I just bought the Coolster 125cc and found the carb has no ability to be adjusted. The needle has one clip setting and the main jet has a 68 stamped on it.

The machine starts without issue and idles great. The take off is very sluggish when in gear. Once the ATV is moving above 10 mph the throttle is responsive and varies with throttle change. This is brand new and the carb jets were open and not clogged. If you get to very slow speed or from a stop, the machine will not rev up and move easily. Takes awhile to get moving. When its in neutral, the rev is fine and responsive. I think someone else posted something about this same problem but responses were to change the sprocket. This is new and should be fine with gearing. The engine should at least rev or struggle if it was gearing.
I am going to try shims under the needle clip to richen that setting and see what happens. No air/fuel screws or anything else to mess with. Any other ideas? How do I find out the pilot jet size? I think the main is #68 when looking at the center up. #89 if reading from outside to center. What is the right reading?

When you rev the engine in neutral it sounds like lots of air suction. It is responsive until you go into gear. A bit confused and needing time to dig into this more. Open to ideas.

 

There is a fuel screw under the brass cap/plug in the spot where a fuel screw would be.. you need to remove the cap and get to the screw..

 

Is the cap on the side with the choke lever or the side as shown on this site. I can not look at the carb at this moment and not sure if multiple choices will be there.

Maxtrade: coolster.com : CARBURETOR PZ20 (CA-17) (FDJ-PZ007) [FDJ-PZ007] - $38.00

Does the new ATV's come with too large of sprockets to brag up the speed of these machines? Right now it sounds like fuel and carb issue but it could be gearing causing this issue?

 

Yes.. its in the lower left corner just above the float bowl..

I have a 90cc mini utility and in my opinion its geared to go way to fast..(no power at all until you hit about 5mph then takes off) a smaller front and or a larger rear sprocket is needed.. but the lean condition you have isnt helping at all.. Id guess it runs better with partial choke on?

 

I hope to look at this tonight. Do you know the jet size? 68 or 89. I am unsure if its a Keihin or Mikuni type jet (or is this the same?).

How do you find out the pilot jet size?

If I was to change the main jet to a large size, how much to go? 95 or 100 (assuming this is a 89).

I do agree with the 5 mph and then it goes. Can't believe the sprocket will be my only issue. The high rpm's do jump without gain in power or speed, so I believe the main jet will need to be replaced too.
Do you think shimming the needle higher will help the low end? I only have one slot for the clip on the needle. If I raise this needle, will this help low end power? Lots to play with and short on time is the current issue. Thanks for the advice on the cap/plug over the screw. I take it that it will pry out easy?

 

it may have been my post you were refering too, if so I still havent figured it out - I am sure it is a jetting issue, but I dont know what kind of jets it would use, and I dont have a shop that is convienient for me to take it to that could help. I tried calling sudco for advice (carb. speciallists) and the second I mentioned a chineese bike the helpfulness ended. I planned on ordering a carb, but they couldnt/wouldnt give me any advice for a jetting starting point and it pissed me off and I didnt order it.

on mine I shimmed the needle to richen it, it helped a little, not much... I tried 1 shim and 2, 1 worked best

I also switched to a smaller front + bigger rear sprocket and it helped a little, but not much. My thinking behind the gearing change was hoping it would accelerate fast enough to basically bypass the bog, becasue it runs great on top - basically I was trying to put lipstick on a pig. I went from a 13/37 to 12/42 sprocket setup, and that should be a huge change.

These things seem to be shipped with poor jetting and poor gearing - I assume the gearing is to help sell it with such a high advertised top speed, like you mentioned.

I am frustrated with it - my next move is to just buy a name brand carb that I can tune better - one with an adjustable needle and cheap easily attainable jets. I just keep going back and forth in my head deciding if I should go to a stock type replacement like a mikuni vm22, or go with something bigger like a vm26, in case I ever try to make this thing fast.


Mine was new as well (I got it late december and am still fighting it). Had I been able to test drive one I wouldnt have bought it, but I am sure I will get it running good eventually.

 

Thanks for the reply High Boost. It was your post I was referring to and could not find it again. This is my third chinese quad. I upgraded them to my Honda EX machines and now back to get my third quad with the automatic transmission for the youngest daughter. She was not ready for shifting the EX. I had a Coolster 150cc DX2 (3150DX2) and it was a great machine! It was bigger than my daughter needed and I tried the Coolster 3125 XR8 now. It is a lot smaller but the carb has no adjustments like the 150cc had. I don't know if changing the carb out is easy to do. What intake manifold do you use? Will it fit? I am going to look for the air/fuel screw and see what that does. I can find lots of main jets on ebay but can't tell anything about the pilot jet.

One other thing I will try is SeaFoam and see if this gives more life to the quad. Stuff does wonders and maybe I do have a clogged jet or something. I could not believe the 150cc power and expected something close with the 125cc. Just listening to the carb when I hit the gas I can tell it is sucking more air than fuel. Need the fuel for explosion and get go! Did you find the air/fuel screw on your carb? Stay in touch if you get the machine going better. I will do the same.

 

I found the adjustment screw, it was underneith a brass plug.
I started drilling the brass plug with a small bit and eventually it just spun out. why they pay extra money to hide the adjustments is beyond me?

I will definately keep you updated.

btw, there is a company on ebay that sells complete jet kits, but they want $30 for 2 jets + $10 shipping = no thanks!

as far as the manifold goes, a mikuni vm22 has a mounting patern only 1 mm off from stock, so I am sure I could bore out the stock manifold and bolt that on easily.. the only "problem" with that is that then you have a 22mm carb going into a 19mm intake.. not sure how bad that is..

there is a company called tbolt that sells performance carbs /w matched intakes for these things (well for pit bikes with the same motors anyway)
they sell the vm22 with a port matched manifold and uni (replica) pod filter for $99, the vm26 kit with matched manifold and filter is $174, but it comes with a selection of 4 jet sizes as well.

check it out
vm22 Mikuni 22mm Carb Kit fits Pit Bikes and other Minis - Pit Bike Engine Parts - GPX YX 124/140 PARTS & KITS - TRC-0107 - TBolt USA, LLC
vm26 TB GPX/YX 26mm CARB KIT VM 26 Mikuni - Pit Bike Engine Parts - CARB KITS & CABLES - TBW9061-0556 - TBolt USA, LLC

buying the vm22 from sudco is $74, so it only costs $26 for tbolt to add a matched intake and filter.. that doesnt seem too bad. From what I understand it is jetted at least close as it ships (100m 30p).

the vm26 is $89 by itself, but at a bare minimum you need a 2 bolt to spigot adapter ($20) and a rejet. I figure running a 26mm carb thru a 19mm hole will represent a problem, so it is worth buying that as a kit if you wanted to go that route - with a carb that size you will definately need a re-jet, it is jetted pretty big stock (190m 22.5p).

I am leaning toward the 22, since I dont plan on doing a big bore kit/race head/race cam anytime soon, I just want it to run good. But I hate the idea of buying something too small, then re-buying again.

 

So the gearing change didnt make that much of a difference?

I could never find jets for my 90cc utility quad.. They can be drilled out to the next jet size though.. a 61-80 set is needed to drill out the pilot jet.. a 1-60 set is needed for the main jet.

How many turns out have you set the fuel screw at? 3? 4? what works best?

 

I dont remember on the fuel screw, I will check when I get home

as far as the gearing goes, the only differance I noticed is that when I am doing donuts, eventually it would bog and sometimes stall.. it is a lot less likely to do that now, but I honestly cant feel ANY acceleration differance.. my thought is that it just doesnt have the power down low, and all the gears do up top is cut top end speed

the other thing I noticed is sometimes I really overload it and ride with my son.. our combined weight at that point is about 275.. in the past I would have to start it flintstone style, but it was fine once we got to speed.. I was hoping that would change, but it didnt

I am happy I didnt put much money into that, but it was a hassle..
1. order sprockets, 2. install sprockets, 3. find out chain is too small, 4. reinstall old ones, 5. order longer chain, 6. install new sprockets again and find out chain is too long, 7. cut new chain, 8. find out chain is now too short, 9. reinstall old sprockets again, 10. order half link, 11. reinstall everything with added half link, 12. test drive and find not much differance, 13. win?


MWQ - do you know how to check cam timing? I just wanted to make sure it was right.. when I googled it everything I found shows a circular plate on the side of the head opposite the spark plug where the cam gear should be.. mine doesnt have that?