These bikes have some very restrictive parts fitted when standard and therefore the first move is to try to remove these parts for better ones. The carburettors and air filters on these bikes mean that fuel and air supply to the engine are limited and therefore the ride will not be as smooth or as powerful because of this.
The first thing to do then is to buy a new race carb, these can be purchased off some websites for around 40-50, this is quite cheap considering the gain in performance it could achieve. After this has been fitted it is a good idea to derestrict the airflow with a free flow air filter (K&N style). This will add extra horsepower to your pit bike and give a great performance boost.
The next part which I would change would be the exhaust and manifold, which are again very restrictive to the engines of these bikes. A performance exhaust can be found on some sites for around 50 but can also retail at a few hundred pounds so be sure to shop around first.
Once these modifications have been made there will be a great power difference increasing low and mid range power as well as top end.
However there is still more to do in order to get the most out of your pit bike, in pit bikes the CDI unit is often restricted. The limiter on these are usually set around 8,500 rpm which means you will not be getting the most out of your bike if this is left standard.
A performance CDI unit can be purchased for these bikes which will mean that these bikes can now rev freely up to about 10,000 rpm which will mean smoother power curves and better acceleration and top end.
Adjustments can be made to the gearing of these bikes as well, in order for you to adjust this depending on the style of track you will be riding on. The best thing to do to change the gearing of the bike is to experiment with the