CT70 k0 Restoration/Mod

Discussion in 'Tech Area' started by socalbuff, Sep 20, 2018.

  1. socalbuff

    socalbuff New Member

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    I have been cruising around these forums for the last month. I then pulled the trigger on an early 70's ct70 k0. It is in need of some serious work but I knew this before getting into this project.

    My plan is to keep the bike as original as possible. I do plan on make a couple modifications (88 kit, clutch conversion, 4 speed, and blinkers)

    I have 4 question at the moment that I hope some people on the forums can answer for me.

    1. The bike came with a plastic gas tank. I can't find information if that is the correct tank for the bike. Was a metal tank the correct tank for this year of bike?

    2. I am planning to put a four speed transmission. I have been able to source both a used ATC70 4 speed transmission and used CT70 transmission on eBay. I could also purchase a new AHP 4 speed which has a lower final gear. This bike will be mainly used on roads with speed limits at 45. Just not sure if I should go with AHP with a lower final gear or stick to more reliable Honda parts. Any input would be great.

    3. The bike wiring is shot and needs to be replaced and while I trying to stick to original. I place reliability and usability above original parts. I am considering converting the system over to 12v system. My reasoning is because of my neighbors ct70 lights and blinkers are weak if and when they decide to work. Also it seems to be easier to get 12v parts. My question is should I stick with the 6v or move up to the 12v?
    If I do decide to go to the 12v will these parts work or am I missing something?
    (http://mytrailbuddystore.com/tb16212vtrouble-freewiringkit.aspx "Wiring harness + parts",https://www.ebay.com/itm/HONDA-CT70...998284?hash=item5409c28e8c:g:2qUAAOxyhS9TfchT "cdi cone")

    4. Last question the fork ears are bent and I am not sure how to straighten them back out with out breaking them. Any ideas would be awesome as I would like to keep the original fork.

    [GALLERY=][GALLERY=][/GALLERY][/GALLERY]

    Thank you guys
     

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  3. racerx

    racerx Administrator
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    Welcome aboard!

    They all had plastic gas tanks until midway through the K1 model production run. You'll want to clean the inside and replace the cap. Since it's plastic, you can use laundry detergent, Simple Green, dishwasher detergent or pretty much any other water-based cleaner and get good results. The caps are a bit perishable...the rubber parts can turn brittle over time, but it takes a long time i.e. many years. Since they are still available new, no biggie.

    Nothing wrong with any of your transmission choices. OEM parts, especially NOS, aren't getting any easier to source, or cheaper. There's nowhere near enough longterm field service history to pass judgment on the AHP tranny. It has been available for a good decade+ now. So, obviously, they don't crumble. What I'm referring to is how they might fare beyond the 5-digit mileage mark. Of course, at a buck-and-a-half, replacement isn't going to break the bank should that be necessary. IMHO, the gears should be fine...it's shift fork durability that's unknown. The one complaint of which I am aware is noise. The OEM Honda gears are, reportedly, quieter. It doesn't seem to bother many riders.

    In terms of what we're calling final drive ratio, that's mph/1000rpm, or revs per mile, in top gear. Doesn't matter what the tranny ratios are, the engine makes a limited amount of torque and that's what determines how tall the gearing can be. For road use, you want to be right at the limit. Stock, these bikes were geared for ~4.77mph/1000rpm...regardless of transmission; they just used different sprocket combos to match the primary + top gear ratio. Adding a 52mm bore-up (88cc) kit, will add a little torque but only enough to upsize the countershaft sprocket by 1 tooth, max.

    Based on what you're trying to do, I'd recommend adding a hotter cam such as the "fast road" cam offered by dratv. It's really just the SL/XL70 grind but, it'll wake-up an 88cc kit, especially when paired with advancing ignition.

    Early CT70 wire harnesses are readily available, as new reproductions. System voltage doesn't really have much of anything to do with which harness you choose. It's more about bulbs, the battery, reg/rec unit. 6v stuff started becoming obsolete ~35 years ago. Now, circa 2018, 12v is the long-established norm so, naturally, that's where the good aftermarket options are. 6v options are limited, to say the least. You're right about the OEM turn signals, the old flasher relays never worked that well, consistently. The switching is done via bimetal strip and getting it to heat, at the intended rate, depends upon running exactly the right amount of current through it. All it takes to derail that train is one blown bulb, a replacement with a slightly different wattage, or change in system voltage. IMHO, a dodgy setup. Electronic flasher relays are wa-a-ay more reliable but...you guessed it...12v items.

    The xrarespres kit, when it works, is the cleanest way to convert a breaker point ignition to CDI. Be aware, this is a newer generation of this seller's product. The earlier one(s) were problem-free. There have been mixed results with the later iteration(s). Some could not be made to work and I'm talking about LH members who know their stuff. IDK if the seller resolved the defect(s)...however...there have been some very recent success stories and failures haven't been reported.

    This is where electrical conversion gets a bit complicated. There are 3-types: 6V/3-speed, 6V/4-speed and 12V. Each requires its own matching crankshaft and alternator assembly. The most common, the 6V/3-speed is the only one lacking spark advance. Changing the crankshaft is a big decision. The only inexpensive new crank is 6V-4-speed type. Next up, it's a toss-up between having a 3-speed crank rebuilt or swapping-in a hondatrailbikes 51mm stroker crank...in your choice of flavor. IMO, the 12V/CDI setup it the best...also the most complicated & expensive. Any 49-72cc motor can be used, for the lower end parts. Only the 1990s era CT70 version had lighting coils...and those are rare. There are aftermarket stators, available as kits, such as those sold by Ricky stator. That's going add a couple of C-notes. The 12V crank also had a longer connecting rod, so you'd have to find a shorter "12V" piston with the revised compression height...that also fits the larger "6V" combustion chamber. And, you'd need a 12V flywheel cover to clear the wider 12V flywheel. See what I mean?

    Before getting any further into an already overly-longwinded post, I suggest closely inspecting your existing crankshaft. Find out if it's in good, usable, condition...or in need of rebuilding/replacement. The answer to that question may help to inform other decisions. Taking things in logical steps will also remove what may, at present, seem like overwhelmingly complicated choices.
     
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  4. Gary

    Gary Active Member

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    I have put in a H motor with a 88 kit and fast road cam. Still using the oem carburetor too. Just run it around the yard with a couple of blasts down the road for good measure. Very happy with the results. If I was to do it again I'd do a crank and bigger bore and most likely not be happy with it because of where I ended up using it....
     
  5. kirrbby

    kirrbby Well-Known Member

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    If you're not trying to do this on the cheap, I'd say cut right to the chase...put your original engine on the shelf and build a 12v stroker from scratch.

    45mph roads means you'll want to be able to go 55 for long stretches, and go 60 in a burst of...get outta this fools way.
    88cc means low 50's tops. Low 50s means you'll likely be pulling over to let people pass you on occasion. And you'll constantly be tempted to run your engine wide open to maintain speed.

    107cc will likely get to high 50's, maybe 60.
    I think you might need to consider 117cc if you plan to ride mostly on the street.
    This will add a lot of parts, work, and money to your project. But, I think that's where you will end up eventually...45mph roads in SoCal? Do folks follow the speed limits out there?
     
  6. kirrbby

    kirrbby Well-Known Member

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    Btw, welcome to lilHonda.
     
  7. OLD CT

    OLD CT Well-Known Member

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    You can go around 63mph on a flat road with a stroker 108cc bore up kit. No need to straighten out the fork ears, shelf them if you want to go that fast. You will need a ''better'' K1 complete front end if you want to ride 45mph or faster on the street. One little pot hole could spell disaster with the front end you have. There is a aftermarket upside down front end available also. This project is going to get expensive quick. Real suspension front and back coupled with 60+ mph top speed costs.
     
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  8. kirrbby

    kirrbby Well-Known Member

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    I'm sure OLD CT is right about that 63mph 108, I just don't trust that I could get mine to go that fast. I think some folks can make a 88cc go 60+. Heck, whereshaldo is rockin 72cc all the way to work and back almost daily, rain, hail, sleet or snow. lol He has been squeezin every bit of speed he can find out of his bike.

    As racerx always says...you have to decide, what are your expectations. Then MAKE it happen.
     
  9. OLD CT

    OLD CT Well-Known Member

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    Pretty sure a remember a CL70 with a 88 kit that went close to 60. Bigger wheels help a little, I suppose.
    I had a 79 YZ80 or ''79cc'' 2 stroke that went 65. it was rated 14.5 hsp on the title.
     
  10. Tripod

    Tripod Member

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    Welcome. Ask your questions, do your research, THEN buy parts.
     
  11. socalbuff

    socalbuff New Member

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    Thank you for all the replies. I am in the process of tearing the bike down and planning on what to do still.

    Also OLD CT I had similar ideas on the front fork and I have been looking into either K1 and K2 fork.
     
  12. racerx

    racerx Administrator
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    K1-`79 fork legs are mechanically identical. The one difference of which you should be aware is the fact that K1 fork tins (HL ears, chrome trim rings & lower covers) can only be used as an assembly; they don't interchange with the K2-`79 fork tins + rubber gators. IOW you can swap K1-`79 fork legs; the only difference there is that the K1s lack locating grooves for the lower ends of the gators. But, if you want to run the gators, which are more durable (rubber doesn't chip or scratch) you will need the K2-`79 chrome trim rings (for the upper gator mounts) and those can only be used with K2-`79 HL ears.
     
  13. socalbuff

    socalbuff New Member

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    "Based on what you're trying to do, I'd recommend adding a hotter cam such as the "fast road" cam offered by dratv. It's really just the SL/XL70 grind but, it'll wake-up an 88cc kit, especially when paired with advancing ignition."

    In my search of the cam I have only found two cams on DrAtv site that seem to fit in the ct70

    Bearing Type Race Cam
    http://dratv.com/bez5atcttrxl.html

    And Longer Duration Power Cam
    http://dratv.com/lodupocam.html

    My question is which of these are cams are better?
     
  14. racerx

    racerx Administrator
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    That's the heart of the matter...with 6v heads, there aren't many cam choices.

    The hybrid (bearing on one end) cam requires valve springs that can accommodate the increased lift without going into coil bind, not a biggie just don't think that you get by with the stockers. It's more of a hp (high-rpm) grind and that will make the bottom of the revband soggy. If the needle bearing race doesn't fit your heads cam journal tightly (i.e. must be pressed into place), the head is too worn and you'll have a mess to deal with. Otherwise, I've installed these into healthy 6v heads without incident.

    The non-bearing cam is usually the better choice, for most, especially if you're going to leave the stock carb & exhaust as they are. With an 88cc kit, expect a top speed around 50mph with no real difference in acceleration/pulling power down low. That displacement increase should allow you to upsize the countershaft sprocket by one tooth...not only optimal gearing but where half of the speed gain comes from. Lower revs-per-mile doesn't hurt, either.
     
  15. socalbuff

    socalbuff New Member

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    I am going to replace the stock carb with a VM20 carb. The Stock exhaust is going to be replaced with a Thumper exhuast. Is the only way to check if the needle bearing cam will fit is to purchase and see if it fits?
     
  16. socalbuff

    socalbuff New Member

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    To Recap currently Engine Parts list

    TB 88 Kit
    New HD Valve Spring
    New VM20 Carb
    43mm K&N Air filter
    Thumper 2 Exhaust
    New seals and Gaskets
    Clutch Conversions
    4 Speed Transmisison (Either AHP or ATC 70)
    CDI conversion
    Changing Sprockets (Currently looking at 16/35 or 16/33)
    Cam (Sounds like Needle bearing cam if fit is correct)
     
  17. racerx

    racerx Administrator
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    Unfortunately, that is the only definitive test that is practical. If that needle bearing is loose, cam "walk" will be excessive. Alternatively, you could source a hondatrailbikes 12v (ball bearing cam) head...not much in the way of port improvements but, 12v (ball bearing) cams are available in more flavors. For that matter, you could source a used CRF/XR50 or 70 and build that; you'd start off with the CDI alternator and everything in 12v...most notably the long-rod crank. You'll need to add lighting coils but that's very possible and a really nice upgrade. I've probably gone too far down that alley already but, we are still discussing things...

    The VM20 carb should be a good choice, plenty of capacity for up to at least 14hp which is way more than anyone's likely to get with less than 125cc, minimum. Not familiar with that exhaust but, it's gotta outflow the stocker.

    Do the math...crankshaft-to-wheel...and make the sprocket selection at the very end. That'll be easy, once you know the primary drive ratio, top gear ratio and tire circumference.
     
  18. OLD CT

    OLD CT Well-Known Member

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    I would avoid the bearing cam. If it walks you will be rebuilding the head quick! As in lunch the valvetrain. Go for the fast road cam. The kinda expensive VM20 is really not needed for 88ccs. You will have to buy a lot of jets to test/tune it. Maybe 40 bucks worth? The jets in it are not going to work. If you did the stroker 108 crank the VM20 is the way to go. I used the very economical dual fuel inlet, left hand choke PZ19 out of the box and the 88cc kit with the dratv fast road cam and reached 53mph. It takes a quarter mile to do it, but it will hit 53..lol. Save the cash for the CDI conversion instead.
     
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  19. racerx

    racerx Administrator
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    IMO, the VM20 is oversized for 88cc but, a number of guys have proved that it's still tractable/tunable with that displacement, leaving "room to grow" should you decide to stroke it to ~110cc. Mikuni jets are easily sourced, as are rebuild parts, should they be needed. With a 108, I'd start with a #130..and keep #140 & #120 at the ready. With 88cc, I'd start with a #110 main and, as OLD CT has pointed out, there'll be some trial & error testing from there...still ought to get you to within 3 jet sizes. OTOH, he's the PZ19 wizard and if 88cc is the limit of your tuning aspirations, there's something to be said for a cheap carb that works well on a known engine combo.

    FYI, a remote, 3-position ("run", "reserve", "off") petcock can be attached to the frame, using the original airbox mounting tab, then you're free to use any single-inlet carb (most are) you choose.
     
  20. socalbuff

    socalbuff New Member

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  21. OLD CT

    OLD CT Well-Known Member

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    As long as you can find a ''left hand choke with the L/S dual fuel inlet'' PZ19. They are becoming hard to find lately. The stock carb or a new aftermarket carb is fine for a 88cc.
    Kitaco is a real good name but 90 bucks shipped is way too much in my opinion.
    Dratv's $46.95 mini monster is a less expensive better buy with the same quality/results.
     
    #20 OLD CT, Oct 2, 2018
    Last edited: Oct 2, 2018

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