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CB750 K7 (1977) Restoration

Discussion in 'Modifications and Projects' at netrider.net.au started by jawntybull, Jun 13, 2011.

  1. My first restoration project - a CB750 K7

    I bought this CB750 last week from a classic bikes club member who had had it in the garage for a year or so since buying it from a guy in Queensland. The bike was meant to be a restoration project for the previous owner, but he also has a CB650 to work on so I picked up this bike in quite good cosmetic condition for a reasonable price (I think)... although it wasn't running when I bought it.

    I like the K series CB750 as it was the first ever production in-line four motorcycle, complete with the first front disk brake and nearly 70 ponies. It was ranked as #3 in the top ten bikes of all time by the Discovery Channel - so it has quite a history.

    At this point I know this bike needs electrical work, fuel work, fork refurbish, and a few odds n ends.
  2. My first restoration project - a CB750 K7

    I bought the CB750 last week from a classic bikes club member who had had it in the garage for a year or so since buying it from a guy in Queensland. The bike was meant to be a restoration project for the previous owner, but he also has a CB650 to work on so I picked up this bike in quite good cosmetic condition... although it wasn't running when I bought it.

    I know it needs electrical work, fuel work, fork refurbish, and a few odds n ends.

  3. First job - electrics. Installed a new battery, cleaned all the contacts, replaced fuses... electrics now working!

    Checked spark plugs; the gap is good and they're in ok condition. Points are also ok, although gaps needed adjusting. Lights work, horn works, hit the starter button and...

    The starter fires up; no fuel yet though.
  4. Checked all the vitals; oil, filter, fuel tank all look like the bike should run. Put in some fresh fuel and... the bike wet itself! Fuel is running out of the overflows on two carbies, and one of the drain plugs looks a bit dodgy.

    Pulled the carbies off, and the float heights were all over the place. Reset the heights and gave the jets a temporary clean


    I ran overflow hoses from the carbies and packed a few rags under them to catch the run off; fingers crossed, hit the starter and eventually got a cough, then a stumble, then brrrrmmm!

    The famous in-line four...
  5. Ok, so the inventory is three out of four cylinders running, two carbies leaking, spark to all four cylinders. Looks like a carbie rebuild.

    Even now though I like the sound of those four-into-four pipes! Click the video link below...

  6. Good for you. It's a great old engine and in the scramble to restore the first models, these later ones are much neglected.
  7. Nice one!

    And this forum is a great place to post your progress and get help when required.

    I also have a CB750, although mine is an F series SuperSport that I am customising.

    I also hang out at the SOHC Four forum, which is basically all about our bikes!
  8. Thanks guys; more progress today. The carbies are holding up ok now, although I still plan to overhaul them with gasket kits. At least they've stopped leaking fuel though, and the bike is starting to run well. My current problem is finding a couple of drain screws for the Keihin carbs. Its an M6x1 customised screw...


    Other progress today; overhauled and bled the front brake.
  9. Good on ya mate. Looks like a great starting point.

    That screw looks more like an idle screw or throttle stop screw? It should have a spring on it probably?

    Your leaking fuel/uneven running sounds like carbies for sure. They hate sitting for any length of time and if the fuel dries up inside the bowls as you have found, it creates a difficult to shift residue that causes the float bowls to stick and jets to gum up. Some carb cleaner will shift the residue and along with blowing all the jets through with compressed air should sort it out.

    I often find when a fuel float sticks like that, if you give the bowl a tap with the butt of your screwdriver, they magically stop leaking! Try it...
  10. Thanks Ged, I've had the carbies off and the floats needed a lot of work; one was not even touching the float valve. I've cleaned them all out using carby spray and now have two carbies working well and two working ok. I'll pull them out again when the overhaul kits arrive and replace all the gaskets too as some of them just fell apart in my hands last time.

    I know the screw looks like an idle screw but its definitely a drain screw; its right at the bottom of the bowl and is screwed in tight. When you open it up the fuel drains out of the bowl. On this particular model of Keihin this screw seats in under the bowl with the tapered end. Its an overkill solution and is giving me grief as these screws cannot be bought on their own; you have to buy the whole float bowl for $70. The last guy who had the bike lost one of them and put a normal screw in the hole, but of course without the tapered end it doesn't stop the fuel running out!

    My fallback is to try and buy a used carbie and salvage the screw from it... any ideas anyone?
  11. Hello Jawnty. Nice work! That's a very tidy looking starting point. Thank you for sharing your work with us.

    My 2c worth - replace the plugs. Don't bother gapping / cleaning / messing about - put new ones in it. Plugs that have been used a bit and then sat around, can and often do go 'off' and never work properly again. They might look perfect - that doesn't mean they are.

    Also - in the pics, there's no airbox or filter. I dunno how well modern bikes run without their filters and plumbing, but I know my '76 CB400/4 would hardly run at all without a filter. No filter can mean the jetting is a long long way out.

    Last tip from an old honda 4 owner - If you decide to adjust the camchain tension, make very f3ckin sure you know the right way to do it, because as a callow 16 year old I broke the tensioner trying to do it in what seemed the logical way to me. All these years later I forget what the proper way is, so look it up, but I went to do it in the way that seemed logical to me, and I screwed it.
  12. Putting kits through them will definately help.

    Hmmm. That screw looks so familiar! Probably common to heaps of keihins from that era. Can you find a manual/info for the carbs that might tell you what it is common to?
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  14. I cant see your pics! and if there are no pics, there is no progress. And i want to see progress! haha...fix pics plz!!!
  15. Thanks for the replies guys. Latest progress is mostly drawing up an inventory and ordering parts. The following are on order or have arrived already...

    Spark plugs
    Air filter
    Carbie overhaul kits (unfortunately don't contain the drain screw)
    Inline fuel filter

    Whilst the plugs look ok, I agree that its best just to change them. The inline filter is to keep the carbies clean once I spend all the time to overhaul them; I've been running the bike a little bit each day and the carbies have been getting progressively worse (particularly cylinders 1 and 3, which are now considerably cooler than 2 and 4); I put this down to crap in the tank.

    I checked the tank and sure enough there was a bit of rust; not too much but the fuel is basically unfiltered in these old bikes. There was even an old marble rolling around in there! I'd run a rust kit through the tank except the previous owner had it professionally painted and I'm nervous about getting the acid on the lovely new paint. My idea for this weekend is to get a bit of light weight chain, attach it to a stiff wire, and rattle it around inside the tank for a while to loosen up the rust. Flush several times then presto! Add the fuel filter, overhaul and install the carbies, then fire her up. Am I dreaming?

    Thanks for the tips on the drain screws. A mate of mine who does up old bikes has access to a lathe, so he's getting some M6 cap screws machined to match. I'm hoping the kits will have the o-rings for the drain screws. Also hoping the kits arrive today of course, otherwise I won't get far this weekend.

    Re the photos oohsam, not sure what the issue is. I can see them fine and I checked on a friend's computer (he's not a member) and they came up fine there too. Anyone else not able to see the photos?
  16. Did you have a look at the link?
  17. Sure did - and visually they are a good match. The US shipping might be high as you have to use "Bongo"? Anyway my friend with the lathe will have a go at making some this weekend, and if that doesn't work I'll order the ones in the link - thanks for the suggestion
  18. Started the carbie reno before work this morning. Took me 10 seconds to get the carbies off the inlet tubes, as opposed to 10 minutes last time - I warmed the tubes up beforehand with a hairdryer and it made all the difference.

    Float bowls all fully disassembled now; float needles were in poor shape, main jets were ok but the pilot jets were partially or fully blocked - this explains the cold idling in cylinders 1 and 3. I tried to unblock them with a fine piece of copper wire but merely managed to block them up further by pushing the crud into the fine jet hole (I believe its only 0.4mm!). Anyway I've got the jets out now (press fit; just pulled them out with pliers) and soaking in carbie cleaner. They're in a little glass jar which I shake every time I get bored with my work; makes a nice little jingling sound!
  19. I suspect it's a standard Keihin item, do you have a parts manual for the bike?

    If so, it will have a breakdown of the carbs & part numbers, then you should be able to find what you need online, or even from Mr Honda.

    Bikes looking good!
  20. You also risk deforming/enlarging them. Not good.

    Lots of carb cleaner and compressed air. Carb cleaner is hideously toxic by the way. I wash my hands everytime I handle anything that has been near it.

    Hairdryer on the rubbers is a good idea! They can be a real pain in the butt.