying. Just get the engine running right and enjoy it. I had a pipe and K&N individual filters on mine. Spend your time and money on suspension and brakes. It has 33mm forks and a crappy front brake. I would upgrade to 37mm front end from a 87 -91 CBR600F and put the rear wheel on too. It will take a little fab work to get the rear disc set-up. Then run a 110-17 front and 150-17 rear tire.
    Then get some rear shocks for it. Dresda used to make a swing arm for it and there may be some floating around. I would alos convert it to a 520 chain. The front sprocket from a XL500s fits as does the rear sprocket from a Suzuki GS500 if you switch to the CBR rear wheel. I had clip-ons and a solo seat on mine.


    Ken
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  3. #3
    Senior Member o1marc's Avatar
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    Keep us posted on this build. I am picking up my 75 CB400F on Wed. What did you give for yours? I traded some powder coating work for mine. My buddy bought it at Barber 2 years ago for $300. He owes me around $400 for some powder coating so I told him to just give me the bike and I would call it even.
    Ken, how much power are you talking about? My buddy built a race one with a bigbore kit and made about 130Hp. I'll definitely be getting his advise through my build. I'm been following the CB400F on CL and Ebay and they are far and few between and bring pretty decent money. One just sold on Ebay and didn't even run and went for over $1k.
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  5. #4
    Senior Member gearheadE30's Avatar
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    That must have been a hell of a big bore kit to make 130+ hp. That's like modified GS1135 range.

    Grant, that's a nice looking find there. I've always liked those sideswept header pipes, and yours don't look too rusty. The first thing I'd do with a bike that's idling funny and having the issues you're describing is clean the carbs. By that I mean take them off the bike, strip them down, and let them sit in carb dip for a few days to get all the crap out of them. My guess is you'll be amazed at how much crap comes out. See if you can't find new o-ring kits for the carbs as well; by now all the o-rings have turned to plastic and don't do much sealing anymore.
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  6. #5
    Senior Member kenessex's Avatar
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    With a 458 kit, big carbs, ported head, yosh cam and yoshima pipe might get you 45 hp, max. It will also have a very short lifespan if you rev it to 12k rpm, which you will need to to make that HP. At that rpm, the rings flutter and break the top off the top ring land. Of course you will also have rod and rod bearing failure in there, too.

    Spend the money on suspension, tires and brakes.
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  7. #6
    Senior Member kerosene's Avatar
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    130+ hp on sub 500cc 70s engine is either a mistake or a silly claim. Ken's 45hp sounds sensible.
    -

  8. #7
    Senior Member braveg's Avatar
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    Ken, thanks for this goldmine of info, will certainly start doing some research around your ideas and figure out what kind of spares I can get my hands on out here. I am joining a few bike clubs in the area and will be attending as many swapmeets as I can to see what kind of goodies are on offer, so will keep an eye out for a Dresda arm if I'm lucky.

    Marc, I bought it for $800 which I know is a lot for you guys, but bikes and cars are generally a lot more expensive out here. Watching US TV shows, I always curse at how cheap second hand vehicles are in the states and UK compared to here, but hey it is what it is. The bonus is that I did get a stripped down second CB400 in spares (minus the carbs) so that does lessen the hit somewhat...

    Gearhead, yeah the pipes are in great shape, pretty much only surface rust, if any, on the bike in general. As for carbs, they were the first on the list for a cleanup which I did over the weekend and apart from a little bit of dirt in the bowls, they look to be in good nick. I could probably stand to change the seals and o-rings, but generally fine from what I can see. I don't think these are the original rings as the PO was fairly active with the maintenance, but once I have run through the basics like new oil, sparks etc I will then look at replacing all the rings and possibly the needles too if need be. Kinda feeling my way through, but having a significant amount of fun doing so

    Suspension, tires, brakes. Check.

    Be yourself, unless you suck...

  9. #8
    Senior Member DesmoDog's Avatar
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    quote:Originally posted by o1marc

    My buddy built a race one with a bigbore kit and made about 130Hp. I'll definitely be getting his advise through my build.
    I'm not sure I'd take his advice, because by the sound of it your buddy likes to tell you stupid shit just to see what he can get away with...

    Oops, strike that, a buddy just pointed out it might also be using ram air on top of the big bore kit.

    My bad.

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  10. #9
    Senior Member braveg's Avatar
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    hey chaps,

    So I got some new plugs today and put them in, the old ones were really sooty and black, which according to the manual means the engine is burning really rich or the heat value is wrong. Would that be as a result of poor carb setup?

    The plug from cylinder 3 was also quite oily from the look of it, does that mean the o-rings are shot?

    Here is a shot of the 3rd cylinder plug:


    Once the new plugs were in I started her up and she ran much better, even managed to idle for a little while but then died. I tried to start it up again but heard a strange squeaking sound on the left side of the bike. The bike has a fairly slow oil leak, so I'm a little concerned that it might be low on oil and that might be causing the squeaking and doing some damage, so going to clean her up real good to figure out where the leak is first, try fix that up and then change the oil.

    Another thing that concerned me is that the plugs didnt seem to go all the way in, the washers still seemed a little loose, but the plugs were tight. I didnt want to tighten further and strip the cylinder head, so hoped for the best. It seemed ok, but if there are any thoughts as to whether this is normal or not?

    Not terribly exciting reading just yet, but it idled for more than 5 seconds, so it's progress
    Be yourself, unless you suck...

  11. #10
    Senior Member kenessex's Avatar
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    The oil leak on the left side between the cylinder and head is from a porous cylinder casting. There was a recall on them in the states in 1976 only on the early 75 models. Don't worry about it, it just makes a stain but really doesn't leak. Just looks bad. The recall was to install new cylinders. Not worth it.

    The CB400F is very sensitive to a dirty air filter, which happens a lot because the crankcase vent plugs into the bottom of the airbox. There is a foam absorbent filter in the bottom under the airfilter and a metal mesh that gets soaked and soaks the bottom of the paper filter, causing the bike to run rich and your plugs look like the one you posted.

    I suggest a compression check and and leak down test. I don't think of anything to squeak on the left side, unless the alternator stator or rotor came loose. Has it been crashed on the left side?
    WERA 119
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    \"Think twice before you speak, and then you may be able to say something more insulting than if you spoke right out at once.\"
    Evan Esar
    Newbies, Geeto and Tex (Bye Tex)hate me!!

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