New 125 Sputters through all throttle/idle

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

  1. racerx

    racerx Administrator
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    I only address starting points with genuine Mikuni carbs. Anything else is "terra incognita"...unknown land.

    With knockoff anything, it's blind guesswork.

    It's not that all cheap carburetors are problematic; some work as they should. My approach is geared toward consistency, over the long term. The VM20 is a good value for $80...easy to tune, consistent quality and parts support. That matters when you need a small part, say, 2 years down the road. Mikuni is still there, as are the major suppliers...like Sudco...long after the ebay sellers are onto selling cheap knockoff POV camera drones.

    The L125 motors out there should be making close to 9hp and a VM20 fitted with a 140 main should be close.
     
  2. TNStreamerDreamer

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    I wish I had taken your advice on the genuine VM20 4 months ago. I've dropped over half that cost on sub-par knock-offs. I finally have a VM22 close running well enough through the full range of throttle but it just isn't "smooth". I may end up dropping the $100ish anyway for the quality of life improvement if the genuine VM20 would help clear that up. Through replacing cheap/unwanted parts, my mistakes, and having very little to start with, I've already got close to $1,900 in the bike anyway. This could be a nice way to ring in the 2K mark!
     
  3. racerx

    racerx Administrator
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    VM22 really needs more like 140cc to deliver seamless throttle response across the entire range. 125cc is the low limit of acceptable, using off-the-shelf tuning parts. 110cc is the absolute minimum displacement and it's a mutha to tune.

    The problem is lack of airflow at low rpm...and the resultant lack of vacuum. Seen, in realtime, via O2 meter the stumbling is obvious...transient lean spots, lazy fuel circuits & poor fuel atomization. Holding steady throttle, these self-correct in 1-3 seconds, an eternity when you're in traffic. And it gets old in a hurry.

    It'd be easy to correct...smaller slide cutaway, different jet needle taper and a different airbleed orifice. Unfortunately, none of those parts are available, despite what the Sudco catalog sez. I used to re-profile the VM22 jet needle...saw about 85% improvement. But, it's almost a black art, working in 0.0001" increments and getting the right section of the needle. All it takes is a couple of ten-thousandths too much material removal and you get a steady-state rich misfire at that, specific, throttle position. Those jet needles aren't always available...without a carburetor attached.

    On the other hand, the VM20 is very easy to tune behind this displacement. All that is needed is the right size main jet and a little tweaking. Without those gaping lean holes to cover, I typically see 15% better mpg numbers...which makes a difference with a whole .62 gallon fuel capacity. If there's any reduction in top speed, it's a couple of mph...above 60mph...traveling downhill with a tailwind. Usable power is actually a little better, without musician-like throttle technique required.

    Cost-wise, it's oftentimes cheaper from the get-go, once the motor is performing as it should, vs the "cheaper" alternatives. Then, of course, there's the part support angle when some little item like a seal, gasket, needle & seat need replacement, eventually.

    Next step up the proverbial ladder is an MJN22 carb...~$350 by the time you add the intake, jets & shipping.
     
  4. TNStreamerDreamer

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    Tuning a VM22 in the manner you mentioned is welllll beyond my level of skill, understanding, or patience. Reading that makes me that much more thankful for guys like you on here who can offer educated recommendations for me.

    If I was running a nice honda engine, I would consider the $350 carb option. My whole dilemma has been carb cost vs that of the motor itself.

    I haven't put enough time on my fake VM22 yet to make a decision on going for a VM20 yet or not. Your suggestion has not fallen upon deaf ears, I assure you. I may end up dropping a Piranha 140 Semi Auto in this thing some day to give a riding experience even further from my street bike so I'm also weighing that in my decision.
     
  5. racerx

    racerx Administrator
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    I wasn't suggesting that you pop for an MJN series Yoshimura carburetor. It is the next step up the ladder from a genuine Mikuni. My point was that the VM20, for your application (or similar) is a lot of value, for a fraction of $350...and really not much more than a cheapie knockoff. Thus, it's a logical option.
     

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