Front forks contain oil which creates damping. You can change the characteristics of the fork by changing the height of the oil inside. If you increase the oil height, then the fork will become slightly stiffer and more progressive. If you lower the height, then the fork becomes less progressive. All front forks have manufacturer's specifications when it comes to oil heights but we will try to tell how to adjust the height easily on a general front fork.
Firstly, you will need to remove the fork from the motorcycle. On some forks, it is possible to adjust the oil height with the fork still on the motorcycle but, for the most accurate measurement, remove it from the motorcycle.
Undo the top fork cap and slide the outer fork tube down so you can reach the spring. Take a spanner, place it between the coils and hold the locknut on the damper rod. Unscrew the top cap from the damper rod and set it aside on a clean bench. Slide the spring out of the fork tube and set it aside. Be careful, as some oil will come out on the spring and if you pull the spring out too quickly you will wear the oil.
With the fork compressed, slide a steel ruler down into the fork tube until it touches the oil, this gap between the oil and the top of the fork tube with the fork collapsed is the oil height.
If you want to lower the oil height, take an oil-level adjusting tool, and set it to the oil height measurement that you are after. Always check the manufacturer's specifications on oil heights before adjusting.
Slide the tool inside the fork and use the syringe to suck out any extra oil until you reach your required oil height.
A simple version of one of these tools can be easily made at home with a piece of rubber hose and a syringe.
If you want to raise the oil height, poor some extra oil into the fork tube and then suck out the excess with the syringe until you reach the required oil height. Make sure that the extra fork oil is the same weight oil as that already in the fork.