Cam chain tensioner removal
While there have been a few reports of failure, the cam chain tensioner is not a big issue for 250 owners. Leon, a long-time club member and multiple 250 owner, says not to worry about it: "I've never had an issue with a cam tensioner on the Ninja. In this case, fixing it before it breaks may be what causes it to break."
How it works
The purpose of the tensioner is to take up the slack on the cam chain as it stretches with normal use. It's designed to only move in one direction. Springs push on a pushrod, which pushes on the cam chain guide. A ball and retainer assembly, in combination with the pushrod and the tapered barrel of the chain tensioner housing, form a ratchet mechanism that allows the pushrod to go out (keeping the correct tension on the chain) but not come back in. For a more technical explanation, see the bottom of this page.
Why this part is important is because if the tensioner goes, the cam chain can slap around within the engine, tearing off bits of the plastic that are there to help guide it and protect the metal. These can float around in the engine until they get stuck somewhere or do damage. The chain can also jump teeth, in which case the timing is off and the bike runs terribly, or you have that unhappy valve-to-piston contact.
The usual noise for a tensioner getting worn is a loud (usually consistent) ticking on the upper right side of the motor, similar to noisy (loose) valves. If you have these symptoms, and you know the valves to be correct, you should replace the cam chain tensioner. A new one is about $35. Part number for the whole assembly is 12048-1106.
The Kawasaki Service Manual doesn't call for any maintenance or lubrication of the cam chain tensioner. It lives in the engine interior and is exposed to oil splashes and vapors, so it receives some lubricant on its own.
Leave it alone unless it becomes an issue, or you perform work that requires its removal. If you remove it, you must reset it for installation (see below). While the tensioner is out, don't allow the engine to turn. You may want to disconnect the battery, just in case.
If you do remove it, inspect the tension rod for damage. If there are signs of scoring, it can be sanded smooth. However, if the rod or bearing are not in good shape, your best bet is to replace the entire assembly.
Sometimes, even though you put it back together right, it may take more than one try to get it to "set" properly. If you install it and it still rattles, pull it back out and try installing it again. Sometimes they like to be loved "just so". To help get it right the first time, make sure the shorter spring (the one that keeps tension on the bearing) stays lined up as close to perfect as possible. If not, it could allow the bearing to cock sideways, which jams up the rod nice and tight. This may be the cause behind sticking rods on reassembly.
The service manual does not mention a need for oil or grease upon reassembly. This has been a subject of discussion on the forum. Members have done both, and both have worked.
The ratchet spring just keeps the ball bearing race in contact with the tension rod and the tapered bore (inside surface of the tensioner housing). In the drawing, the pink is the housing (metal) and the purple is the bore (absence of metal) and the inside surface is at the edge of the purple area. The washer in the diagram represents the bearing cage. Its purpose is to keep the bearings distributed around the rod and to transfer the force of the ratchet spring to the bearings, pushing them toward the narrow end of the taper.
When the rod moves out (left in the diagram) the bearings roll toward the wider end of the tapered bore. At some point they will lose contact with either the rod surface or the bore surface, will slip, and will be pushed back in contact by the ratchet spring. Hence the rod can move out with essentially no resistance.
When the rod moves in (right in the diagram) the bearings roll toward the narrow end of the tapered bore. They won't be able to go far before the space between the rod and the tapered bore is smaller than the bearings and they get wedged into this decreasing space, preventing the rod from moving further in (unless something slides instead of rolls).