New 125 Sputters through all throttle/idle

Discussion in 'Lifan' started by TNStreamerDreamer, Jun 19, 2019.

  1. racerx

    racerx Administrator
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    All you need, mechanically, to do a rebuild is a pair of new seals and about 8oz+/- of fork oil. I'd start with 15W filling the fork legs to within ~3" of the top, fully compressed; then, fully extend the leg and install the cap, to trap air. If the fork hydrolocks before it can bottom out, remove 5cc of oil and retest...until it can just bottom out. If it doesn't hydrolock, more oil can be added to further firm-up suspension action. Stiffening it beyond that limits your options to 20 fork oil.

    Grab some cleaning solvent. Once the inners are removed form the outers, you're gonna find a lot of crap inside, that's gotta go. It's easier to metalfinish & polish the aluminum lowers while they're apart, same goes for cleaning/removing the rust from the upper portions of the inners. A little grease, or clearcoat will keep the steel from re-rusting.
     
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  2. OLD CT

    OLD CT Well-Known Member

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    Racer did you see the Tenn form? It asks to list the engine#. I don't think a VIN # will fly in the engine# section..
     
  3. racerx

    racerx Administrator
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    No, I missed that. Good call, sir(y)

    At first, this seemed odd but, if they're asking for both VIN and Engine SN, it makes more sense. Have to wonder if that would cause problems if/when the engine is changed-out, subsequently.
     
  4. OLD CT

    OLD CT Well-Known Member

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    Every state is different, I never was asked to list my engine#. BUT it sounds like a good idea to help verify the bike if it is stolen/recovered. If I had to list my engine # I darn well better store that info somewhere at home. I really only know by heart my HKO engine# that I have owned since the 70's! Problem is, it's not in the bike! LOL!
     
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  5. TNStreamerDreamer

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    IMG_20190731_201619.jpg @OLD CT @kirrbby @racerx
    Well guys, here is my loot today!
    I am not sure what can be done with the front forks. It was in a bit rougher shape than I thought. I was thinking of just hitting the rust spots with a wire brush and lightly clear coating the bare metal and rocking the distressed look.
    Hubs are in rough shape. One handlebar is bent. Gas tank is pretty rusted (just replace with a plastic alternative?). Might only be working with the bare frame and swing arm here really. What do you guys think is worth salvaging?
     

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  6. OLD CT

    OLD CT Well-Known Member

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    Too much rust. I would have left it. It's been outside too long.
     
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  7. TNStreamerDreamer

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    Well...I've got it now.

    The body seems pretty decent really. I haven't found any weak spots. I figured $180+gas was worth a usable frame and being able to title it.

    I'm thinking new forks are a must. I've got a pretty good set of smaller dirt bike forks but I think I would have to run a slightly slimmer front wheel to use something like that. Any major downfalls of that?
     
  8. kirrbby

    kirrbby Well-Known Member

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    Naahh...it's just a little patina :)
    If that frame isn't rotten, you've got a frame.
    If those forks aren't pitted in the area that passes thru the oil seals, you've got a good, rebuildable front end.
    You can do a lot to save the hubs, depending if there is enough steel left in the brake drums, or if they are too badly rusted.
     
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  9. TNStreamerDreamer

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    Well guys, and update...
    I got it registered and title is coming! I had a very patient cop who actually walked over to the clerk with me to make it happen because the clerk's office didn't have a clue. But, $47 late I have tags in TN!

    I've gone ahead and ordered the following since it was missing or rusted to the grave:
    Inverted front end w/ disk brake
    12" Single Piece Wheels
    New Seat
    New Fold Down Bars
    Piggy Back shocks
    Rear drum brake hub w/ 31t sprocket
    Stock Style Tail Light
    and some misc electrical items and such.

    I spent all weekend killing the rust with Evaporust and products that turn it to a black paintable surface. Also "Seal-all" worked wonders on my gas tank! I highly recommend it for a $5 fix on small holes.

    Right now, I've got the bike looking very much like it did with a light clear coat to prevent rusting where I had to clean it up. I'll probably run this distressed - modernized hybrid look until I do/don't decide to powder coat it down the road.

    TLDR:
    Bike is titled, registered, and well on its way to being a solid commuter.


    I haven't found 100% either way yet, but do 120/70-12 tires fit in the stock K1 swing arm? If not, can I go different tire size in the back or am I stuck needing the aluminum arm?
     
  10. OLD CT

    OLD CT Well-Known Member

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    A 31T rear sprocket will make 4th gear on a L125 useless. Stick with a 35 and run the largest countershaft sprocket it can handle.
    For instance, a 16-35 might be a good choice. The smaller the countershaft sprocket is, the more wear on the chain will occur.
    With a 31T rear you will need a 12 to 13T counter. That would wear out a chain quickly running 12-31. A 31T rear is for a bike that makes a HELL of a lot more power than a L125.
     
  11. TNStreamerDreamer

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    Shoot alright. I was just doing some rough numbers and with a 17/31 the other bike was running around 6,000rpm at 40mph on 15.5" OD tires. My new rear should be 18.6" OD so if I did my rough math right I was thinking I would be able to cruise 40mph at around 5,000 rpm with the same tooth ratio. I don't expect I could ever hit 7,500 rpm in 4th with the bigger wheels and same ratio though. Is that a big no-no?
     
  12. racerx

    racerx Administrator
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    Optimal gearing boils down to one thing...what the engine can actually pull. This can be affected by more than just horsepower and displacement. The key parameter is crankshaft revs per mile; that is much easier to understand when expressed as mph per 1000rpm, in top gear. You're heading in the right direction, applying some basic math to figure out where things stand...and which direction they need to be moved toward. Knowing the actual horsepower & torque numbers it's easier to guesstimate a starting point and limit the trial & error testing. This is where I usually cringe when someone makes blanket recommendations about sprocket combos. Ideally, you want to be within 1 tooth at the wheel sprocket; being 1 tooth off at the countershaft will cost you. But, the L125 is a known quantity...more or less. Thus, we can skip a lot of the theory and deal with known values. A garden-variety L125 motor should work best geared around 6.8-7.2mph/1000rpm in 4th gear. Taller than that, it'll fall flat on its face...shorter and it'll be busier than it needs to be, i.e. wasted revs.

    Going by your stated numbers:

    15.5" tall tire = 48.7" circumference. A brand new Trailwing will fall between 56-56.5" circumference. I dunno where one would find a tire with a 48.7" circumference; that's smaller than any 3.50-10 that I have ever seen. 18.6" diameter = 58.4" circumference; that's closer to the 120/90-10 tires I've seen. You really want the gearing to be within 3% of optimal, for a road bike. That's a lot easier to achieve by measuring tire circumference; being off by as much as an inch will introduce less than 2% error...easy precision just about anyone.

    I would strongly suggest slapping a measuring tape over your existing tire(s). Assuming your numbers are good enough 58.4/48.7 = 1.20. So, if you had the gearing optimal with the existing tires, you'd multiply that sprokcet ratio by 1.20 to compensate for the taller tires.
    Thus, if 17/31 was optimal with those short tires:

    17/31 = 1.82
    1.82 x 1.20 = 2.19
    2.19 x 17 = 37.2 would be (mathematically) the matching wheel sprocket. But before you order a 37t wheel cog, let's get a little more accurate.

    40mph @ 6000rpm = 6.67mph/1000rpm, pretty close to what I'd expect this motor to pull. 40mph @ 5000rpm = 8.00mph/1000rpm I'll guarantee your motor won't pull that very well, except going downhill with a strong tailwind.

    Best guess, your motor will run out of power around 55mph and out of breath around 8000rpm, with peak power coming on ~7500rpm. OLD CT gave you the quick summary and I agree with all but one of his points, especially running the largest C/S sprocket that will fit (16t or 17T) to extend chain life...and provide swingarm clearance. Once you move away from a stock size tire the mph/1000rpm ratio is changed. You're now talking about 20% taller gearing...from your tire size change. His numbers are based on stock-size tires, which also are shorter than your bike's new shoes...by enough to make a noticeable difference. If you want to dial-in your gearing perfectly, get some accurate measurements, then break out the calculator. At this moment, I don't know that your tire size numbers are accurate as they could/should be...
     
  13. TNStreamerDreamer

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    Man, thanks for digging in on this with me. So my original numbers are from a Z50 clone tire that I was running the L125 on to get my 40mph around 6,000 rpm range. I roughly laid a tape across the tire and it looked around 15.5" OD.
    I was hoping to just get my cruising speed (~40mph) down to a range where the engine wasn't buzzing so hard to keep up. It sounds like that may be a wishful thing for a L125. I felt the engine was happy up to about 5,500 rpm, then really whined over that. I felt like my low-end was way peppier than necessary so I figured I could gear a little taller.

    I do expect the 17/31 was overly optimistic. Perhaps going to a 15/31 could find me a happier cruising point, sacrificing performance to overcome hills and such?

    I guess another big question is... would a motor prefer to run higher rpm in a ratio it is more suited for over the full range, or be fed more fuel at lower rpm to TRY and achieve the same speeds?
     
  14. racerx

    racerx Administrator
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    Having the mph/1000rpm number is pure gold, for a tuner. Having the numbers OLD CT gave, for this engine Trailwing tire size combo is, as well. If your current combo has been tested & works well...and you know the engine rpm at any given speed...that's even better. All you have to do is work with percentages, based on tire sizes.

    The simple answer to your gearing question is: there is one optimal sprocket combo with ~6% spread (+/-3%) to accommodate rider preference, that's it. Usually, I'm digging deep into every imaginable scenario & situation. Gearing doesn't need all the longwinded explanations. The engines power curve determines optimal gearing, period. You are torque limited and that's that.

    I learned how to dial-in gearing about the time when the first silver CT70 K0 model rolled off the line. It comes down to basic physics. With these little bikes, what matters most is getting the final drive (top gear) ratio right (crankshaft-to-wheel) based on what the engine can pull without falling on its face in top gear. Believe me, with just a bit of experience, you'll know when that point has been reached. With this level of engine output, that happens when you're ~3.5% overgeared, in absolute terms. Once you get that sorted, the rest of the gear ratios take care of themselves. The engineers did get the overall ratio spread (first gear to top gear) right. It even translates from road to trails. The only exception I've found is super low-speed riding with lots of stops & starts...like a swap meet. An engine can only pull so much gear (torque) and deliver so many mph (horsepower). Carburetion is a totally separate parameter, that needs to be dialed-in the same, regardless of operating conditions; any "adjustments" to riding style are, well, throttle application.

    That's all, folks! Stick to this and you'll have your bike run optimally in no time...and...it only has to be done once.
     
  15. OLD CT

    OLD CT Well-Known Member

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    I just measured my tire and it is somewhere between 17.5 to 18. ''4.00-10 Zippy2's'' I base my rough numbers of the area I like to ride. If you have all flat areas my #s should be close. I run 16-33 and can pull any big hill in 4th with my stroker 125 ''hi compression'' build that can rev to 12 grand ''billet clutch'' equipped, in first second and third.
    17-31 would be a BIG OLD dog or a big steaming pile of :poop: with a L125... Agreed? 6 teeth off from racers math. Enjoy!
     
    #75 OLD CT, Aug 6, 2019
    Last edited: Aug 6, 2019
  16. racerx

    racerx Administrator
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    If that combo works...it works. Replicate it, i.e. build another engine of the same configuration (bore x stroke, cylinder head, cam, piston, etc) and you'll know precisely how it needs to be geared, from the get-go.

    What I am saying here is that changing a single parameter can affect gearing, via sprocket combo, dramatically. So, be careful about making assumptions. While "displacement equals torque" and "HP equals MPH" are both valid, neither quite tells you everything you need to know to realize optimal results on the first try. The most obvious variable is gearing, from crank-to-countershaft; this is why I state final drive gearing (crank-to-wheel) in mph/1000rpm. Less obvious is the fact that two engines of the same displacement can have significantly different power curves...and with them, gearing requirements (i.e. optimal mph/1000rpm gearing).

    To be clear, I am NOT disputing your results. I am attempting to further clarify/explain for the benefit of those either new at this, or are about to begin tuning a different engine combo. Since the first Chinese 110s & 125s hit the market, I've seen optimal sprocket combos ranging from 16/35 to 17/33, paired with Trailwing tires. Those differences are bigger than you might think and displacement is the least of them. It is possible to get the same displacement by increasing bore, or stroke, or both. Increasing bore size has a modest impact on torque & gearing. OTOH, increasing stroke length is like night & day. For example, the ever-popular "88cc kit", a.k.a. 52mm bore, increases torque enough to allow upsizing the C/S sprocket by a single tooth. Retaining the stock bore paired with a 51mm crank yields 89cc...and gearing requirement gets pushed close to what you're running with your 125cc tune. The more typical 108cc tunes, based on Trailbikes 52mm bore-up + 51mm crank, work well with a 4-speed tranny paired with 15/31 cogs...and Trailwings. 16/33 = 2.063, 15/31 = 2.067...a difference of 0.2%. That's negligible and too close to explain why your 54x54 tune wants the same gearing as a relatively mild 52x51tune. I know of at least one 110cc motor that works best with 17/31 sprockets, mine; changing from Trailwings to Bridgestone road tires, both "4,00-10" required changing to 17/32 to maintain the same mph/1000rpm. At the ragged edge, where you want a road machine to be geared, it is that critical.

    No need for a deep dive into the physics, here. The concept remains simply that any given engine can only pull so many mph/1000rpm. Beyond that, it's overgeared and will suck out loud. The "shortcut" is going in knowing what a specific engine can pull, after someone else does the homework for you. The next best approach is an educated guess based on stroke length, displacement and gearing (primary drive + top gear combo), unless one has a tachometer. Here's a "cheat sheet", expressed in mph/1000rpm final drive: stock CT70 gearing ~4.78mph, w/88cc kit ~5.0mph, 52x51/52 (108-110) ~6.6mph, L110-125 ~6.8-7.0mph. Use these figures as starting points; the further you deviate from stock, the more they'll diverge. And, Chinese engines have come in many different flavors, over the past 18 years, so be aware.
     
  17. kirrbby

    kirrbby Well-Known Member

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    *** PURE GOLD ALERT***

    This IS the gold that racerx mentioned above. He is kind enough to share it with all of us. This is the info that I write down in ink...you should too
     
  18. OLD CT

    OLD CT Well-Known Member

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    I didn't think you were disputing, I actually agreed. The main point is I highly doubt 17-31 will work for a L125/ I wanted to save TNSD the major hassle of removing the rear wheel after a 5 min road test. My recommendation would work much better than 17-31.
     
  19. racerx

    racerx Administrator
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    Oh yeah...17/31 is way beyond what this motor can pull with a CT70 sized tire.
     

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