1971 ct.70 - throttle assembly too tight - best modification to fix "no spring back"

Discussion in 'Modifications' started by Larry Word, Jan 11, 2012.

  1. racerx

    racerx Administrator
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    I ran the same engine & front end combo, circa 2003-04. K1 fork is better than you might think. Give it a rebuild...which means replacing the oil seals and cleaning-out the grunge. Then refill using 15W fork oil. Experiment with the fill quantity. Increasing the fill stiffens fork action...going too far will cause problems, like hydrolocking before full compression can be reached. Following reassembly, fully compress a fork leg, then add oil until there's ~6" of air space remaining to the top. Repeat for the other side. Then allow the fork to decompress, reinstall the caps and test ride. You can increase fill level (fork fully compressed) to within 3-4" of the top. If you go too far, you'll get unwanted bouncing and an overly stiff ride. Go way too far and you'll lose fork travel.

    For footpegs, I've modded OEM Nice. It's a lot of work. The fixed pegs are ankle-breakers. I lop those off, then weld spring-loaded pegs in their place. Only other alternative is modifying , i.e. cutting, sectioning, welding-up, your existing footrest asm.

    I suppose you could cut the heel portion of the shifter. I've seen it done. Can't help you much with this one. I am sqaurely in the heel/toe camp and probably loathe the toe-only setup as much as you hate the heel/toe.

    There are a number of kickstarters that fit this engine. Takegawa might make the most sense for you, due to you location. Japan isn't that far, by air, so shipping shouldn't be much. Comb through Web!ke, they'll probably have cheaper knockoffs, too. A foldable lever fits these little bikes better than the OEM Nice unit.

    Those Trailwings are okay for dualsport use, though the rear will only last about 4500 miles. The stock drum brakes are okay, too. Above 55mph, you'll want to engine-brake before grabbing a handful of brake lever. Just make sure they're clean and in good working order. Below 50mph, they work quite well. With these tires, 17/31 is optimal gearing; run the best quality, full-roller, chain you can source. I run D.I.D. NZ3 series. And, lastly, replace the rubber sprocket dampers. with this kind of power, their service life is 5000-6000 miles. They'll still look good but will allow excessive sprocket movement on the hub nose.

    All-in, this ought to get you a 60-65mph bike that will sustain 50-55mph all day long...and somewhere around 100 miles per gallon/60 miles (100km) per tank.

    I have serious reservations about that carburetor...it's too big. The intake port is 22mm. Running a 24mm carb makes no sense. Best carb available is an MJN22. Best inexpensive carb is a Mikuni VM20.

    The stock CT70 K1 rear shocks are lame, too weak for an adult weight rider. Best shocks available are Ohlins & YSS...$750USD + shipping. Takegawa lists a set of Dax shocks, with adjustable compression, rebound & preload for $350USD...last I checked.
     
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  2. RMHRC

    RMHRC Member

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    Brilliant

    wrt the carb - should I downsize to the pe 20?
     
  3. racerx

    racerx Administrator
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    I've not used a Keihin, too pricey.

    VM20, #22.5 or #20 pilot jet. Main will usually be #150. Go with a 20 or 22mm intake. 20mm preferred, for higher airflow velocity and superior fuel atomization. Heat insulator between carb & intake.

    You should find this easy to dial-in, crisp throttle response, top speed in the low 60s and 110mph+/- 5. Total cost around $100us. Should be able to reuse your UNI filter.
     
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  4. RMHRC

    RMHRC Member

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    Wonder why they seller sold me the pe24 then.
    Apparently it was the go to carb.

    Must admit it’s pretty doughy in throttle response

    My PE24 has an intake bell OD of 44mm

    do you know the intake bell od of the mikuni vm20? I’d really like to keep my brand new uni filter.
     
  5. RMHRC

    RMHRC Member

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    Ok Bob I’ve done some more local research...

    I can secure a vm20 locally for $100 USD so not a bad deal.

    only issue is I need to also secure a new manifold as the vm20 is smaller and has a bolt on flange. I’ll chase locally.

    Seems the micky also has same intake bell dia as the PE 24 which is good.

    I’m at sea level.

    Any idea Bob why Asian concepts would have sold me such a wrong sized carb? It’s a genuine Keihin PE24.
    I’ll pull the trigger on this carb - for the record you’re def saying it’s a much better carb than the PE24?

    My engine is completely stock Nice 110.
     
  6. racerx

    racerx Administrator
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    Look at the exhaust port/headpipe interface. The gasket is far larger than the port, the pipe slightly larger. The "step" created by the ID mismatch, acts as a one-way check valve for the exhaust stream. Running a carb & intake with this configuration creates unwanted turbulence that ruins fuel atomization and impedes dynamic airflow.

    Before you pull the trigger...a.k.a. "add to cart"...check what you have. Is the intake larger than the port, i.e. do the two diameters match at the port/intake interface? If the intake overlaps the port, then not only is the port the bottleneck, you have a flow obstruction and it's the exact opposite of an anti-reversion baffle. At best, the carb is actually no larger than 22mm. Next best, it's truly a 24mm carb and the intake is...more or less...funnel-shaped, 24mm at the carb end/22mm (and port-matched) at the head end.

    Terms such as "better" are relative. Keihin makes quality products, always has. The operative term is "suitability", i.e. "better" depends upon the application. Honda installed an 18mm round bowl Keihin carb on these engines and the Honda Nice is a 65mph bike. Unfortunately, the OE carb is virtually impossible to tune and, as-supplied, jetted to the lean limit. Installed on a CT/ST70, Z50 or other model with free-breathing exhaust and an open-element air filter, it's too lean. At 4mm smaller than port ID, the carb becomes the bottleneck. As counter-intuitive as it may seem, it's actually preferable to running a carb that's larger than the port. The Nice is a stomper, not a screamer. That small carb gives EFI-like throttle response and allows enough peak hp to reach close to 60mph on a CT70. Basic physics tells us that going from 60mph to 65mph requires 30% more power...all else being equal. Everything has its limits. Ever come across someone asking if taller gearing will increase top speed? The answer is: "only if the engine makes enough power to still reach the same rpm". There will always be a number of builder-tuners out there who think that their stock bike will reach 60mph because a gearing chart equates speed & rpm. The same "bigger mean better" mindset applies to carburetion, as well.

    As to why AC and others sold PE24s and larger carburetors...they also had large catalogs of tuning & race parts. They listed "big valve" race heads and offset crankpins (to make stroker cranks), as well. Not every part listed work with every other part. Their BV heads were notoriously bad. Offset crankpins make for short-lived crankshafts. Some of their early big-bore kits left less than 1mm cylinder sleeve thickness, and would shatter at the spigot end. First AC motor I sourced was a f****ng nightmare; I ended-up replacing the crank, transmission assembly and the entire top end. To be fair, their "reconditioned" motors did improve toward the end. The last two I saw, about decade ago, were actually okay. The owner of one damaged the end of the crankshaft and I ended-up doing a complete teardown to fix it. There were a lot of used parts but most were still in good condition. The crank bearings were still fine, like new. The shift forks had nothing more than light scuff marks. Cylinder bore was still std. and within spec. The valve seats were still in excellent condition, the exhaust needed only a very light cut. The bad: the cases were stained like nothing I've ever seen...had to be walnut-shell blasted to clean them, internally. The oil pump gerotor was scored badly. First and second gears had some pitting. $600 worth of parts and a lot of cleaning later, that motor was literally like new. Thailand is very different from the west...and that includes Oz.

    The best carburetor out there is the Yoshimura MJN series. Running an MJN26 on a stock Nice would be foolish...it's too big for the engine. MJN22 is the smallest size offered and it does work very well, it's also the biggest carb that can be run on this engine. I wouldn't go so far as to say that you could expect solid speed gains from it. I plan to replace the VM22 on mine with an MJN22. The VM22 is nearly impossible to tame on a 110cc motor. There's simply not enough airflow velocity to get seamless response below 4000rpm, especially at low throttle openings. I ended-up re-profiling the jet needle, removing ~0.0006" of thickness in the section corresponding to 1/8 throttle to minimize a transient lean spot. Can't completely smooth-out that hiccup. Testing revealed that as little as 0.0001" too much thinning of the jet needle resulted in a steady-state rich misfire at one, specific, throttle position...and at very low rpm. By contrast, the VM20 has no transient lean spot, is very easy to tune...and is about 15% more fuel efficient. If the VM20 costs any peak horsepower, it's not a factor below 60mph. How much time will you spend at that speed, or faster? On the seat-of-the-pants dyno, there's no difference below 60...other than not having to modulate the throttle.

    So, for me, it's a choice between the Mikuni VM20 and Yoshimura MJN22. The latter is a much more sophisticated piece, beautifully engineered...and ironically...using Mikuni jets. Fuel consumption numbers are slightly better than the VM20, too. The question is how much better and at what price? The VM20 is a $70 carb, an MJN22 is more like $300.
     
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  7. RMHRC

    RMHRC Member

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    Wow - awesome reply. I’m fine to try the new carb as I trust your logic over the Thai companies any day. At least I don’t have to clean my pe24

    so intake is 22mm at the engine block ?

    is the manifold shaped internally so that it goes from bigger carb to smaller at the engine block? Or is it just straight uniform ID?

    Any ‘step’ would be terrible for smooth jetting i agree.

    Is there any particular manifold you recommend for the micky? (I need an angled offset one.)

    tbh the throttle response has never been crisp on my bike so a smaller carb makes sense

    I’ve ordered new fork seals so forks will get a refresh

    Bob do you know the stator lighting output on the Nice110?

    I have a Baja designs LED headlight that I think will work awesome on the CT but it needs 80watts...
     
  8. racerx

    racerx Administrator
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    I've no idea what your intake is. You'll have to check & verify. Real estate is limited on this bike. LH turnout style is your best bet, 22mm ID end-to-end, and port matched at each end. Tbparts just fits a VM20 on this motor, with the heat insulator located where it really should be...at the carb end. Having the intake mounted directly to the head, just the gasket in-between, allows the intake to heat up...improving fuel vaporization enough to allow one main jet size smaller to deliver optimal air:fuel ratio across the revband. If you can source a 20mm intake that will fit, that'd be even better; the smaller volume will "see" higher airflow velocity, which improves fuel atomization.

    Electrical output is a topic that's taken up a lot of my time. I'd estimate total alternator output at 70W, it may be closer to 90. I know someone who claimed to run a 45W H4 automotive HL bulb off of direct alternator power. Here's the catch...there are a pair of 3-coil arrays that comprise the 6-coil radial stator. One powers the charging circuit, the other powers the HL. The problem is that each is run to ground, with a single lead supplying AC power. A simple diode is used to provide DC to the battery. A conventional incandescent bulb can...and does...use AC. That's incompatible with LEDs. It is possible to lift the stator grounds, solder them together, leaving two AC outputs (the stator thus turned into a single 6-coil, floating-ground array) that can be fed to a full wave diode bridge regulator/rectifier unit. One must know what he's doing to effect this conversion modification. Done correctly, it works really well...everything powered from the battery and tight voltage regulation to boot.

    But...an 80W LED headlight?!!!! I'm running an 16/24W LED headlight "bulb" array, it's brighter than most car headlights. I've seen some fairly impressive LED headlights in the 6-10W range. If you go with an 1157 array for your tail light, that should leave 15-17W "free" to charge the battery; then you could either go for a lower wattage LED headlight or, run a partial-loss lighting/electrical system. I did that for over a decade, using a 35W headlight bulb. Only problem, no daytime running lights. Alternatively, you could feed the HL lead, from the stator, to a second diode, install a second battery, and use that to power an LED HL.

    If you want to run anything that draws more than about 18W, you'll have to go full wave and that means modifying the stator, as mentioned above. That'd give you at least 70W of DC power, probably more. When guesstimating electrical output, I tend to be very conservative.
     
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  9. RMHRC

    RMHRC Member

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    Thanks again

    I have removed the battery and run genuine Honda regulatir rectifier off my xr so I have full dc power with no battery weight.

    Australian xr250/600 ran an engine based dc batteryless system
     
  10. racerx

    racerx Administrator
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    Be very careful in your cross-model assumptions. Those larger models have far more robust alternators. And, there's a world of difference between grounded, half-wave, systems and floating-ground, full wave. The former loses more than half of its total AC wattage as waste heat, using a simple diode rectifier. Thus, 2-35W three-coil arrays, one is used to supply the full 35W, AC power to a HL; the other feeds the same amount of current into a diode rectifier with 17W left to charge the battery...if you're lucky. Alternator output falls-off precipitously at low revs, with these machines. Just my opinion but, there's far more to be lost than gained by omitting a 1kg battery. That said, it's your bike...and your call.
     
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