Go-kart-seats are arguably one of the most important parts of the go kart. In go kart racing there are no seat belts so a properly fitting seat is of extreme importance. If it is too small it can hurt to squeeze into it and the bolts holding the go kart seats might bruise you. If it is too big you will get tossed around on the turns and bumps.
kart seats are typically made out of fiberglass or even carbon fiber for durability and to keep the weight down. If you buy a new seat you will have to drill all the seat mount holes yourself. You may even have to bend your frame to fit a small or large seat! We would recommend you get help from an experienced kart racer or mechanic if you are going to bend your frame. These people are experienced with go kart seats and know how to properly align the frame.
Pro Tip: If you find your go kart seat hard or uncomfortable some buy a pre-fitted seat cover for comfort. You can always make your own out of padding and black duct-tape!
Most people add their lead weights to the seat as well (by drilling holes in your weights and in your seat). These weights are bolted on, make sure you use washers! If you are adding weights you should be familiar with your leagues karting regulations.
Also remember to check your seat mounts regularly. We have had our seat come lose and scrape the ground as we race! Not only did our bottom get really warm, a hole melted into the seat!
Check for cracks regularly as well. A crack is a simple fix. All you need is a fiberglass repair kit and follow the instructions.