How to replace oil rings on yamaha aerox tech 3 yr 2003?

If it is a four stroke scooter please don't touch it without a mechanic handy.
The valves will have to be aligned before dismantling.
They will also have to be re-aligned when putting it back together.
If it is a two stroke, no problem. No aligning to do.
You can check engine compression by removing the spark plug, putting your finger over the hole and slowly turning the engine over. If you finger is blown off the hole you have heaps of compression.
Unfortunately there is no easy test to confirm if the oil rings have had it.
Do some simple maintenance first...
There should be a breather pipe for the engine/oil.
Make sure there is no road grime built up in any of these pipes first (an air compressor works fine).
Change your oil (if you haven't already). The filter could be blocked.
Hope some of this info helped. If not...
This could help...

http://scooter-manuals.spb.ru/Manuals/Yamaha_YQ50_Aerox_97_Service_Manual_ENG.rar

Tony
August 2008