Today’s motocross bikes and the ultra light power plant engines that power these bikes share many traits of a Formula 1 race engine including titanium valves, ultra tight clearances, aluminum head, and high compression pistons. All these parts make for one amazingly fast and light motocross bike, but they also mean more maintenance. As an owner of one, you are probably well aware of the maintenance required on these bikes. One of the most common issues that occur with these engines is that the valves tend to wear/settle and get out of spec, due to the characteristics of the Titanium valves. On today’s tech tip Tuesday we take apart a Honda CRF250R, check the valve clearances, and show you how to re-shim the valves so that it will once again purr like the tiger it used to be. If you have an air cooled bike or one that doesn’t use shims for valve adjustment, check out our other post on adjusting non shim dirt bike valves. Though we are working on a CRF, the process and procedure for changing out the shims is the same for the Honda CRF450r and other shim type valves bikes such as the YZ450F, KX450F, RM-Z450 and more. Most other motocross bikes such as the YZ will have two cams instead of the single overhead cam that Honda bikes have, but they still use the same type shims to adjust the valve clearance.
What you’ll need:
- Socket set
- Valve type feeler gauge set (with mm measurements)
- Torque Wrench
- Extra shims
- New engine oil
- Bungee or cord (to keep cam chain from falling into case)
- Digital caliper gauge (for measuring shims)
- Magnetic pickup tool (optional but very helpful)
- Cam Chain tensioner tool (optional)
- Pen and paper
- Beef jerky (you might need a snack mid job)
Okay first things first you’ll need to remove everything that will keep you from getting to the valves. Start with removing the seat, plastics around gas tank, gas tank and hoses going to carburetor (don’t forget to turn petcock to off). Once you’ve got those out of the way, carefully remove the spark plug wire/boot and set it to the side so it’s not in the way. At this point, make sure the top of the engine and valve cover is very clean. Clean it and spray it off with compressed air so that no dirt will fall into the top end of the motor after you take the valve cover off. Remove the breather hose from the back of the valve cover and proceed to remove the bolts on the valve cover. At this point, if you have been using power tools to remove bolts, then put them aside and use only hand tools. That will keep you from messing anything up with too much torque. Once you have the bolts off the valve cover, remove it and place it with your other parts on a clean surface or shop towel. At this point you can check the valve clearances, but you will need to put the engine at top dead center. In order to do this, remove the inspection hole on the left side of the crank case.
Then take off the the service hole on the right side of the crank case where you can turn the crank shaft with a hex bit. Turn the crank shaft until the mark inside the crankcase lines up with the mark on the crank case cover. The cam will also have two marks on it which will line up with the top of the head. There are two Top Dead Centers, one at the top of the exhaust stroke and one at the top of the compression stroke. You want to have your engine at the top of the compression stroke in which the cam lobes will be outward and up as shown in this pictures (no valves are being pushed down on). Now you can check the valve clearances with your feeler gauges. You will check it by inserting the feeler gauge between the cam and the shim bucket which is directly beneath the cam. For Honda that have a single overhead cam such as this CRF 250R, you will measure under the rocker arm for the exhaust valve clearances (as opposed to under the cam on DOHC engines) as shown in pictures. The feeler gauge should have slight drag on it when you are measuring the clearance and it shouldn’t have to be forced between the bucket and cam in order to be measured. As you measure each valve, record the clearance in a notebook. On this particular CRF, the exhaust valves are in specs, so we won’t be adjusting about them, but the intake valves are too tight to get a feeler gauge in them so we will have to put a smaller shim in them to get them back to factory specs. Once you have the measurements, compare them to specs given in your repair manual (if you don’t have a repair manual yet, buy one as it will most likely pay for itself in one repair job). If your valves are out of specs, you’ll need to adjust them by replacing the current shim with a smaller or larger shim to make up the difference. In order to do this you’ll need to now remove the cam shaft(s) so you can get to the buckets and shims. The first step in doing this is to take the slack off of the cam chain by using a cam chain tensioner tool or by simply removing the cam chain tensioner altogether if you don’t want to buy the tool. Once the chain is loose, remove the bolts securing the cam shaft journals. Be extremely careful when removing these as there are thin c-clip guides beneath them that can fall down into the crank case, making for a much longer job (see photo). Once the caps holding the cam shaft in are off, remove the chain from off the end of the cam sprocket(s). As you do so it is a good idea to tie something on the chain or hold it with something to keep it from falling down into the crankcase. You may need to slide the bearing down the shaft a little in order to get the chain off. Once the chain is off the cam, secure it so it doesn’t drop in the case, then carefully remove the cam shaft and place it on your cloth with the other pieces making careful note of which cam goes where and which cam cap goes in which spot. At this point you should now see the buckets that are covering the adjustment shims. To remove these buckets and the shims, take your magnetic tool and place it in the center of the bucket and lift it carefully out. With luck, the shim should stick inside the bucket, but the don’t always so take care that you keep your hand under it as you are removing it so you don’t lose one falling into your engine or crankcase. At this point you can use a digital caliper gauge to measure the shim(s) (an analog caliper gauge as well, but the digital ones work good enough and can be had for less than $15). We only took out the intake valve shims, since our exhaust valves were in spec, but the process is the same for those. In order to figure out where to go from here, take the actual clearance of your valve and subtract it from the value of where it should be, then subtract that value from the size of your current shim to give you the value of the new shim you will need. Here is the formula:
Current Shim – (Clearance Spec – Actual Clearance) = Size of new shim
Here’s how our bike calculated: 2.10 – (.15 – .00) = 1.95mm
Since both of the intake valves had no clearance, they measured 0. In our case, we know that the bike is running pretty rough due to the valves being out of spec and they are most likely more than .15 out of spec so we are going to put in a size 1.85 shim to go even smaller. Since it likely has a negative clearance. Most shims come in clearances of .05mm increments so measure everything exactly and round down to the nearest .05. When in doubt, it’s always better to have too much clearance than too little since valves get tighter as they wear.
Once you’ve figured out which sized shim(s) to replace your old ones with, do so carefully. It’s a good idea to measure the new shims and not just go by what’s printed on the surface. Install the shims and put a dab of fresh oil on them before you replace the buckets. From there, carefully replace the camshaft and chain making sure that the camshaft marks are lined up correctly with the top of the head and that the cam lobes are facing out/up (not pushing on the valves. Put back the guides and journals and carefully torque down to specs (check specs in your specific manual) with a torque wrench in a crisscross pattern. At this point, the cam shaft should move slightly when you wiggle it back and forth at the cam gear. If it binds then you didn’t put everything back together properly.
If it looks good measure the valves again and see if they are in spec. If they are still tight or off, you’ll need to recalculate the shims and try again until you get them within clearance. Sometimes this can be a tedious process, but the end outcome is worth it. Once you have achieved correct valve clearance, congrats! It’s all down hill from there. If all the valves look good, take time to slowly crank the engine over by hand a few times using the side access hole in the crank case and make sure everything moves and looks normal before putting the valve cover back on. If everything is good, follow the reverse steps to button the bike back up (valve cover, breather tube, spark plug wire, gas tank, gas lines, plastics, seat, etc). Make certain that you torque the valve cover on to the exact torque specifications alternating between bolts a few turns at a time until they are at the right torque specs. Once you’ve got everything back on, your bike should be running like it’s former beast self, so go out and ride!
Do you have a tech tip question you need answered? Email us at firstname.lastname@example.org and we might answer your question in our next Tech Tip post.