Gearing...Sprockets

[MXonaKawi]

So for those of you that don't know, I ride a 2000 KX 125.

My 3rd gear seems really short.
I would ideally like to get more power out of it. I have to shift to fourth way too soon it seems.
I am currently running a 51 tooth rear sprocket and a 12 tooth front sprocket.
I head into most turns in 2nd gear, shift to third and hit powerband while exiting and then have to hit 4th for most jumps.

I've never ridden my bike with the stock 12/49 gearing but my current 12/51 setup doesn't really feel like it has a ton of bottom end (which was the goal). In my mind, I'm considering either going larger or smaller on the rear sprocket because I don't feel like I'm getting optimal performance out of my current setup.

I'm wondering if I should get a 53 tooth rear sprocket with the purpose of riding in a higher gear all around the track (taking my turns in 3rd and shifting to 4th on exit and staying in 4th for the jumps as well).

OR

Going for a stock 49 tooth rear sprocket and hopefully getting more top speed out of 3rd gear?
Is my thinking right?

[KevinTwoStk]

I run 12/50 on the same bike. I've never run the stock 12/49 either.

I think going to 12/53 isn't going to do what you want to. Yes, you might be able to pull 3rd through some corners that you run in second now, but each gear is going to be that much shorter.

On my bike I have to enter most sharp corners in second and immediately shift to 3rd upon exiting the turn. If it's a long sweeper, I have to shift to 3rd midway through the corner.

I would try 12/50 and 12/49.

Finally, I don't think you'll get much low end out of your engine no matter what you do, mostly because it's a 125. The best way to broaden the powerband and add some low end would be to go for a 144 kit. No replacement for displacement.

One other consideration is a flywheel weight. This will smooth out the power and make the powerband feel broader. I haven't tried one, but most who have rave about them.

[gonzo435]

Ive, never owned a kx 125 , but ive had several yz125, and ktm 125 over the years,  13/50 or 51 always seemed to work well for me. Running a 12 on the front just seemed to make it go through the gears to fast. The nice thing about playing with gearing is a fairly low cost for sprockets, id get a couple choices and play around till you find your favorite combo. Good luck! 

[KevinTwoStk]

The internal ratios might explain why a 13T worked well on the YZ125 and a 12T felt too low. That's only a guess without doing some research.

Yeah, gearing is pretty cheap to experiment with compared to a lot of other mods. No harm getting a 13T front and trying it out. I've read that one tooth up front equates to about three out back. There's a chance two different-length chains might be needed to try out all the ratios.

[MXonaKawi]

Awesome. Thanks for the insight guys. I ordered a $20 brand new 50 tooth sprocket off ebay to try out that gearing. If I like it and it gives me that boost of top end and more space at the top of each gear I need, then I'll order a better quality Renthal 50T.
If it feel like it's going in the right direction but not enough of what I want, then I'll try a 49T.
I'll keep you posted.

[KevinTwoStk]

I'd like to humbly suggest a Renthal Twinring sprocket once you've found your preferred gearing.

I also suggest using an O-ring chain. I didn't feel any loss of power compared to a non O-ring chain. What I have noticed is that I don't have to adjust the chain very often, and that my chain doesn't get noisy even after I ride through mud or in wet conditions.

The countershaft sprocket will still wear out, though, and a 12T wears out faster than a 13T model. Since countershaft sprockets aren't too pricey and also wear out faster than the chain and rear sprocket I replace my countershaft regularly.

[MXonaKawi]

UPDATE!!!

The 50T sprocket made it harder for me to clear long tabletops out of corners. I'm going to go back to the 51T and work on my jetting a bit to get it closer to perfect. If I still want to mess around with the gearing a bit more once I get my jetting perfect, I might try a 49T sprocket to see how that feels. I think the reason I didn't love the 50T was because it wasn't enough of a difference. The only difference I noticed was the inability to clear certain jumps. I'm also curious on how the 52T or 53T would feel.

[MXonaKawi]

UPDATE!!!

The 50T sprocket made it harder for me to clear long tabletops out of corners. I'm going to go back to the 51T and work on my jetting a bit to get it closer to perfect. If I still want to mess around with the gearing a bit more once I get my jetting perfect, I might try a 49T sprocket to see how that feels. I think the reason I didn't love the 50T was because it wasn't enough of a difference. The only difference I noticed was the inability to clear certain jumps. I'm also curious on how the 52T or 53T would feel.

So my bikes jetting is great now. I've got it dialed in to where I'm super comfortable and I'm getting faster and staying on the pipe all around the track. I'm going to give a 52T sprocket a shot just to see if I like it better. I think a 53T is too dramatic of a difference. I'll let you guys know how the 52T feels.

[MXonaKawi]

I threw on a 52T sprocket this week and it was interesting. Maybe I was just vibing really well with my 51T setup but it took a few hours of adjusting to get used to the 52T. Overall the acceleration was better and I was able to get more speed before jumps out of corners but I also had to shift a lot more...A LOT...which was surprising because I was expecting to shift more but just a little. I'm still not sure if the upside is worth the trade-off in comfortability and not having to shift as much. However, if after a few weeks I'm just as comfortable with the 52T setup as I was with the 51T, then I will just keep it and get used to the shifting to take advantage of the added acceleration.

Thoughts?

[Motorrad]

I run 15-42.