'71 CT70 ...saved from the grave yard

Discussion in 'General' started by scott s, Oct 24, 2017.

  1. scott s

    scott s Member

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    I pulled this out of the local bike boneyard yesterday. It's rough as a cob, but for only $40 I couldn't resist.
    Almost everything on it is stuck, rusted or rotten, but all I really wanted was the frame and some hardware.

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    So far, all I've done is dragged it into the garage....because it won't roll...and sprayed everything with PB Blaster in preparation for disassembly.

    I have lots of questions and could use some guidance. Below is a list of things I'd like to do and a couple of pics for inspiration. Anything you can tell me is appreciated.
    I'd like to do:

    * Fork swap. USD or conventional hydraulic?
    * Front disc
    * Aluminum swing arm, probably the extended one. What do you do about the brake rod and stay with the extended arm?
    * 12" wheels. I re-use my old rear hub. Do the disc swap front hubs accept the 12" hoops?
    * Big motor....140-160cc. Would also like some suggestions on brands/size for this. Lifan, Piranha, SSR, etc.

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  3. racerx

    racerx Administrator
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    Congrats!

    Looks like a lot of usable stuff for $40. What catches my eye is how straight everything looks. It's also amazingly complete. First step is determining how much has succumbed to tinworm infestation. Remove the seat & gas tank, then take a close, look at the inside of the frame. If it's solid, with no heavy, flaking, rust...or rust perforations...you've bagged a screaming bargain. Ironically, this machine appears to be a better restoration candidate than most...provided that the rust hasn't gone further than I expect, at this early, minimal-information, stage. Whether, or not, the frame passes muster, you're sitting atop a nice collection of parts. It's a question of what's usable/restorable/repairable/saleable. There will be some surprises. Starting with the worst of it, I'd toss the wheels. Don't even bother trying to remove the old tires. There's a good chance of one, or both, axles being rusted inside the bearings. I wouldn't be surprised to find the swingarm pivot bolt rusted inside the swingarm pivot bushings. You should be able to break it loose using an impact gun, which I prefer over a breaker bar...if you have one. Enough dis-assembly discussion for now, assuming that you'll ask questions if you run into something beyond your present CT70 skill set.

    IMO, if you're only looking to use the frame, tank, seat latch & hinge, floating brake lever (the one needed for the original type pedal), TL bracket, rear hub & brake assembly, there's probably 15X the acquisition cost of the base bike in saleable parts. See your custom project getting a little less expensive?;)

    As for chassis upgrades, more than just a good idea, imho. Stuffing horsepower into the engine bay is the easy & attention-grabbing part of the project. Once you add enough power to cruise at low freeway speeds, they are essential for safety...and usually given short shrift. A stock K0 rolling chassis has enough suspension for maybe 45mph, over very well-maintained pavement. K1-`79 can be oil-tuned well enough for 60-ish use. The stock drum brakes are okay up to ~55mph; with a little riding technique, that can be stretched to ~60mph, maybe a little more. The stock rear shocks...are useless for this kind of riding or/and riders whose weight exceeds ~130lbs. So there's your baseline numbers.

    I'm a fan of inverted fork legs. That said, I use them only because the best suspension performance available comes packaged in this format. If conventional, a.k.a. sweeper style, fork legs had the better damping and excursion numbers, I'd be using them instead. Can't tell anything about fork legs from photos and they're anything but created all the same. Quality and associated cost varies greatly; you get what you pay for, with any suspension item. The fork legs in your photos deliver far more visual appeal than performance. Unless the specs have recently changed, in a big way, they're too soft and come up short in excursion. A front disc brake, however, is a big improvement...something one discovers with their first 60mph panic stop...and you're talking about 60mph+ power. The rear shocks are critical, as well...if you're building this machine to ride.

    As for Chinese engines, I leave the recommendations to others who've gone that way and have accumulated some real mileage on them.

    For the moment, I'll stop here. This is a process that's too long for a single post. Figure out what you're starting with, set some goals & a budget, then we can help you one sub-project at a time.
     
  4. scott s

    scott s Member

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    Sounds like we're on the same page so far. With my limited knowledge of this model and the road I'm taking, anyway.

    I was hoping to save the rear hub to bolt up to a 12" hoop. The front disc kits come with a hub center. Even if the rear hub is too far gone, those are readily available.

    If I'm not mistaken, this model has the plastic gas tank, so that's a (possible) plus. The rust isn't too bad, with only a few crusty areas but no real "cancer". I'll know more after a day of taking everything apart.

    Part of me wants to just polish out the paint and treat the exposed areas and keep it just like it is. Another part of me says "That's a tiny frame....it wouldn't cost much to have it painted some wild metal-flake color"!
     
  5. racerx

    racerx Administrator
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    I've no idea where you're located. That said, here in the "Great pothole state" of MI (where pavement goes to die), the fine art of self-preservation is not to be taken lightly. For that reason, I want all the sidewall height I can get...which isn't very different from stock. That means no 12-inchers. That extra inch, or so, of tire protects the rim & hub...also adds some much-needed plushness to the ride quality. I've bent rims and a stock rear hub, shod with 4.00-10 Trailwings, from uneven expansion joints at sub-50mph speeds.

    You can use the stock rear hub, I did so for better than 20,000 miles. As long as the brake drum surface is reasonably smooth and within max ID spec, you're good to go. Adding 2-3x stock power changes things dramatically, but the indications that something is nearing the end of its service life can be hard to recognize, without experience. Just keep close tabs on the hub nose (that centers the sprocket), rubber dampers and C-clip land. The sprocket should fit snugly on it's bore and have minimal lateral wobble. When the hub nose is worn out, the sprocket will wobble excessively. That'll first manifest as uneven chain tension (when you do the adjustment) followed by progressively heavier shaking at high speeds, from chain whip. A certain amount of uneven chain tension is normal & unavoidable. It's learning to ID when it's gone too far that matters. Use quality Japanese (NTN, Koyo, Nachi) or German (FAG0 bearings and replace the dampers at 4,000 mile intervals and you should be fine; they should still look okay and be fine for a stocker. Draw a pencil line through the C-clip gap, then check for C-clip rotation after each ride. Won't take long to learn what excessive rotation (in degrees) looks like. 1mm of sprocket clearance is ragged-edge, any more unacceptable, imo. Dry-fit the sprocket, without the rubber dampers; if you can feel any radial slop, by hand, source a new hub. If the snapring no longer fits snugly in its groove, the hub is toast. This may come across as alarmist and it's not meant to be. The hubs are typical Honda quality, rarely fail, and few have anywhere near enough miles on them for this to be an issue. As best I've been able to determine, rear hub life is more than adequate. I replaced mine after 19,000 and have no idea how many miles it had covered before I bought it. I've only seen a single example that failed...mileage unknown. Few of us have ridden to this mileage level. The rider noticed the rear sprocket was about to leave the hub and replaced both before disaster could occur. The point of these ramblings is that it can happen, so be aware. Mine was nowhere near that stage but, I lived with chain-whip (and the annoying high-speed shake) for 3 years.

    BTW, I forgot about your swingarm question/comment. Unless you're going to run a significantly taller-than-stock tire (120/90-10 is the practical limit and has other issues) run a stock length swingarm. Aftermarket aluminum alloy has its advantages, especially the quality JDM one...most notably G`Graft. There's no paint to chip (they have plastic rub strips), the pivot bearings are superior (improving stability) and, let's be honest, they're nice bling. Even with a ~19-20" tall tire (the absolute limit) anything beyond +2CM looks ridiculous on a CT70...like a foreign object being dragged behind the bike. The CT70 frame imposes some strict limits on what can be successfully executed. IMHO, it's working within those tightly-restricted parameters that makes a well-done project satisfying.

    As for color choice...it's your bike and since it's a full-on custom...whatever you like best is the right one. Not everyone will agree with you. I reckon that universal agreement on color is never going to be possible, even with a 100-point purist restoration (meaning a stock color, too) so yours is the only opinion on the subject that matters.:whistle:
     
  6. scott s

    scott s Member

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    I'm in SC and the roads aren't too bad around here. There are two man reasons I'm thinking about the forks/extended arm/tall wheels combo:
    - Extra stability at speed with the increased displacement
    - Extra height, as I'm pretty tall
     
  7. racerx

    racerx Administrator
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    Yes, without the heavy-duty freeze/thaw cycles and road salt damage, you must have some nice roads to ride. That makes a difference.

    In terms of bike-to-rider fitment, you're really more affected by the handlebar and seat configurations than anything else. That said, an inch or two, here & there can really add-up - potentially into a well-tailored fit. The biggest impacts are to be had with handlebar configuration - height, width, fore/aft grip location (relative to the rest of the bike). Some cutting, sectioning & welding can easily get you added height & grip width. I'd recommend 26" overall width, it's possible to go beyond that but, that's the widest I've seen, where stock K0 was closer to 21". Height is fairly straightforward, too. Moving the grip centerline forward, however, would be much trickier and other angles could be affected by changing this. If you're dedicated enough and have access to the fabrication talent needed, a pair of custom-fitted bars could more than worth the investment. BTW, width and fore/aft positioning can have a major impact on stability, as well as comfort.

    Next up, the seat. It's possible to "sneak in" an extra inch+ of foam and gain some height...along with comfort. A real pro could configure the foam and stitch up a custom cover. Guys like that are few & far between...underrated for what their talents can do to improve the riding experience, too, imho.

    Which brings me back to the suspension. Stock fork length is 600mm. You can go to ~625, (650 is possible but, you'll get a chopper-like stance, shifting the weight rearward in the process (...just what you don't want) as long as you source taller rear shocks. Stock shock length was ~330mm, you'd be surprised just how much difference going to a 340mm shock makes. You can go to about 350mm, before chain clearance becomes an issue. Taller tires will make a difference, too. Stock Trailwings are ~18" tall, 20" is the practical limit. I've split the difference and gone to Bridgestone ML16/17 series tires which are right in-between...with the same nominal 4.00-10 size.

    Stock saddle height is ~28", on a K0. My customized daily rider has a saddle height just over 30"...running 340mm shocks and fork legs with an installed length of ~625mm. With 650mm fork legs, 350mm shocks and taller tires it should be possible (and require an extended swingarm) to get another inch, or so, of saddle height. That's what I meant by those one-inch changes adding-up. You still won't really need more than a +2CM swingarm; the rear fender will have to be relocated to clear the tire.

    There's another factor lurking in suspension, namely, compressed static (rider aboard, bike not moving) height. With the stiffer spring rates needed to match adult-weight riders, the suspension will sag less, regardless of unladen height (no rider aboard) giving you functionally taller ride height.

    It's all about small(ish) changes, in multiple areas, to get the desired overall improvements and it can be done. You seem to be thinking along those lines from the get-go, an excellent head start. That kind of overview helps to bring your goal(s) into focus.
     
  8. scott s

    scott s Member

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    Man, I completely failed at keeping this updated. :whistle:

    I ended up stripping it down to the frame. That, and the gas tank, are about all that was really usable.

    Rat's nest inside.

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  9. scott s

    scott s Member

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    Had it painted Purple Haze and installed new decals (a hint of what's to come).

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  10. scott s

    scott s Member

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    New EVERYTHING followed.

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  11. scott s

    scott s Member

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    And there she is!
    Now titled, registered, insured and street legal.
    New USD forks and front disc brake. 12" wheels and tires. +2" swing arm. Piranha 140cc with Mikuni VM26 carb and CHP exhaust. Gearing to take it to 60+ MPH.

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    Since those pics were taken I've updated the kick and shift levers and the drive chain.
     
  12. scott s

    scott s Member

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  13. scott s

    scott s Member

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    It won first place in its class at Cyclemania last summer and got a feature in Full Throttle magazine.

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    And I hauled it down to the Barber Vintage Festival and used it to get around the facilities for the weekend.

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  14. scott s

    scott s Member

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    I used the carb from eBay. It's a genuine Mikuni, but it's my understanding that there are two versions. I'm thinking about getting the one from T-Bolt supply. I think that the one I have has a different needle and slide and it can be a little hard to start at times, especially after it sits for long periods.
    I've tuned it and jetted it as best I can. It runs like a Banshee, but there's a mid range "boost" when the slide and needle really come into effect. And the mileage could be better, I think, even though it's not particularly rich.

    I'm told, but I don't know, that there's a 2 stroke version of the VM26 and that may be what is being sold on eBay. The needle and slide are wrong/different, and there's an "air jet" nipple on the carb that isn't on the T-Bolt carb.
    If $110 will get the right carb and make it even better, that is worth it to me.


    I'd also like to switch to LED lights for a little better battery life. There is a charging coil and it charges, but it's marginal and the battery is tiny. I only have brake/tail light and headlight.
    Does anyone know of an LED set up for that headlight bucket?
     
  15. AtLarge

    AtLarge Member

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  16. cjpayne

    cjpayne Well-Known Member

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    Im a purist at heart, but this bike rocks!!!!!!!!!!!!!!!!!!!!
     
    #15 cjpayne, Feb 12, 2019 at 1:25 AM
    Last edited: Feb 12, 2019 at 3:31 PM
  17. MountainMini

    MountainMini Member

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    Congratulations on the first place trophy. A very stunning bike. Nice work.
     

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