If you want to think through your sprocket changes a bit for efficiency (the only concern I am addressing), here are some notes. Of course they are not terribly expensive and relatively easy to change, but anyway.
first go to jtsprockets.com (
JT Sprockets: Catalogue ) and looked up your vehicle in the catalog and see what the typical range of teeth choices are for front and rear sprockets for your bike, then look in the buyers guide to see some other models to google/ebay for.
for my american 1975 cb125s it was:
Front sprocket available in: 12, 13, 14, 15, 16, 17 teeth
Rear sprocket available in: 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 49, 50, 51, 52, 56 teeth
Then go look at your bike and confirm how many teeth are on the front and rear (I used a flashlight and a bit of chalk to count the teeth on the front sprocket quickly)
I had 15 teeth on front and 40 in the rear
then figure out your approximate current gear ratios. I drove around a little in each gear at 5000 rpm and noted my mph on the 125, dont crash
gear,mph@5000rpm
5,32
4,27
3,22
2,17
1,12
ok, now search/phone/ebay around for the biggest front sprocket and smallest rear sprocket you can find, possibly someone has a kit with a chain also.
I was able to locate a 17 tooth front for a few bucks stateside, and a 35 rear from thailand (apparently the cb125 is alive and well there)
So now for some fudge, try starting your bike in 2nd gear. Lets say this will be the "worst case" scenario, for launching. Give yourself plenty of tries to get used to it and see if it is something you could live with or at least get a feel for how close it is to something you could learn to live with. For this exercise we won't select a new combination of sprockets that would make the "new" first have less than the "current" second gear launch. Of course if your conditions allow less launching torque compromise for, say, less rpm at cruise plus aeromods and minimal stop and go or whatever, feel free to select any ratio you want.
to see the effect of the front sprocket change, take the tooth count of the largest one you can find and divide it by your current tooth count.
mine is 17/15 or 1.133...
then multiply the mph figures by that number, i.e.
5,36.3
4,30.6
...
1,13.6
since my new mph in first is 13.6 and my old mph in 2nd was 17, and launching in 2nd was barely tolerable, I should have no problem getting going with just the 17 tooth front sprocket change.
to see the effect of just the rear sprocket change, take the tooth count of your current tooth count and divide it by the tooth count of smallest one you can find.
mine is 40/35 or 1.14
and again multuply the original mph figures by that number, i.e.
5,36.5
4,30.8
...
1,13.7
And again, the mph/rpm ratio in first is not terribly close to second (13.7 vs 17), so I should be fine with just changing the rear and not the front also. Also note that 4th gear is ~1.5mph below the old 5th gear mph when we change either sprocket, so 4th is basically the new 5th and we have some noticable overdrive in 5th now.
Finally we want to consider changing both sprockets, so we take the original mph figures and multiply by both numbers, 1.133... and 1.14 here.
5,41.35
4,34.9
...
1,15.5
So we can see that in this case after changing both sprockets first gear would still have more torque than launching in 2nd had, so we should have a better feel how the low end will be affected, and we have added about 10mph at 5000rpm. Even 4th is higher than the original 5th. Of course this is a dramatic change and you should expect the bike to feel very differently, and you might need to shorten your chain too, but easy sprocket changes is one of the charms of motorcycles.
FYI, I have a 17 tooth front on order for my 125, will consider the rear especially if there is one stateside (don't like waiting weeks).