The clutch in the CT-90 and CT-110 Hondas are 3-plate centrifugal clutches, which is why there's no clutch lever over on the left grip. There are a few ways the clutch can fail, and while it's relatively easy to fix a clutch in a CT-90, you should know something about what you're doing.
You should not even go near a CT-90 without an impact driver. The bike is practically sewn together with many, many Phillips screws. All the side covers, most of the accessories, and a lot of other parts, are screwed on with Phillips. If you attempt to remove a side cover with a plain Phillips screwdriver, you will mess up the slots in the head and you will never get the cover off. Therefore, before you even finish reading this page, go to the store and get an impact driver with a full selection of screwdriver and nut-driver bits. If you don't know what an impact driver is, basically it's a big lump of machined steel with a special fitting inside where if you smack the butt end of the driver with a mallet, the screw tip turns maybe 1/8th of a turn. However, the act of smacking it with the mallet also drives the bit down tight into the slots in the screw head, minimizing the chance you'll chew up the screw head. These are typically about $20 and will save you much more than that the first time you
mess up a bunch of weird machine screws. Two cautions: don't get the web of your hand (that part between your thumb and forefinger) between your hammer and the butt of the driver, and don't stick the driver up on a surface where it'll roll off and fall on your head. I've done both those things. They hurt.
I once spent an entire day in Youngwood, Pennsylvania, taking the left side cover off a 1970 CT-90 to free a chain that had jammed between the sprocket and the side cover. Lacking an impact driver, I ended up having to use a hacksaw to cut flat slots into all the screw heads and turn them out with a big flat-blade screwdriver turned with a wrench. It was an incredibly mess, though I did succeed.
To do clutch work, you will also need a special tool called a "locknut wrench" or a "pin spanner." This is not commonly available, and the easiest place to find it is a Honda dealer. It is Honda part number
07716-0020100, WRENCH (16mm)
. What this thing does is remove the locknut down in the center of the clutch. That locknut is like a threaded washer with four small notches cut in its rim at 90 degree angles. You cannot take it out any other way -- resist the temptation to use a screwdriver and mallet. You will foul up the locknut. Anyway, this locknut wrench is about $29 and usually a special-order item from Honda. Order clutch discs and a right side cover gasket while you're there, and some replacement side cover screws. If you want to save yourself hassle later, consider replacing the side cover screws with Allen-head fasteners. That way you can skip the impact driver next time. The clutch discs for the CT-110 and most CT-90s are Honda part
22201-121-631 DISK, CLUTCH FRICTION
and the gasket is
The locknut wrench is 1/2" drive and is also a 14mm locknut wrench if you flip it around. Once you see it and see the locknut, its use is self-explanatory.
If the clutch slips (you crank on the gas but the bike either doesn't move forward in gear or moves forward only if you crank on the gas way too much, or if you smell what smells like burning cork, the clutch is worn or misadjusted.
If you can't kick start the bike -- if the kicker seems to slip and not turn then engine over, the clutch is probably slipping.
If you can feel unusual vibration in the gear shift pedal when you change gears, the clutch could be warped or loose.
Any loud clacky noises from the right side of the bike probably mean that the clutch has come loose or come apart. Don't run the bike, or you're liable to crack the (very expensive) side cover.
The kick-start on the CT series works through the clutch in sort of a backwards mode. When you kick the starter, the starter drives a large gear. That gear meshes with a smaller gear on a sort of spiral spline. The act of kicking is supposed to force the clutch into the "engaged" position and thus spin the crank, to which the clutch is attached. If the clutch slips, the clutch does not engage the crank and the engine doesn't turn over. You might be able to start the bike by push-starting it, but the right solution is to fix the clutch.
There were at least three different styles of clutch used on the CT-90 and CT-110 bikes, and maybe more. Some parts are interchangeable between the SL-90 and ST-90 and the CT-series. If you have the Clymer manual, you're probably looking at what they call a "Type III" or "Type VI" clutch, and sometimes a "Type V." Most of the other types listed by Clymer were for 50cc or 70cc bikes.
Tearing Into Things
To service the clutch, you need to remove the footpegs and sidestand. There are 4 bolts directly under the engine. The engine shield can stay; the brake pedal can stay. Remove the kick lever -- the setscrew is
so don't lose it. It requires a 10mm wrench. Once you take the lever off, put the screw back in it so you don't lose it.
Remove all the side cover screws using your impact driver. Be careful -- a stray smack with the mallet and you could crack the aluminum casting. Take
on where the screws came from. They are
not all the same length
and must go back exactly where you got them! At left is a picture of a 1986 CT-110 with the sidecover removed. The kick lever is back in place (I removed it to remove the side cover, then replaced it to check some stuff). The large round donut is the clutch assembly... it spins when the engine is running, and as it spins faster, the springs in its face gradually let the clutch discs inside apply pressure to the driven plates and the bike can move forward.
Once the screws are out, you can pull the side cover. Watch out for small parts which may tumble out when you slowly and carefully pull the side cover away from the engine, and if you can, avoid damaging the gasket. You should replace the gasket when you're ready to finish the repair, but if you need to reassemble and test before you're finally finished, no sense messing the good gasket up. Once the side cover is off, there will be some clutch-release parts in the "well" in the cover, and some still on the side of the clutch. Keep very careful track of them.
To remove the clutch release bearing, you need a small Phillips bit in your impact driver. There are two of them, they're flat-headed and small. Save them, and pop the release bearing off the side of the clutch. That should expose the center hub of the clutch. Take a small screwdriver and pick the "keeper" tabs out of the notches in the locknut way down in the center. Using a large pair of ChannelLock pliers, hold the clutch housing still and use your 16mm locknut wrench to undo the locknut. Honda makes a special tool to hold the clutch still while you do this; pliers work fine. If you're reading Clymer, they suggest sticking a penny between the big kickstart gear and the small one, or sticking a screwdriver in the spark plug hole to keep the engine from turning over. If your clutch is slipping, neither of these suggestions will do a damn thing. Hold the clutch housing, or make a tool to hold the clutch.
At the left is a picture of the clutch still in place, showing the slots that the special clutch tool will fit into on the locknut. If the locknut is out, you should be able to ease the clutch off the crankshaft. There's a bushing left on the crank. Leave it there unless it's damaged. Don't touch those Phillips screws you see (4 of them) around the face of the clutch assembly, at least not yet. The whole thing comes off like a big donut. A big inedible donut. Tap it off and make sure it stays together for now.
Take the clutch someplace you can work on it comfortably... there are small springs and stuff in it, and you must not lose them, so do not work on a grass, dirt or gravel surface. Spread a towel out if you have nothing else, but put something down to catch small parts.
The clutch is held together with an enormous circlip. Remove it, and the clutch discs should pop a little ways out courtesy some small return springs. If you can see problems already, or smell them (burned cork smell) you might as well just take the whole thing out at once. Grab the "gear" in the middle of the clutch and lift straight up. All the clutch discs will come out with the gear. Save all the little springs. Make careful notes about how the metal discs and the friction discs and the little springs related to each other.
If you think the clutch discs are OK, measure them with a micrometer. Lay all three friction discs (the ones with the ridges and grooves around the rim) together and measure how thick they are. On mine, the clutch slipped with the old ones and did not slip with the new ones. The old ones totaled 7.41mm, and the new ones 8.07mm.
was enough to keep the bike from working
If in doubt, replace them. Always replace all three, no matter what the "good" ones look like. Save the best for emergency use but replace all three as a set during a normal repair.
If you see severe burning or metal distress on the clutch plates (the shiny metal discs with ears on them) the clutch probably suffered burnout and you should consider replacing the plates, too. If the heat was severe, as in catastrophic clutch failure, the springs themselves may have lost their zing. Replace anything that looks like it had a hard life. This is not a job you want to do twice. Another thing to check is for any cracks in the clutch housing. If that thing comes apart at high RPMs, you could end up with a piece of cast aluminum coming through the side cover and into your leg. That would hurt.
Clymer illustrates pretty well how to reassemble the clutch plates, but if you took careful notes when you took it apart, you don't need Clymer.
Once you have the clutch back together, stick the circlip back in (you need to compress the clutch slightly). Stick the clutch back on the crank, put the keeper back into the hub (the thing with the bendable tabs on it) and then torque the locknut back in place and bend one of the bendable tabs into place to keep the locknut from working loose.
Reassembling the clutch release is kind of a trick, and the illustrations in the Clymer don't help a lot. I may add some photographs here to illustrate how to reassemble it, but one tip I can make is that if the bike is tilted to the right side, the side cover and the clutch release parts go back on easier.
Once the side cover is back on, you need to adjust the clutch. To do this, release the locknut that's under the rubber cover (on CT-110) or out in space (CT-90) on the side of the cover. Back the setscrew off until it turns freely, then screw it back until you just begin to feel a little resistance. What that screw controls is the "pre-compression" of the clutch. Too tight, and the clutch will slip and burn out. Too loose, and the clutch won't release properly when you shift, and the gears will grind. Just make sure the locknut is tight. Hold the setscrew while you tighten the locknut.
Reinstall the kick lever, making sure to reinstall the setscrew and tighten it properly, or you'll trash the splines on a very expensive and hard-to-replace kickstart shaft. Start the bike up (assuming you fixed the clutch right) and you need to adjust the clutch.
Start the bike and put it in gear. At idle, the CT-90 should never "walk" forward in gear. It should only move when you actually rev the engine. If the bike attempts to "walk" at idle in gear, adjust the screw in the sidecover "tighter" until the engine doesn't try to walk the bike. If the clutch is already too tight, to where it doesn't start moving until you practically crank the throttle wide open, back the screw off some. Eventually you'll be able to feel this easily.