2009 Klx 250s forks are too soft

Was thinking of getting some stiffer springs and/or thicker oil. Has anyone done this mod or have any tips?

If you want people to help you, you need to help them. There is a TON of information missing from your post. How much do you weigh, what do you use the bike for, what does "too soft" mean, etc.

As per RaceTech's calculator, the stock 0.43 springs in that bike are about right for a 140-150 pound trail rider. Their website will calculate the "right" spring rate for your application (some will debate the accuracy of their calculations, but it's a place to start).

Changing oil weight will change the damping of the forks, but not linearly across the speed range. Viscosity has a more significant impact at lower damper velocities (because the flow area is smaller and thus there is more shearing). Depending on what you want to achieve, this can be a good thing or a bad thing.

Those are probably cartridge forks, if they are they can be revalved by a suspension tuner or a competent DIY'er, if necessary.

I am 145lbs and I do alot of off road riding. The times I knotice it the most is on woops and on hard hits. It bottoms out alot and actualy affects my handling on single track. I have adjusted the rebound all the way too

At 145 pounds, RaceTech says that the stock fork springs should be about right for you. It says they are 0.43s, but I don't know that bike to be able to confirm it.

When you say you adjusted your rebound "all the way," do you mean all the way in (stiff/slow) or all the way out (soft/fast)? Adjusting the rebound is probably not really going to help with bottoming on big hits, and going to fully slow on the rebound might make it worse (because the forks might pack down due to slow rebound).

If I were you, this is what I would do to try to address the situation:

1. Measure your static & race sag to determine if the springs are approximately the right rate.

2. Try increasing the oil level by 10 or 20 mm to add a bit of bottoming resistance.

3. Revalve the forks or have them revalved

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