Coolster 125 Pitbike bogging issues

[Dusman]

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]

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]

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]

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]

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

Thanks again.

[dirtbkr188]

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]

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]

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]

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:

[Dusman]

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

[dirtbkr188]

I went browsing and found THIS on jetRus...




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

[GrandKidsMX]

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]

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?

[dirtbkr188]

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.

[GrandKidsMX]

Originally Posted by dirtbkr188

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.