125cc LIFAN stumbles

i dont know if im in the right sub cat but here it goes

i just bought and installed a new 125 lifan for my bike

the motor stumbles in the mid range

i have richened the mix 1 full turn

it came with a new carb it was the same

size as the carb i had on the 110 so i swaped em out

same prob

when im cruising like 30MPH and turn the throttle it stumbles

if i ease into it it dont happen

it is like this with both carbs

any ideas ??? any help is welcome

and thanks for letting me join your forum

The fuel adjustment screw ONLY controls like idle to 1/4 throttle and that's it. The needle controls 1/4-3/4th throttle, and the main jet mainly controls 3/4-full throttle.

I'd try richening up the needle jet and see what happens. If it gets worse, try leaning out the needle jet. If that doesn't help, you might look into changing out the main jet.

I'm not sure what size jets that bike is supposed to run on or I could give you more specific help

+1 to what Towlieee said above, and post up a clear pic of the carb you have, possibly be able to help with jetting from the pic. The standard carb that is included with most Chinese bikes and engine kits is the 22mm Mikuni-clone carb, it looks like this one...


thanks for the replys explains a lot

the carb i have is the one shown in the pic posted

by dirtbkr188

the needle is set to the 2nd groove

from the bottom

witch way is richen withch way leaner ??????

That carb has a fuel screw located in front of the float bowl, under the intake manifold. You turn the screw OUT (counterclockwise) to richen it, and IN (clockwise) to lean it.

At sea level, the optimal jet sizes for that carb seems to be a #17.5 or #20 pilot jet, and a #100 or #105 main jet, but you'll have to fine-tune to your area.

thanks agian dirtbkr

the needle jet is now on the last grove now

seems a little better but nit quite there

the idle screw is 2 turns out

the pilot jet is 15

and the main jet is 100

would you say going to 17.5 or up to 20 on the pilot

and 105 on the main

would sovle my prob ????

thanks agian you have me going on the right track

I would try a #17.5 pilot jet, reset the jet needle in the 3rd or 4th notch from the top, and see how the bike runs then. You might also try a #105 main jet for the upper RPM range. Set your fuel screw at 1-1/2 to 1-3/4 turns OUT from seated, and make the adjustments in 1/4-turn increments and then see how the bike runs. Having different sized jets on hand allows you to fine-tune it even better.

You say you're cruisin along at 30MPH and it stumbles, you have to realize that 30MPH in 2nd gear is probably 3/4 to WFO, whereas, 30MPH in 4th gear is probably just shy of 1/2-throttle, if that. The pilot jet circuit controls from idle to 1/4 throttle, the jet needle covers from 1/4 to 3/4 throttle, and the main jet circuit covers 3/4 to WFO. Also keep in mind that those ranges overlap, and that you cannot blip the throttle to WFO and expect these little carbs to react instantly; they will not do so, simply because they do not have an accelerator pump, like the bigger 250 and 450 4-stroke bikes.

I'm not saying that my suggestions are an instant cure-all for your particular problem, it requires the fine-tuning at your end, based on the conditions where you live.

One additional thing to check is the intake tube size. Make sure the intake tube openings match the cylinder head and the carb. If the tube internal diameter at the head is 22mm and the opening in the head is 26mm (big valve Lifan engines) it's never going to run right.


idle mix 1-3/4 turns out

pilot jet 17.5

needle 4th notch from top

this solved my top end stumble 100%

now i had a stumble 1/4 to 3/4 range

went back and changed back to the #15 pilot jet

this solved my stumble 95% hardly notice it

im happy with this cause this is a converted moped :smirk:

i ride on the street and it spends most of its time in top gear

i guess the only thing left to do is port the intake

as hugemoth said

i may do this in the future but for now im happy with it as is

thanks to everyone who helped


Glad to have been able to help you get it dialed in to suit your needs. Ride safe and enjoy! :smirk:

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now