When upgrading the KLX to make more power, especially the big bore kits over 300cc, the stock clutch may not hold up very long and will start slipping. This requires that the clutch springs and/or friction plates be upgraded.
If your stock friction plates are in good shape, a simple upgrade would be to swap 3 of the stock clutch springs with 3 Barnett 38lb springs from a kit for a KTM. I’ve included the barnett part number and some references below. The spring kit comes with 5 springs, you’ll only need to use 3.
Another popular option is to use the clutch friction plates from the EBC DirtRacer kit and a Barnett KLX spring kit. The EBC kit does come with springs, but the Barnett springs are stronger and will hold more power. This kit of Barnett springs is specifically for the KLX, and will replace all 6 stock springs.
Another potential solution, though not as popular, would be to use clutch friction plates from the 1993 KX 125. The 1993 KX 125 uses a similar clutch design, but with thicker and larger surface area friction material. It is apparently possible to use the stock 1.6mm steel drive plates if the “judder spring” is removed from the clutch assembly, but it is probably a good idea to use 2 or 3 thinner (1.2mm) steel plates and 4 stock plates. This upgrade can also be combined with an upgraded Barnett spring kit.
Reference the kawasakiforums post or Planet KLX for more info: