Coolster 125 Pitbike bogging issues
 

Hi fellow ChinaRiders,

I have a 2016 Coolster 125 pit bike that had a hard time starting, continuing to idle once hot, and regardless of whether it was cold or hot, it would begin to bog down when you reached 3/4 throttle. Instead of trying to crack the tamper proof stock carb, I simply purchased a PZ22 mikuni clone on Amazon and installed it today. Carb got installed, idles like a champ now, starts no problem, but the bogging is still there at 3/4 throttle and its slightly worse now. I fiddled around with the mixture screw and it didn’t seem to really make much difference so I left it at about 2 turns out.

I did also try to check the valve lash, but if you are familiar with these bikes, you can’t get a standard feeler gauge in there since the valve tappets are somewhat sunk down into the top of the head inside of a @ 1.5 inch valve cover hole.

It seems to me at this point it either has to be (1) valves or (2) jets.

What kind of feeler gauge do you use to check your valve lash on these bikes and what size pilot and main jets do you use?

I’m approximately 980 ft above sea level.

I use this set of flexible feeler gauges shown HERE, they work real well with the recessed tappet adjusters.


If your Mikuni-clone carb looks like This One, I have a baseline setting that I go by. I go up to a #95 or #100 main jet, a #17.5, #20, or #22.5 pilot jet, I set the jet needle clip in the 4th slot from the top, and start with a fuel screw setting of 1.5 to 1.75 turns OUT from seated. Keep in mind that it is a FUEL screw, you turn it OUT to richen the mixture. I'm at sea level, the above settings should work for you.

I think the main jet is to small. If I remember that carb comes with a 95 main, and a 15 pilot. I'm at sea and ran a 112.5 main in my apollo 125. You could also try to lower the needle clip ( the needle with the e clip) that will raise the the needle up a little allowing for more fuel

Here’s the Mikuni clone I installed:

XLJOY Dirt Bike 22mm Carb PZ22 Carburetor Intake Pipe Filter For 110cc 125 cc Pit SSR Trail https://www.amazon.com/dp/B073RGBM64..._8Y6xDb1P60G41

I assume this *is* a Mikuni clone. If not, I'll need to know. (sorry, I'm new to this)

I’ll order those jets as soon as I find out what I need and the gauge set.

Thanks for the follow-up and info.

What should the intake and exhaust valve adjustments be for this? .003 intake/.004 exhaust?

Thanks again.

Normally, .003" for the intake and .004" for the exhaust is the preferred specs for the 125cc engines.


That carb is actually a Keihin-clone made by Molkt, it is a bigger knock off version of the carb used on the Honda XR/CRF70 for many years. As far as jetting is concerned, I can't help you there, I've never used one.

I received the new jets yesterday, but interestingly, they didn’t fit though they were for a Keihin-clone 22mm type carb. These are M5 sized in diameter and I need M4s as that is the size of the stock main jet.
By the way, it’s no wonder this thing is bogging, the stock main jet is a 75!

Can you take a minute to post some pics of the jets from that carb, if you don't mind? I'm curious to see what style jets that carb uses.
Also a couple of pics of the jets you ordered, as well, for comparison

The stock 75 main jet that has an M4 thread: Attachment 18220

Attachment 18221

Below is one of the jets I ordered that is a 110. It has an M5 sized thread and won’t fit. Above it is the stock 75 main jet for comparison: Attachment 18222

I’m returning the ones I showed you and ordering these instead. They are exactly the same dimensions as the stock main jet:
https://rover.ebay.com/rover/0/0/0?m...2F192971544166

I went browsing and found THIS on jetRus...




...apparently while you were posting at the same time...lol

We run the PZ22 on our 125cc New Cheetah. Our jets are 100-Main & 42-Pilot. Air screw (it's not a fuel screw) is 1-1/4 out. Needle clip is on the bottom notch (richest setting).

We are at 4300' elevation so a 105-Main might be what ya need if your are down closer to sea level???

Here are the proper jets.

https://www.jetsrus.com/a_jets_by_ca...low_27-xxx.htm

https://www.jetsrus.com/a_jets_by_ca...and_38-xxx.htm

Update:

(1) I changed the main jet to a 105, seemed too rich, and so I went down to a 95. We originally started with a 75 main.

(2) I turned the air screw out 1.25 turns and voila, the engine woke up!

This thing screams now. It can wheelie into 2nd gear with my 170 lbs on it and frankly, it’s “scary-fast” for a little pit bike.

However, we have an ongoing problem: before, if I opened it up wide open throttle down the street for @ 150 yards it ran fine and then started spitting and sputtering. It is *still* doing this. I even had to push the bike home as it wouldn’t restart until it cooled off a bit. Thus, that sounds like the valve lash needs adjusting. I’ll try that once my smaller-sized, angled gauges come in.

Does this sound like I’m on the right track?

try lowering the clip on the jet needle down one notch, to raise the needle and slightly richen the mixture in the 3/4 to WFO throttle range. If it improves and you can, lower the clip again and see if it improves more. If you run out of notches on the needle, you may have to go up the next size on the main jet, and relocate the clip to the 3rd (middle)notch, and re-tune the needle position accordingly.

+1 on that advice.

If the bike is getting as hot as descdribed it needs to to be richened for sure.

Another point to consider is that even though the mixture adjustment screw is towards the engine/intake side, it may actually be an air screw, and turning it OUT may lean the mixture, and, as such, will make the engine run hotter than normal. The engine will run strong...but not for long.

Update # 2
 

I pulled the plug today, and sure enough, the electrode was showing a very lean mixture as it was whitish.

1. I lowered the needle clip to the bottom setting per ya’ll’s advice. There is only three settings and it was already in the middle setting. It ran like a champ.

2. I put the K&N style filter back on, and it ran like dirt. Sputtering and spitting in 1/2 to 3/4 throttle range. Pulled the plug and it was sooty and black. Too rich.

3. I decreased the main jet from a 95 to an 85 and it ran similarly, but was lean again per the plug.

4. I put a 90 main jet in, and it ran a little better, but still bogging at 1/2 to 3/4 throttle.

Yes, I adjusted the mixture screw each time to get it “right”. It seems to like 1.25 turns out.

After that, it was time to quit.

I am at a loss as to what to do at this point.

A couple of questions, if you don't mind...


1. Why did you switch to a K&N filter if the bike ran good after the needle clip adjustment?

2. Why do you make such big swings in jetting changes? Going from a 95 to an 85 is five size differences, why not go to a 92 and then a 90 and then an 87? You would then see if those slight changes are for the better or worse.

To answer your questions:

1. Because the K&N filter set up is the stock set up on the bike.

2. Because I don’t know what I’m doing.

Thanks for your input. It is much appreciated.

1. If you're riding off-road, you should be using a two-stage foam pod air filter that is properly oiled for better engine protection.


2. Since the bike seemed to run so good with the 85, if you think there is room for improvement, try the 88 and reset the needle clip back to the center notch, and adjust from there

Did you get the valves done yet? They usually come pretty tight from the factory. Getting valves set correctly should be your starting point. Not saying it's your issue but it's one less thing to worry about when getting things set up.

I am waiting for my feeler gauges to come in from eBay. I bought the flexible ones recommended earlier that allow you to access the recessed tappets that come with these bikes.

I gotta tell ya, this is a wonderful learning experience. It can be frustrating, but I *do* enjoy it as I like a challenge and am mechanically inclined.

If your carb is a china PZ22 your needle should have 5-notches for the c-clip. Wonder why your's only has three???

Not sure. My carb is a "Molkt" PZ22 clone (Keihin clone).

Adjusted valves...no change.
 

(1) I adjusted the valve lashes to proper specs: .003” intake/.004” exhaust. I couldn’t get the gauges in at all upon initial inspection. They indeed do come from the factory too tight. Now they lightly sound like a shutter on a sewing machine.

I ran the machine and it still bogged at 3/4 throttle. I pulled the plug and the top was still slightly white. This was with a 95 main jet.


(2) I then moved the needle clip to the middle position and while it would idle just fine, it wouldn’t even run when in gear but *completely* bogged down and cut off. I pulled the plug and it was still a little white but frankly I don’t think it ran long enough to get a good read.

(3) I changed the jet to a 98, and now, it really won’t run without starter fluid and is very hard to kickstart.

I had already worked on it for about 3 hours, so it’s time to quit today.

Making some progress...
 

1. I rechecked the valves and while the exhaust was in spec, the intake was still too tight. I must have accidentally set it too tight by turning the tappet while tightening the valve lock nut. There is just a little play in the rocker arm when I move it with my hand when in TDC.

2. I couldn’t get the bike to idle longer than a few seconds, so I checked the carburetor bowl drain screw to make sure I was getting fuel. We are getting gas. The idle screw was properly set too.

3. I checked the pilot jet to see if it may be clogged since it controls idle to 1/4 throttle (or a little more). Sure enough, the pilot jet was clogged. I blew it out with my air compressor, reinstalled it in the bike, and viola! She idles great now.

She runs great from idle to 3/4 throttle. She loses power and bogs down at 3/4 to WOT. That is with a 98 main jet, middle position on needle (only 3 positions) and mixture screw @ 1.5 turns out.

I’m going to stop tinkering with the carb at this point as I’m going to order a 2 stage pod air filter.

My suggestion to you after you've oiled and installed the pod filter would be to leave the settings as they are now until you see how it reacts with the pod filter in place.

If your below 800 ft sea level, you will probably need to be around 110 -112.5 main jet.

Thanks Douglass. I'm definitely keeping the current "tuning" the same until I get the pod filter installed. I should be able to get it installed in the next few days as it is coming via Amazon Prime. I am currently at 938 ft above sea level according to Google.

I appreciate your input. All of this is very helpful! Thank you.

Pod filter installed, won't start
 

(1) I got the pod filter installed, but now it won't start.

(2) I left all of the carb settings the same as before the install (95 main, needle clip in middle setting (only 3 notches on the needle vs. 5).

Interestingly, the bowl had a bunk of junk floating in the gas in the bottom of it and the in-line filter is getting the same junk in it.

Thus, it's time to pull the tank for a thorough washing out with mineral spirits, replace the fuel lines, and get a new in-line filter. I may even purchase a genuine Mikuni carburetor in a few weeks as I'm not sure of the quality of this Molkt carb (Keihin knock-off).

QUESTION: Does anyone here know off the top of their heads what the typical size of 125cc pit bike fuel lines is?

It probably won't hurt to pull the carb apart and clean it as well, if the debris was in the bowl, chances are it could still be in the carb passageways too.

I had a similar problem on my 125cc Apollo ATV. You will need 5mm sized fuel line, and go to autozone or orileys to get a new filter. Take the carb apart, and blow everything out with carb cleaner. What I do after EVERY time I finish running the bike for the day is while the engine is running I drain the fuel bowl.

Hope this helps

Douglass

Here is the link for the fuel line I replaced the factory lines with.

https://www.ebay.com/itm/Motion-Pro-...72.m2749.l2649

Were you ever able to get this sorted out?

I’m sorry I haven’t gotten back on here. Been busy dealing with some health issues in the family. I’m currently rebuilding/cleaning the old PZ22 carb, putting new lines on, & cleaning out the tank. I should have it running by the end of the week. I’ll let you know how she goes.
Thanks so much for inquiring about what’s going on with this little bike. Y’all have been such a big help!

I'm sorry if I missed it, but you got this bike new in a crate?
I've seen where people get these bikes out of box to find hella metal shavings in the fuel tank..

Really? That’s just crazy. I didn’t get my bike in a crate, but bought it from a friend who’s grandson drove it for 2 minutes, wrecked it, and said it scared him and he didn’t want to get back on it. I think the bike sat for a long time before I bought it, and as you know, motorcycles sitting for extended periods of time and not being ran will totally screw up carburetors, especially if you’re using regular fuel. Moreover, I don’t think this bike was ever properly prepped, so I’m left going back and doing that. The valves alone were so tight I’m surprised the thing ran!

@Dusman....any updates?

Update
 

FINALLY the weather is decent enough here to get the 125 Coolster pitbike back together today (I don't have a garage). Here's what I did and here's what happened:

1. Ultrasonically cleaned the carb and rebuilt her. I left the 98 main jet in.

2. I rebuilt the entire fuel system (i.e., new lines, new inline fuel filter) and cleaned the tank thoroughly. The dang line out from the tank had a pine needle in it! No wonder the thing was behaving like it was starving for fuel at times! The tank was filthy and had various junk in it. The kid that owned it before apparently, at some point, left the gas cap off or something and some trash got in that tank. The old fuel filter was filthy. I'm amazed the thing ran at all.

3. I got her all back together, got her fired up, adjusted the air/fuel mixture screw, the idle screw, and she starts right up now.

HOWEVER, she still seems to want to bog down a little when you get to 3/4 to full throttle. When I ran her up the street tonight, again after running her full throttle, she ran well, but she didn't run long. She started bogging down and just wanted to cut off and then finally did, even when I let up on the throttle.

It is indeed as if the valves swelled (or something expanded) and the internal engine dynamics changed. That's quite weird considering right before I wintered her I adjusted the valves to spec. I even double-checked them to make sure I did it correctly.

She'll run ok for the 11 year old, but if you really want to open her up, she's just gonna act funny until I can get this figured out.

Finally...Mikuni PZ22 success!
 

I wanted to post a final update here on the Coolster 125cc Pitbike.

With the extensive help from dirtbkr188, I adjusted the Xjoy carburetor (a Molkt PZ22 Keihin/Mikuni mix clone) up one side and down the other and just never could get it to run correctly. I purchased a genuine Mikuni back in October, but really wanted to try to get this cheaper carb to run right for the purposes of getting a better carburetor education.

However, after cleaning, rebuilding, rejetting the Molkt every which way but loose, and trying just about everything, I simply couldn't get it to run right after hours of working on it.

Yesterday, after working on it for a few more hours, I took a break, got some lunch, and said, "I've worked enough with this clone; I'm putting the Mikuni on."

The Mikuni came factory jetted at a 95 main and 15 pilot, which should have been about right for our elevation.

I put her on, gave the bowl time to fill up, and she fired up on the first kick. The idle was *ridiculously* high, so I turned the idle screw out and it still didn't seem to want to idle at a proper rate. I took the Mikuni off and made sure that the jets were seated properly, the needle wasn't floating around, and that there wasn't something I overlooked, such as a vacuum leak at the intake manifold or the pod filter. I tightened everything back down, fiddled with the idle screw a bit more (it's a weird one, never seen anything like it), and finally she started to settle down.

She runs like a top now and will get up to @ 42 mph (GPS confirmed). No bogging, no hesitating once she's warmed up, and she truly runs like a champ with that Mikuni on. I'm not exaggerating when I say this, but if you buy a Coolster 125cc pit bike and you're having bogging issues, I'd recommend just going ahead and purchasing a PZ22 Mikuni or getting this kit from t-boltusa.com:

https://tboltusa.com/store/mikuni-22...nis-p-107.html

Below are a few videos of the information and some seat-time action. In these videos, I put them together to update dirtbkr188, who has been a *tremendous* help through these last 6 months. That dude has given me a "college education" on carb tuning! Seek him out if you are serious about tuning your bike. He is a huge help and enjoys teaching other folks how to do this:

https://youtu.be/PxBQ5T6OIGE

https://youtu.be/RUd2eXvPUE8

https://youtu.be/WvnzaOyOQgk