Coolster 125 Pitbike bogging issues

Dusman · 08-22-2019, 09:41 PM

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.

dirtbkr188 · 08-22-2019, 10:14 PM

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.

Douglass · 08-23-2019, 01:18 AM

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

Dusman · 08-23-2019, 05:15 AM

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.

Dusman · 08-23-2019, 07:32 AM

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

Thanks again.

dirtbkr188 · 08-23-2019, 08:21 AM

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.

Dusman · 09-01-2019, 05:17 PM

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!

dirtbkr188 · 09-01-2019, 07:13 PM

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

Dusman · 09-01-2019, 09:29 PM

The stock 75 main jet that has an M4 thread: Name: E95C8408-80A8-4A32-A2AB-FAD085ADCA95.jpg
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Name: D23D33A9-964E-4E20-B000-36CC7AC7BE71.jpg
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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: Name: FF874178-5F21-4805-8CE6-B80C42C26F0D.jpg
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Name: FF874178-5F21-4805-8CE6-B80C42C26F0D.jpg
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Size: 19.8 KB

Dusman · 09-01-2019, 09:41 PM

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

Dusman · 03-07-2020, 09:01 PM

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.

Dusman · 03-29-2020, 09:39 AM

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:

dirtbkr188 · 09-01-2019, 09:47 PM

I went browsing and found THIS on jetRus...

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

GrandKidsMX · 09-02-2019, 09:38 AM

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

Dusman · 09-07-2019, 09:00 PM

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?

08-23-2019, 05:15 AM	#4
Dusman Join Date: Jan 2018 Location: NC Posts: 216	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. Last edited by Dusman; 08-23-2019 at 08:03 AM.
	0

08-23-2019, 07:32 AM	#5
Dusman Join Date: Jan 2018 Location: NC Posts: 216	What should the intake and exhaust valve adjustments be for this? .003 intake/.004 exhaust? Thanks again. Last edited by Dusman; 08-23-2019 at 08:19 AM.
	0

03-07-2020, 09:01 PM	#11
Dusman Join Date: Jan 2018 Location: NC Posts: 216	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.
	0

03-29-2020, 09:39 AM	#12
Dusman Join Date: Jan 2018 Location: NC Posts: 216	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:
	0

08-22-2019, 09:41 PM	#1
Dusman Join Date: Jan 2018 Location: NC Posts: 216	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.
	0

08-22-2019, 10:14 PM	#2
dirtbkr188 Join Date: Aug 2009 Posts: 511	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.
	1

08-23-2019, 01:18 AM	#3
Douglass Join Date: Aug 2017 Location: Bay Area, CA Posts: 375	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
	1

08-23-2019, 08:21 AM	#6
dirtbkr188 Join Date: Aug 2009 Posts: 511	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.
	1

09-01-2019, 05:17 PM	#7
Dusman Join Date: Jan 2018 Location: NC Posts: 216	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!
	0

09-01-2019, 07:13 PM	#8
dirtbkr188 Join Date: Aug 2009 Posts: 511	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
	1

09-01-2019, 09:29 PM	#9
Dusman Join Date: Jan 2018 Location: NC Posts: 216	The stock 75 main jet that has an M4 thread: 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:
	0

09-01-2019, 09:41 PM	#10
Dusman Join Date: Jan 2018 Location: NC Posts: 216	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
	0

09-01-2019, 09:47 PM	#13
dirtbkr188 Join Date: Aug 2009 Posts: 511	I went browsing and found THIS on jetRus... ...apparently while you were posting at the same time...lol
	1

09-02-2019, 09:38 AM	#14
GrandKidsMX Join Date: Sep 2018 Posts: 27	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
	1

09-07-2019, 09:00 PM	#15
Dusman Join Date: Jan 2018 Location: NC Posts: 216	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?
	0