Towards the end of last riding season, I noticed that when starting my WR250R (2008) after it had been sitting for a while, it made a metallic clattering sound from the area of the cylinder head. The jangling lasted only a few seconds before fading away to the usual, quiet putt-putting sounds. Some Googling of the symptoms and a little intuition led me to suspect that the cam chain was loose, causing the noise. As engine oil pressure built and fed the oil-assisted automatic tensioner, it would take up the cam chain slack and cause the noise to quickly fade away.
The tensioner protrudes from the engine case right above the starter motor, and right below the cylinder head.
The automatic tensioner contains a spring and a winding mechanism, and is supposed to retain tension on the cam chain after the engine is shut off. However, something must not have been working right with this mechanism, allowing it to slowly lose tension over time until returning oil pressure could make up the difference again. I suspected a weakened spring or binding of the mechanism.
Apparently, this is a known problem with some WR250Rs and can eventually lead to the chain skipping and losing cam shaft timing. Yamaha seems to have issued a recall for the problem around 2010-2012, in which they replaced the automatic tensioner. However, I wasn't aware of this recall, so I was left with two options to fix it myself: (1) order a replacement tensioner from Yamaha; or (2), install an aftermarket manual tensioner. Although I've read many positive comments about manual tensioners, I wasn't keen to go that route. Further digging found that Yamaha issued a new tensioner part number around 2012, which hopefully solves the problem. I decided to order the new tensioner. Worst case, I'll get 27,000 km out of it--just like I did with the first one.
The original Yamaha part number for the automatic tensioner assembly is 3D7-12210-00-00. The newer version is 3D7-12210-11-00, which I ordered from Babbitts Online along with a new fibre gasket.
You'll also need to replace the copper gaskets/washers under the M6 bolts, as they are well crushed from the original installation and shouldn't be reused. These gaskets are the same type as used on the coolant drain M6 bolt and are approximately 11.5 mm diameter with a 6mm hole. The Yamaha part number is 90430-06014-00, and the same part from Honda is 90463-ML7-000. You may also be able to find them at a specialty fastener or auto parts store.
Here's the full assembly (my old one):
So, how to replace this thing?
Initially, the job may seem rather intimidating, but it's actually not too bad and could be done in an evening or two assuming you have all the parts and nimble fingers. No fancy tools are required, although it helps to have various lengths of 8mm sockets and drivers. If you're super careful, you could probably replace the tensioner without removing the valve cover, but I opted to take this opportunity to check the valve clearances and inspect the cam chain in case my original assumptions about the tensioner were wrong. I also decided to replace the cooling hoses, since I would need to drain the coolant anyway. The original hoses, while seeming to be in good condition, are now 10 years old and my bike will be called upon to withstand desert heat this summer. I'd rather take my chances with new hoses so I ordered the DRC kit.
The following steps are adapted from various internet sources and are meant to complement the Yamaha WR250R shop manual. I won't replicate what other sources explain; I'm just adding some points to help clarify or simplify. These steps assume you've already removed the seat, side panels, and gas tank.
Before you unbolt anything
1. Disconnect the battery and remove the ignition key. This will prevent you or some "helper" from accidentally trying to start your bike until you've completed and double-checked your work.
2. Unscrew the circular covers for the crankshaft and generator rotor on the left side so you can access the crankshaft bolt and find the TDC mark on the generator.
3. Using a socket wrench on the crankshaft bolt, and while looking through the generator hole with a flashlight, slowly turn the crank counterclockwise until the TDC mark (a single engraved line) aligns with the mark on the view hole. (Note that the TDC mark is just clockwise of an engraved "H" mark on the generator, which indicates the ignition point.) Aligning TDC now will save you some hassle later.
Now you need to clear a path to access the two 8mm bolts that secure the tensioner to the cylinder head. This means removing the starter and some other stuff.
Remove the tensioner
4. Remove the exhaust header pipe.
5. Loosen the muffler from its frame bolts. You don't need to disconnect the EXUP valve actuator cables on the muffler. You should be able to just swing the muffler out of the way of the starter bolts and hold it to the frame with some wire, while being careful to not crimp the cables.
6. Disconnect the electrical wire on top of the starter. The rubber boot just pushes up out of the way. The boot may be full of crud, so get a toothbrush in to clean it out.
7. Unbolt and remove the starter by pulling it straight back. Be careful to not allow accumulated gunk to fall into the open hole. Clean all grime off the starter, especially around the O-ring where it mates to the engine case, and on the electrical terminal.
8. Back on the left side of the bike, remove the actuator mount for the clutch cable. It'll facilitate accessing the lower tensioner bolt, and you might as well inspect and clean the clutch actuator while you're in there.
9. By now you should be able to loosen the two bolts holding the tensioner. Note that the tensioner will be trying to push itself out of its mounting position because of its internal spring. The two bolts are installed with crush washers and may take a bit of force to crack loose.
At this point, you could jump to step 21 and install the new tensioner. However, there's a risk you may inadvertently allow the cams to skip out of correct timing when re-setting the tensioner against the chain, which would mean having to remove the valve cover to check and reset the timing. That would be a hassle, so I recommend just carrying on with removing the valve cover, which can be done from the right side if you clear a path as follows.
Remove the valve cover
10. Drain the engine coolant. As noted above, the water pump drain bolt uses the same size crush washer as the tensioner.
11. Unbolt the metal clip that holds the coolant hoses to the frame next to the radiator (it also secures the radiator to the frame), then disconnect the hoses from the rad and gently bend them down and out of the way of the valve cover area.
12. Remove the top bolt holding the radiator to the frame, and loosen the bottom bolt. This should give you enough freedom to swing the radiator slightly forward and out of the way of the valve cover.
13. On the left side of the engine and near the top of the valve cover, there's a thick wiring bundle and vent hose zip-tied to a tab on the frame. Cut the zip-tie and disconnect the vent hose from the forward assembly in the left shroud. (By the way, this assembly directs fresh air from the air box into the exhaust port, to assist in combusting unburnt fuel.) Tuck the hose down and out of the way.
15. It's a good idea to now clean the area of debris, blowing it out with compressed air so there's less chance of grit falling into the soon-to-be-exposed cam area.
16. Remove the spark plug.
17. Unbolt the valve cover. You should be able to wiggle it up, right, forward, and then rotate it clockwise to get it out of the frame. Mounted on the throttle body is a little vacuum widget that looks like it will block the cover, but you actually do not need to remove this part and can wiggle the valve cover around it. In the picture below, I've already removed the valve cover, but the widget is the thing with two Phillips (actually JIS) screws and two hoses going into it.
Now the valves are exposed, and you can check clearances with feeler gauges. The clearance specs are:
Intake (rear of bike): 0.13-0.20 mm (0.0051 - 0.0079 in)
Exhaust (front of bike): 0.23-0.30 mm (0.0091 - 0.0118 in)
I was able to easily insert a 0.10 mm feeler under the intake cams, but a 0.15mm feeler was tight, indicating they were probably right on spec around 0.13 mm.
The exhaust cams were easy with a 0.20 mm feeler, but very tight with a 0.25 mm feeler. So, probably a bit too tight vs. spec, but that's how it came from the factory and 27,000 km later it's running fine, so I'm not going to change anything.
Confirm your cam timing
18. With the crank at TDC (you can double-check TDC by looking in the spark plug hole and confirming the piston is up), check the timing of the cam shafts: the correct position is with the lobes facing away from each other, and the timing marks on the cam gears parallel with the top edge of the cylinder head. Note that it is surprisingly easy to skip the timing chain on the cam gears, so you definitely want to confirm the positions before reinstalling the valve cover.
20. Double check the position of the TDC mark down in the generator hole. At this point, everything should be perfectly aligned.
Reinstall the tensioner
21. First you need to compress the tensioner piston and lock it into place. You can do this by squeezing it between your thumb and fingers, with the piston end at your thumb, while rotating the body of the tensioner (the piston only retracts with a winding motion). Once the piston reaches the bottom, hold it there and gently squeeze the little wire clip at the end to lock it into the groove, and gently release your thumb pressure. The piston should stay in place. If this method proves too tricky, you can gently squeeze the piston in a non-marring vise. Note that applying slight pressure to the end of the piston releases the wire clip and allows the piston to extend--you'll need to do this after re-installing the tensioner.
22. Insert the tensioner into the engine case, using a new gasket if necessary (wasn't in my case) and install only the front mounting bolt, using a new crush washer. Snug the bolt just past finger-tight, but not so you crush the washer. Depending on how setting the tensioner goes in a moment (e.g. if you goof up and jump the timing chain), you may need to remove and reset the tensioner, so using just one bolt to hold it in place will avoid a lot of fiddly frustration.
23. Now you need to release the tensioner piston so it presses firmly against the chain guide and takes up the slack in the timing chain. The trick is to do it without skipping the chain: hold the left (exhaust) gam gear firmly with your left hand, and slowly turn the crankshaft counterclockwise while keeping hand-pressure on the chain. After about half a revolution of the crank, the chain should tighten up enough that you hear a slight "click" as the tensioner release the piston, and it'll be obvious to see the chain slack slowly disappear.
24. Carefully turn the crank by hand counter-clockwise over a few revolutions to confirm that the TDC mark and cam timing positions are correct, and chain tension is firm. In my case, after setting the tensioner I found that both of my cam gears were just slightly off horizontal by half a tooth, even though the TDC mark was in the right place. This appears to be within normal tolerances, as experimenting with moving the cams by a tooth either way to try to correct the angle only made it worse.
25. Install the second tensioner bolt and torque both bolts to 10 N-m.
26. Inspect the valve area for debris. If there's any grit, wipe it away carefully.
27. Reinstall the valve cover. The manual says to replace the valve cover gasket, but if it's clean and in good shape, you should be able to re-use it without issue. For extra security, I ran a thin bead of silicone gasket sealer (Hondabond HT) on the underside of the gasket before mounting it back on the head.
Reinstall all the other bits by following the instructions in reverse order.
It's a good idea to change the oil after this procedure, to flush out any grit that may have fallen into the valve area.
I'm happy to report that after reassembling my bike, it fired up perfectly the first time and purred like a kitten. The start had none of the jangly sound that set me on this service path, and the engine just sounds really good.
As a footnote, I disassembled the old (left) and new (right) tensioners to see if I could notice any differences.
Other than the date code and part numbers on the case, the parts seem identical, although the new tensioner is noticeably easier to compress the original one.