Change Carburettor

I change a 24mm carburettor for my ST70, but the max. speed only 60 to 65km. How can I turn the carburettor to increase more speed.

You can't.

- 24mm is the absolute max. for a stock OT70 head / inlet
- A decent 24mm carb is quite hard to set-up correctly, a reproduction 24mm is extremely hard to set-up right on a stock OT70 head
- your ST70 engine is most likely restricted by the flat Hitachi ignition. You can't fit an advancing ignition if yours has a S-type crank.
- more fuel + more air = more power
You can't get much more mixture in the 72cc engine.
Make the mixture richer doesn't always provide you with more power.
- I suggest to check the colour of the spark plugs tip.

Thanks for your reply. I checked already the colour of the spark plug, it’s black. So I need to change a smaller size or the original size carburettor.

Or you try different jets (main or idle), needle heights,...
A genuine Keihin PC20 or a CR20 mini (expensive!) are the best carbs for the OT70 heads.
If you want a cheaper option, go with the Walbro or Shengway 20-22mm

Thanks your opinion.

It's not an opinion, it's science.