Re-jetting my Lifan Carburetor

I fired up my Lifan 125 for the first time today, and almost immediately you could tell the bike does not like idling without the choke. Even after getting up to temp, and putting the idle screw all the way out, the bike idles for a few seconds without the choke, then sputters and dies. With the choke the bike seems to idle quiet well. Even with choke off and a little gas, the bike runs rough and you can hear slight popping/coughing noise in the intake.

So I'm assuming I need to find some larger pilot jets, but here is my problem, the seller on eBay I bought the engine from says the carb is a "no-name" aftermarket carb and does not have a brand name. So how should I go about finding jets?

Also, can I still do some WOT runs to get a plug reading on the main jet? That way when I open the carb I can change more than one jet at a time?

So, I'm gonna answer some of my own questions for anyone who might see this in the future and needs help. It appears that the carb that came with my Lifan 1P54FMI kit from eBay is a "K/F Standard of Japan" PZ19. It's common on cheap pocket bikes, scooters, small motors, etc. I read that it is kinda a copy cat of Keihin CV carbs. Looking back, I now wish I would have went with buying the motor separate and getting a Mikuni VM22 carb since it is name brand and probably a nicer quality.

Oh well, after learning some more about carbs, I now realize all I had to do was play with the A/F mixture screw as it was too far out from factory. Now at 1 and 1/4 turns out it idles very nicely without the choke. I will go on later to do some WOT plug chops and see if I need to increase the main jet.

The pilot airbleed adjusting screw is just that; it regulates the amount of air entering the pilot (idle) circuit. The further out from seated (counterclockwise) the more air and the leaner the mixture. You can richen the idle mixture until the screw is ~1/4 turn out from seated, beyond which point you'll cut off airflow completely and the pilot circuit will be effectively shut off. At the opposite end of the range, roughly 2-1/2 turns out is the lean limit.

Let's see a picture of the carb. If it really is a PZ19, it is waaay too small for a 125cc engine. You would be better off with a real VM20 and a 17.5 pilot and a 140 main to start with. They come from the factory with a 22 and 180 that is way too much for your engine.

Agreed, FWIW. However, I'd start with #20 pilot, one size larger in a VM20. I'd also order both #140 & #150 mains; cheaper than postage and a lot more convenient to have at-the-ready.

I will get a picture tonight, really wish I could find the source for these carbs, searching the web doesn't really bring me to a source of where they are produced or anything. I'm definitely a novice in the field, but it doesn't look THAT small...It looks the same if not a little bigger than the stock carb on the original 70 motor, and it idles well now that it's adjusted (about 3/4-1 turn out), and on its first ride yesterday it never hesitated like it wasn't getting enough fuel.

Obviously I won't know the whole story until I finish the bike and start getting some plug readings.

It doesn't hurt to grab a pic off ebay. I did. this is one version of a PZ19. It probably is a really good carb to break in the 125 as it will limit the top speed of a 125cc. Better to go easy on it for at least 500 miles before hammering it to let all the engine parts get used to each other. This is a perfect carb for the job.

If you like it and want to run it, then scooterpartsforless.com has the jets. After buying the real VM20 and jets you would most likely spend almost 1/2 of what you paid for the whole china 125 engine.
Although you will have a great carb if you buy a VM20 and get the true top speed out of the 125. a ''real'' VM22 is a little too much for a Lifan 125. There are a lot of fake VM22's on ebay. All China carbs are rated on the generous side, ie PZ19 is a 16mm. Some 26mm carbs are around 24mm.

OLD CT & RacerX, y'all know WAY too much about these bikes, LOL... all I know is how to wreck 'em, AYE???

Here’s the pic, it does look pretty much just like the one you posted old CT. But man I feel like 16mm would be noticeably way tiny...everything I’ve read says 19mm, but like you said, if it’s on the generous side then it’s likely a bit less, I’m going to run it for now if I can get it tuned right, and maybe up the road I’ll upgrade

That carb has a bigger slide so you are good kid. As a matter of fact I have not seen one like that yet. I can't say for sure if the jets will be the same as are in that carb now. Looks like a nice carb for the bike. The jets I mentioned work in the knockoff tiny pz19 like I posted. Try and get some #'s off that carb kid. Thanks. That bike has nice paint, btw.

OLD CT said:

It doesn't hurt to grab a pic off ebay. I did. this is one version of a PZ19. It probably is a really good carb to break in the 125 as it will limit the top speed of a 125cc. Better to go easy on it for at least 500 miles before hammering it to let all the engine parts get used to each other. This is a perfect carb for the job.

If you like it and want to run it, then scooterpartsforless.com has the jets. After buying the real VM20 and jets you would most likely spend almost 1/2 of what you paid for the whole china 125 engine.
Although you will have a great carb if you buy a VM20 and get the true top speed out of the 125. a ''real'' VM22 is a little too much for a Lifan 125. There are a lot of fake VM22's on ebay. All China carbs are rated on the generous side, ie PZ19 is a 16mm. Some 26mm carbs are around 24mm.

Click to expand...

Thought I would add I have bought many cheap China carbs on eBay,you NEVER get the same carb twice.If I was having problems with a carb and couldn't adjust it,I would just throw on another one for only $13.00 delivered to your door.

Finally got some pics of my jets, the pilot is stamped k32, and the main is stamped 83, I cannot find mains that look like this anywhere at all though??

Main:




Pilot:

I have been messing with everything and changing my sprockets and different carbs.Im still using the carb that came with my Lifan 125 engine kit.I pulled out the slide and removed the needle valve and set it to the lowest bottom setting.My bike now runs like a raped ape and pulls the wheel going into 3rd in wide open throttle.I have never even opened up this carb,other than the top and slide.Pretty happy with what I getting out of a cheap disposable carb.Seems to have a dead spot at low rpm,but don't care when it performs like it does wide open.

If you have the jet needle C-clip in the lowest groove, then it might be worth experimenting with the next larger size main jet. It'll have to be paired with a different jet needle height setting. The impact on WOT mixture will probably be around ~3% (educated guesstimate), the biggest change(s) will be at part-throttle where, somewhat ironically, it may actually run leaner. Generally speaking, jet needle height is used to tweak throttle response. With many carbs, WOT mixture is also affected. Not knowing anything about your, specific, metered air & fuel leak, I've no idea if yours works this way. Odds are high that you're in a good place with the WOT air:fuel ratio. That said, this is the peak of summer and you've no real way of knowing what, exactly, is in the tank. A subsequent fill-up may result in a leaner, or richer, mixture. Then, there's the cooler temps of late August to the end of the season, depending where you live & ride. Cooler air is denser air and that means leaner A:F ratios. That's the reason why I would want to do a little testing with the larger main jet...to have a safety/adjustment margin, on standby, ready-to-go if needed.

As for your low-rpm stumble, set the pilot airbleed (idle) adjustment screw 1/2 richer (clockwise) from its present setting. As long as it's at least 1/2 out (counterclockwise) from seated (closed) you should be okay. That will richen the mixture from idle to ~1/8 throttle. Hopefully, that'll at least minimize the hiccups...if not cure them. OTOH, if you're expecting EFI-like response, where the the throttle can be snapped open...fuggeddaboudit...aint gonna happen with a suction carburetor.