88cc upgrade

Discussion in 'General' started by whereshaldo, Dec 26, 2018.

  1. whereshaldo

    whereshaldo Member

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    I just rode to work on my newly upgraded C88 and while it has not turned this little machine into a crotch rocket, I have to say it does make a decent improvement in the overall performance of the bike. A hair quicker off the line, definitely pulls stronger, and a noticeable improvement in holding speed on moderate grades. I used the Dr. ATV 88cc kit, and overall found it to be a very easy changeover. In all, I'd say it took around 7 hours start to finish, but I split it over xmas eve and some of xmas day. The carb will take some additional re-jetting and I'm hoping a 5mm Keihin jet is a 5mm Keihin jet.

    There is something so satisfying and simple about these bikes that really belies the first glance. The minimal amount of tools required to do anything is a remarkable feat of engineering.

    Anyway that's my success story. Will follow up when I re-jet it again.
     
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  3. racerx

    racerx Administrator
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    Congrats, sounds like the process went well, especially for a first-timer.(y)

    All things the same, the stock 16mm Keihin carb should be adequate to feed all those added, fire-breathing, cubes;). There is some top-end power potential in a larger 18-20mm metered air & fuel leak (VM18, or VM20 mikuni, 18mm Kehin)...especially when paired with a more aggressive cam. Consider, however, that those gains are going to mimic what you got from tuning the stock 72cc for more hp; they won't register as obviously on the seat-of-the-pants dyno, as the effects of added displacement...even if the tuning level is taken out to 55-60mph levels, the practicable limits of 88cc.

    I'd suggest going up 1 tooth on the countershaft sprocket. With a CT70, that's usually the limit of 88cc torque and well worthwhile for a road machine. Worst case, the machine feels a little overgeared for your use...there's still the rear wheel sprocket, which could with be upsized for the larger C/S sprocket, or downsized 1 tooth for the stocker. Might seem like hair-splitting but, with small displacement & minimal torque, dialing-in the gearing makes a difference. With the kind of constant use and the miles accumulating, you will notice and enjoy the improvements. Small improvements, multipied by mileage equal cubic satisfaction...:yellow70:
     
  4. whereshaldo

    whereshaldo Member

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    I'm currently running a 14 x 37 which is just slightly longer than the stock but shorter than running the 15 x 36. I'm trying to find that balance between the extra top speed with the longer gears but not too much sacrifice on the torque for the hills i encounter. Once I get the carb sorted, I may move to the 15x36 and find there is enough to keep me moving. I realize a stroker would help, as would a 125 Lifan, but I want a stock look and I'm not really looking for more speed, just a little more guts. Above 40mph, the brakes and front suspension start to get a little squirrely and to fix that it starts to become a custom cub rather than a fairly stock looking setup.

    Hell, I'm just trying to talk myself out of buying the $150 125cc Stella thats for sale in my hood. 1 old car project and one old scooter project are enough. Beyond that I run out of room and time in equal measure.

    Hal
    .
     
  5. whereshaldo

    whereshaldo Member

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    So I've rode this around a bit and put in a 102 main jet over the weekend (up from a 98). The 102 is definitely too big, I think I'll dial it back to a 100, it seems a bit slower at WOT than it was with the 98 almost a bit constricted. I'm still not happy with mid throttle however -- from about 1/8th turn to about just under 1/2 turn the bike does not cruise but sort of surges a bit. Its not bucking but it doesn't feel smooth either. To me this sounds like the pilot jet, but I don't really know what I'm talking about and I haven't found a good description of the PB32C in detail. The other thing could be the needle clip position but my understanding of surging at mid throttle is a fuel starvation issue not a fuel oversupply issue.

    I wish I had 2 carbs so that I could just swap between the two rather than break the whole thing down every time I want to change a jet, but buying a chinese carb (cheap or decent) won't give me apples to apples unless I buy two. These pilot jets from Jets R Us (here) do press in but when I'd replaced the ugly pitted #35 with a new one the bike didn't really run great (smaller holes around the skinny part) and it doesn't take many jets before I could have bought two cheap chinese carbs.

    Any thoughts or advice?

    Hal
     
  6. racerx

    racerx Administrator
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    I've lost track. What carb are you running now?

    You can pretty well forget about the pilot airbleed adjustment. By 1/8 throttle, its an insignificant portion of the overall A/F balance.

    Time to start trying different jet needle heights. I'd start by raising the c-clip 2 grooves...which will lean-out the mid-throttle A/F ratio a bit. See what that does...
     
  7. whereshaldo

    whereshaldo Member

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    It's a stock Keihin PB32C. Will adjust the clip at lunch today.

    H
     
  8. whereshaldo

    whereshaldo Member

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    Ok, I've been tweaking the carb all week, playing with different needle positions and different jets. I finally went and double read the Honda Motorcycle Carbeuretion manual (available in the C70 Yahoo Group Files). With that, I tore back in today and went back to the 98 jet, which is what I had before. The 100 and 102 were too big and its hard to describe the feeling but once you know what over-jetted feels like its pretty obvious when you get it right. So now with needle clip in the middle position, 98 main jet, and air screw turned out 2.25 turns the bike just hauls. Its really a pleasurable ride. One small thing that I also changed which did make a slight but notable difference was the slide valve. I'd put in the Dr ATV heart transplant kit, which has an adjustable needle and a new slide valve. When I had everything apart today I compared the original slide valve to the Dr ATV and the slice out of the slide valve is slightly taller on the Dr. ATV valve than the OEM Honda valve. Putting the OEM valve back in smoothed out the intermediate throttle ever so slightly.

    Thanks all for the continued advisement.

    Hal
     
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  9. racerx

    racerx Administrator
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    Bravo...well done!(y)

    Clearly, you've learned more than just the basics. This level of expertise is enough to allow one to tune most carb/engine combos and with a minimum of guesswork. Yes, at times, it can be easy to mistake rich for lean; that said, once you've learned to navigate that fork-in-the-road, life gets a lot easier.

    One parameter you've noted (great observation, btw), but probably don't fully understand is "slide cutaway" size. In a nutshell, the smaller the cutaway, the faster the fuel flow increases with throttle opening. IOW, smaller cutaway will deliver a richer mixture at low part-throttle. This can go a long way toward eliminating that dreaded transient lean spot, from off-idle to ~1/4 throttle...depending upon the cutaways size/carb and engine combo. Unfortunately, there aren't many slide choices for carbs smaller than 32mm anymore.

    Now...enjoy the ride:yellow70:
     
  10. Roy Chambers

    Roy Chambers Member

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    Let's fine tune it even more Bob!

    whereshaldo , since you not going to use the Dr ATV Slide , why don't you try your hand at modifying it.
    This practices has been done for years. I even did it to my CT70 when i could not tune around the same issue.

    You can change the Cutaway with a fine File! This is how it works, if you want it Richer ( your case ) you file the bottom of the Slide, if you want it leaner, you file the Cutaway itself.

    This is how I like to do it. I use the Draw file method. You clamp your File in a vice a pull the part toward you, try to keep it as straight as you can! If the File is to aggressive, and the part is jumping and hanging up, stop and get a piece of White Chalk and File the Chalk down into the teeth. This will help a lot. Take a Little at a time, remember you can't put it back on. Give it a try and see what it does.

    RC.
     
  11. racerx

    racerx Administrator
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    Interesting, Roy. I've never attempted this. Of course I've also not encountered a transient lean spot with a stock 16mm Keihin carb, that couldn't be tamed with "non-invasive" tuning methods. It's always been the bigger carbs, VM22/26, feeding 110-140cc tunes...especially the true horsepower tunes, i.e. big valves/big ports or/and carb sized the same as port ID. At one time, Mikuni offered slides with cutaways ranging from .5 to 2.5; now, slide (as well as jet needle & needle jet) availability is spotty, at best. It's frustrating when a 1.5 cutaway is creating an intractable transient lean stumble and a slide change would cure it. With a spare (and otherwise useless) slide on hand, it might be worth a try...maybe even better if one has access to a lathe. I have modded jet needles, with considerable success. That's how I was able to use VM22 carbs on engines with a 22mm ID intake port. That said, it's damned ticklish (read: "high-precision") work. Seriously, being off by as little as 0.0005" can be the difference between best (most linear) throttle response and a steady-state rich misfire at a specific throttle opening.

    Not raining on your parade here, you may have the answer for a few tuning problems. The skillset + knowledge required is likely going to be beyond a lot of otherwise accomplished shadetree tuner/mechanics. The best low-buck solution is usually sizing the carb 2-4mm smaller than port ID. That's enough to keep airflow velocity high enough to activate the fuel circuits and maintain atomization quality. The best solution, if a 22mm or larger carb can be used, is a Yoshimura carburetor. Their emulsion tube setup is highly efficient...unbeatable, imho...for a price.
     
  12. Roy Chambers

    Roy Chambers Member

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    Yep, there is another trick that you can do also. But some times it does not work out and sometimes it works great.
    You can take a Hacksaw and cut a small grove it the center of the front of the Slide! It acts as a Bleed / Bypass, like the small holes that are drilled in some Carburetors. When this works right, it's kind of like a small accelerator pump! In some cases it really smooths out the transition. But if doesn't work, you have a junk Slide.

    Also: when you do the File treatment to a Slide, you want to try to start with a Cutaway as close to right to begin with. Trying to change them too much knocks off the relationship / of the Needle.

    Carburetors are neat, they take care of themselves all the time, As the Bore wears out and the Slide wears out, so does the Needle and Needle Jet. So in most cases the A/f Ratio stays the same.

    Another thing i have encountered over the years with Carburetors, A given Brand / Style / type/ that works great on one kind of Engine, will give you all kinds of problems on another type of Engine. Making you work for days on end to try to get it right. You would think, the Engine is just a pump, and if it worked good on one Engine it WILL work good on another.
    Not always the case.

    Thanks Roy.
     

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