Help with Carb Selection (Mikuni?)

Discussion in 'Lifan' started by illuminance, Sep 1, 2020.

  1. illuminance

    illuminance New Member

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    So, I was not able to get my CT project running by myself, and resigned myself to taking it to a shop. I had advanced the project from a bare frame & swing arm to the opponents 4 yard line, but couldn't punch it into the end zone. *Sigh*. Anyway, they got it running, but mentioned the cheap $12 carb as a problem. Of course the next day it won't even start for me. Folks on here have touted the Mikuni VM20 as the best fit for a stock Lifan 125. Not terribly worried about the price, just want to make sure I get the right one. The ones I see on Ebay are VM-273...is that the one I should get? Here is an Ebay link:

    https://www.ebay.com/itm/Mikuni-VM2...192544&hash=item33bebe29b2:g:AMgAAOSwTRZeWVjy

    And maybe a 140 main jet?

    Thank you, and stay safe.
     
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  3. OLD CT

    OLD CT Well-Known Member

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    Yes, that is a real VM20. You will need to change the pilot jet that is in it 22.5 to a 17.5 and buy a trialbikes VM20 main jet assortment. I cant tell you what you will end up using but 135 or 140 is a good start. The main jet will be way to big that is in it. You go up one main jet size at a time starting with a 135 or 140 until you hit rich misfire, a ''stumble'' at ''3/4 and up throttle'' then go back down one size.
     
  4. racerx

    racerx Administrator
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    I'm not familiar with your, specific, L125. They've been sold in a number of different flavors, over the years.

    Speaking in educated generalities, I'd expect a 125cc motor inhaling through a VM20 to want a 22.5 or 20 pilot jet and a #150 main. I'm basing this on actual output, as much as displacement. Usually, increased displacement "wants" a larger pilot jet. Idle airflow is affected by displacement. Peak airflow (and with it, fueling requirement) depends upon hp, not displacement. That's oversimplified, but adequate at this point in the conversation. Without diving deep into jetting theory, suffice it to say that there's no substitute for trial & error testing. Two engines of the same displacement, manufacturer, model etc running the same carbs, intakes, air filters can sometimes end up needing different settings, or even different main jet sizes. We hope to guess right, from behind a keyboard. More likely, the guess(es) will get you to within 2-3 jet sizes.

    The pilot circuit is easy to tune. Engine up to full operating temp, you should be able to dial-in a stable idle with the airbleed adjusting screw at least 3/4 out from seated, no more than ~3 turns out. Turning the screw clockwise richens the mixture, CCW makes it leaner.

    With the main, jet needle height affects fuel delivery from 1/8 open throttle through WOT, with a Mikuni. Varying the jet needle height, it is possible to go from overrich to terminally lean with the same main jet installed, if it's the best size for the application. It's usually best to find the rich limit...beyond which the engine will go into rich misfire at WOT, before it can reach peak rpm. It can be a little tricky figuring out if you need to go one size larger or smaller. The telltale indications: lean typically comes with bogging/stumbling/weak throttle response. Rich usually delivers EFI-like throttle response, before the engine goes into rich misfire. Since the jet needle has a substantial effect, it's a good idea to try adjusting jet needle height before changing a main jet size. If the engine runs rich, drop the needle (move the c-clip to the top groove); if it's lean, raise the needle (c-clip in the lowest groove). You may find the optimal result with a jet needle height adjustment...or if that won't do the trick, then you know it's time to try a different main jet.
     
  5. illuminance

    illuminance New Member

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    Thank you Old CT and RacerX. I ordered that carb and the main and pilot jet kits you suggested. The old saying "You get what you pay for" certainly applies to the $12 carburetors.
     
  6. illuminance

    illuminance New Member

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    Ok, I have obviously done something wrong. I ordered and received the VM20. Changing the pilot jet was no problem. The main jet? It looks nothing like the replacement assortment I bought from TBparts.com. Either they sent me the wrong carburetor, or I ordered the wrong jets. Any thoughts?
    -Thank you- DSC06764.JPG
     
  7. kirrbby

    kirrbby Well-Known Member

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    At the bottom of the float bowl there is a 10mm hex head "screw". It's not a screw, it's actually the main jet holder plus drain screw. Remove it, then swap the jets, and put it back in...done.
     
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  8. illuminance

    illuminance New Member

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    Perfect, spot-on knowledge, it's now in there, thank you
     
  9. racerx

    racerx Administrator
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    BTW...be very gentle when tightening the jet holder into the bowl. That "dead soft" aluminum sealing washer is just that...dead soft. Give it just a little too much torque and you'll have a leak that won't go away, until you replace the washer.
     

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