NOTE: the Honda XR and Suzuki DR have rocker arm valves which work fine for low-revving, high output engines, but the KLX-300 uses a bucket-under-shim design with fewer moving parts for higher, quicker revs, and more reliablity. The YZ/WR 400/426/450 and DRZ 400 also use a bucket-under-shim design, but the KLX 250/300 did it first.
Before getting started your bike should be cold; i.e. not run in the last 12 hours. And clean -- KLX'ers keep their bikes clean, right? You don't want any crud falling into your engine while you're working on it. This may be a little intimidating at first, but it really is easy, so get out your service manual and tools and let's get going!
The first step is to remove the seat, radiator shrouds and the fuel tank so you can have good access to the cylinder head. Remove the valve cover and the spark plug.
Use a 17 mm socket to turn the ignition rotor so you can bring the motor to T.D.C. A notch in the rotor should line up with the notch shown.
Note the position of the cams. The lobes of the intake cam should point aft. On the cam gear you will see a small line next to the letters "IN". This line should be parallel to the to top of the head. On the exhaust cam the cam lobes point forward and the line next to letters "EX" should be parallel to the head.
Measure the clearance with a feeler gauge. If they are within spec, put everything back together, and go for a ride. If not, keep reading, you still have some work to do... :))
Intake => 0.10 - 0.19mm
Exhaust => 0.15 - 0.24mm
Remove the eight bolts that hold down the camshaft cap. Two of the bolts are longer than the rest, but more on that later. Put a piece of safety wire on the cam chain. I didn't have any handy so I used a long tie wrap. This is so you can tie up the cam chain after the cams are removed. You don't want to drop the chain down in the motor. Retrieving it is no fun. Before lifting the cams out remove the cam chain tensioner. This gives you some slack in the chain.
This is a shim and bucket. Usually the shim comes out stuck to the under side of the bucket.
The shims have their thickness stamped on one side but you should measure them just to be on the safe side. If you know the size of your current shim and the measurement you just made the table on page 4-18 and 4-19 of your service manual will tell you what new shims you will need to be within spec. Now off to the dealer to buy new shims.
With the new shims in hand, place them on top of valves with the numbers facing up. It's a good idea to write down the sizes of shims you are installing to make your next adjustment a little easier.
Put some assembly lube on the lifters before putting them back in.
Now put the cams in. With the lines on cam gears lined up with cylinder head and the motor at T.D.C. count the cam chain pins between the lines. You should have 32 of them. See the figure at the bottom of page 4-9 of the service manual.
Remember the two extra long bolts for the camshaft cap? The cap is marked with a number next to each bolt hole. The longer bolts go in the number 1 and number 6 holes. Note the small dot next to those numbers. Follow that number sequence to tighten those bolts to 8.5 ft-lb.
Now all you have to do is reinstall the cam chain tensioner, valve cover, spark plug, gas tank, radiator shrouds, and seat. Now that wasn't hard was it? Next time it will be even easier.