125's have a low first gear so you should be able to pop the front wheel pretty easy
You should be able to notice any gearing difference's by having a straight line race with your mate, both holding your bike's at wot in top gear.
Whoever's bike is fastest has the tallest gear's, but the faster bike will also be slower taking off.
You can lower your final drive gearing pretty easily, take the stator cover off the left side of the engine. There will be 2x 10mm headed bolt's holding the sprocket retainer onto the output shaft of the gearbox.
Take the 2x bolt's out and turn the retainer so it come's off the shaft, sprocket can come off now.
If you replace the front sprocket with a smaller one then this will lower your gearing.
Most 125's come with a 14 tooth or 15 tooth front sprocket, replacing the sprocket with one that's 1 tooth smaller will lower the gearing, fitting one that's 2 teeth smaller will be quite noticable.
By fitting a smaller front sprocket you should be able to adjust your chain tension to take up the slack without having to buy a shorter chain.
A front sprocket can usually be picked up off ebay for around $10 inc delivery.
Make sure you check which chain your bike sue's, and the diameter of the output shaft as well.
There are 2x common chain's used on the 125 engine's, a 420 pitch, and a 428 pitch.
The 428 chain is a couple of mm's wider than the 420, and the output shaft size can be either 17mm or 20mm depending on the engine.
You can also lower your gearing by fitting a bigger rear sprocket, it's a slightly dearer option though.
you need to get one with 3 or 4 teeth more than your original sprocket though to equal dropping 1 tooth off the front sprocket, or 6-8 teeth more to equal dropping 2 teeth off the front sprocket.
This usually require's a longer chain too
I'm presuming you've done stuff like replace the cheap china spark plug with a decent NGK C7HSA
Do both bike's run the same style carby, both run oiled foam air filter's ?
Does your bike need to be warmed up for a little while before you can ride away?
If you start to blip the throttle soon after it's started does it hesitate, and want to die ?
If so then the pilot jet in the carby is too small, or maybe the pilot circuit is dirty or gummed up
I've found with the Lifan 125's the standard Pilot jet in the carby is a touch too small.
They usually come with a 15 pilot jet, replacing it with a 20 and adjusting the mixture screw to suit will change the way the bike start's and ride's, it will start a lot easier, and feel a lot better to ride.
That's another thing, if you bike hasn't been tuned by adjusting the carburetor then that could be where you're lacking power.
Also it could be your bike need's to have the valve/rocker clearance's checked and adjusted, that can make a big difference if your's are tight.
Adjust the clearance's to .003" on the inlet and .004" on the exhaust, make sure the engine is at tdc on the compression stroke before attempting to adjust the valve's
Here's a good video that help's explain the procedure.
[video=youtube;QJciyJRJh8Q]https://www.youtube.com/watch?v=QJciyJRJh8Q[/video]
