Would appreciate some help from you mini-sled veterens. I'm replacing the gears and chain on the kids '05zr120. Also adding a real chain tensioner. One kid weighs 50 lbs, the other 80 lbs. Have added RecMotor governor bypass spring, but would like sled to be a little faster.
Question: is there a gearing chart out there to make comparisons to? What seems to work best for those of you who have made these changes? Thanks for any help you can give, I'd like to store the sled for the summer with upgrades already done.

Recmotors has a gearing chart online. We run a 10/26 combo with no problems. I have the 6.5hp Honda but even with the stock motor it worked great. Weight of the kids really won't make too much of a difference. I have a son that's 80lbs and another that's 45lbs. They both pull me on a sled behind them with the other one on the sled with me. We've got over 300lbs between all 3 of us and it still goes like a bat out of hell.

Thanks for the reply, Guenther, that sounds good. I imagine you put a different chain tensioner on also, and if so what brand did you go with?

No, I didn't. I put 2 zip ties through the stock tensioner spring (front & back of spring opposite of each other) to stiffen it up and get the correct tension. It actually works great and cost nothing. It's been like that for 3 years and still working great.

I tried your 10/26 combo works great going down hill mind you I can't get back up the hill Did I miss something?

I tried a 13/46 (3.53) using a noram race clutch, #35 chain and power intake kit on a stock 120 XC and found it affected the bottom end too much for snocross style riding, especially in deeper snow. Dont get me wrong, it went really well once the track was moving but if he stopped going up a decent hill, or was buried in snow up to the running boards it had a hard time spinning the track again. I had to go back to 4.15 to get the power he was used to with stock set up (10/42). Saved me pushing him out of power requiring spots. maybe it had something to do with the 13 tooth front, I don't know, but the 4.0 range worked better for his riding style (between 45 and 55 pounds).

Just my experience.

i run 15/44 on the polaris works great and 15/56 on the cat and it goes good until you come to a hill

I run 10/30 on the trails. Goes good does about 20mph. In deep snow he gets stuck easier, so I change to a 10/36. Has better low end and goes 15-17 on the trail. I use the rec. motors chain tensioner. My boy has over 120 miles so far this year on the 120.

Hmmm, I don't know. We never had a problem even with just the stock motor and bypass spring. I don't know what to tell you, sorry!

It might have to do with Geunthers 6.5 HP motor in his, stock I believe is 4.5. I installed a Power intake kit with Gov bypass spring and adjustable Main jet then went to 10/38 gearing my daughter rode next to me at 18 mph on my speedo, and she has plenty of torque to get through the hills.