ok i got a little lost with all the cog sizes, but to calculate the gear ratio you divide the rear sprocket by the front sprocket so your current gear ratio is 38/14= 2.72 so for a higher gear ratio you want it to be over 2.72,
and with a 12 tooth front its at 3.17
and
the 42 tooth with the 14 tooth would only give you a 3 so you are slightly better off with the 38 tooth rear with the 12 tooth front.
this should hive you a nice bit more torque then you already have.
hope this helps.

 

I would give the 12 tooth a try first.. I was thinking it had a 12 tooth stock.. dropping 2 teeth on the front is a good start.

 

I have the same problem here's the solution and it worked well in my 125cc with me at 180 and my son at 30 on the atv. Front sproket down one rear up 5-7 I've noticed major change an I did this before I did other mods such as better coil ,plugs ,bigger carb, and performance CDI fter the upgrade is really moves and low end is waayyy better then stock And like other states this 125cc hauls on a stright away with me and my son on it.

 

I know this is a old topic but PBI sprokets is a good place to get your sprockets from

 

1st timer,
Do you know what part numbers you used for your sprockets or how you knew which ones to get. Where did you get your new chain?

 

I ended up getting mine from and ebay seller "whygostock"
Motion Pro, Hyosung items in WhyGoStock store on eBay!
they can get you just about any size you need at a good price, but you may have to call them

 

On a coolster 125CC. Made a noticable difference. I would suggest the mod

 

I bought a 11t for mine going from a 13t. The one on there has 2 bolts through it with a plate. New one doesn't have the 2 bolt holes. Can I run a simple snap ring in place of the plate?

 

Yes a snap ring should work fine.

 

I purchased a mikuni vm22 for my 125 along with the 26mm intake manifold and it helped low end quite a bit. You could change gears though. The 125 xr8 I have will do 34mph at the top end (ve70ified via gps on my phone), which is a little much for the kids. Although it is nice for them to be able to keep up with me on my 150. actually when I raced my daughter she was walking away from me in the beginning. just remember, these things only have like 5hp. You kind of need a running start to get up and over things as they do not have a cvt style clutch. For comparison, the 125 with my daughter (~70lbs) was able to get up and over a hill that was to steep for my dodge ram to get up.