The 125/150 battery is larger than the one supplied with the 50. I have heard others have replaced the 50 battery with the larger one with pretty good results.
I think the stock 50 battery is really marginal. When I wired my deadlights in as running lights, I found that my battery discharges after about two weeks of commuting. Others have said this problem goes away with a larger battery, or at least you get a lot more time before the battery needs to be topped off.
Also, it is worth noting that performance is noticeably better with a fully charged battery. When my battery is too dead to hit the starter, the scoot is slower. If I don't put it on a charger at that point, eventually it will get to where it misfires not far above idle and is unrideable.
I would really think a sea level 50 would do better than an indicated 35, which is really about 30- the speedometers are optimistic by about 10%.
I would check to see if the scooter is still restricted.
The easy way to check is to unbolt the exhaust at the cylinder. If it is restricted, you will see the exhaust pipe is between a quarter and a half-dollar wide, but there is a washer between the exhaust and head that has an opening the size of a dime. If it's there, take it out and throw it in the trash.
If that one is there, then there is also a washer in the variator that limits the top speed gearing. You can see how to remove it by watching the dealer prep video.
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Doing both of these will bot improve the power by 30-40% and allow you to hit a higher gearing.
While you are in the variator, you may want to change the rollers. 7 gram rollers will give you a lot better acceleration.
