The Honda CT90 “Trail 90” uses a semi-automatic centrifugal clutch instead of a hand-operated manual clutch. When the friction discs wear out, the clutch will begin to slip, especially under load or acceleration. Rebuilding the auto clutch restores proper engagement and protects the rest of the engine from friction debris. This guide walks through removal, disassembly, cleaning, reassembly, and adjustment of the CT90 auto clutch assembly.

Understanding the CT90 Auto Clutch

As engine speed rises, the centrifugal weights swing outward and clamp the clutch pack together, transmitting power from the crankshaft through the transmission to the rear wheel. When engine speed drops, springs pull the weights inward and release the clutch.

When to rebuild the auto clutch

Common signs that the auto clutch needs service:

• Engine revs increase but road speed does not keep up, especially under throttle.

• You feel slipping when shifting, or the motorcycle hesitates before fully engaging.

• Metallic or burnt friction smell in the oil.

• Visible friction debris inside the clutch basket when opened.

Tools & Parts

• Metric socket set 

• Metric combination wrenches

• Allen keys / hex drivers (if your cover bolts have been upgraded)

• JIS-style screwdrivers or impact driver for original clutch and filter screws

• Small flat-blade screwdriver or pick for snap rings and springs

• Brass or plastic mallet

• Needle-nose pliers

• Torque wrench (for reinstalling the oil filter nut and cover fasteners)

• Electric impact driver (for removing the oil filter nut)

• #9005 oil filter castle-nut socket

• Replacement auto clutch friction discs (x3)

• CT90 Oil Change Gasket Set

• Mineral engine oil plus zinc ZDDP) additive 

• grease (for gaskets and shift mechanism pieces)

• Anti-seize Lubricant

• Parts cleaner / solvent and brushes

• Shop towels, drain pan

Step-by-Step Instructions

1) Drain the Engine Oil

1. Warm the engine briefly so the oil flows more easily, then shut it off and support the motorcycle securely.

2. Place a drain pan under the engine. Remove the 17 mm drain plug and allow the oil to drain completely.

3. Inspect the drain plug head and threads. Plan to replace a badly chewed plug or damaged aluminum crush washer.

2) Remove the Right Side Cover

1. Locate the rubber cap on the right side cover over the clutch adjuster. Pry off the rubber cover and set it aside.

2. Remove the stamped cup-shaped washer and locknut from the adjuster screw. Keep track of the small O-ring that seals the adjuster. Note: The clutch will be re-adjusted at the end of the repair; exact adjuster position now is not important.

3. Remove all the right side cover bolts. These can often be stuck so have an impact driver handy if they have never been removed before. We recommend replacing the factory JIS screws with our stainless steel Allen head bolt kit for easier service next time. Pay attention to bolt lengths. This cover uses two longer bolts at the dowel locations; group them so they go back in the same positions.

4. Gently tap the cover with a soft mallet to break it free. Do not pry between the gasket surfaces with a screwdriver. If the gasket surfaces were previously coated lightly with grease, the gasket may release intact. If it tears, plan to replace it.

5. Once free, carefully remove the cover, watching for the small spring and “three-ball” auto-clutch adjuster parts on the inside of the cover. Set these small parts aside so they are not lost.

3) Remove the Auto Clutch Assembly

1. With the side cover off, locate the centrifugal oil filter cover on the front of the clutch basket. Remove the two screws securing the cover.

2. Pull off the cover and gasket.

3. In the center of the clutch basket is a four-prong nut that secures the clutch basket / oil filter to the crankshaft. You may have to clean out junk in the oil filter to be able to see it. Use a punch and hammer to bend the tabs of the multi-tab washer away from the nut so the nut can turn.

4. Because the clutch is slipping, you usually cannot simply wedge gears to hold it. An impact driver is the most efficient way to remove the nut:

   • Use our #9005 oil filter nut tool on an electric impact driver to easily loosen and remove the nut. If you do not have an electric impact driver, you will need to find a means to hold the clutch basket from spinning while you loosen the oil filter nut. 

5. Remove the oil filter nut and the tab washer and set them aside.

6. Slide the entire clutch basket assembly off the crankshaft.

7. Remove the inner bushing / bearing from the center of the clutch and the separate crankshaft bushing if it comes off with the basket. Keep these bushings clean and oriented correctly for reinstallation later.

4) Disassemble the Centrifugal Auto Clutch on the Bench

Place the clutch assembly on a clean work surface. Maintain the order of parts as you remove them.

1. On the face of the clutch basket, locate the large snap ring that retains the clutch stack.

2. Pry the snap ring out of its groove using a small flat-blade screwdriver, working around the circumference until it lifts free. Under the snap ring is the first steel plate. Lift it out and note that the basket may contain loose friction debris. As you remove each part, stack them in order to mirror the way they came out.

5) Identifying Steels, Spring, & Other Components

Inside the clutch pack you will find:

• Three friction discs – all identical.

• Four steel plates – each one is different:

  • A base steel with six spring pins.

  • A steel with all tabs fully notched.

  • A steel with alternating notches and small round holes in the tabs.

  • A steel with round holes in every tab.

• Six small coil springs:

  • Four long springs that run the full length of their pins.

  • Two shorter springs that are about half as long and sit on the center pins.

Opening the basket and auto-clutch core

To thoroughly clean the unit, the clutch basket halves must be separated:

1. Identify the four screws on the back side of the clutch that hold the auto-clutch core to the basket. These screws are frequently tight and already chewed up. Use locking pliers and/or an impact driver as needed to remove them. Plan on replacing them with new hex or socket-head Allen bolts.

2. Use a small flat head screwdriver to pry out the four small carrier springs that sit partially in the posts.

3. As the screws loosen, spring pressure will push the basket halves apart.

4. Separate the basket from the auto-clutch core and remove the four large clutch springs beneath the screw heads.

5. Do not disassemble the centrifugal weights and large snap ring inside the auto-clutch core unless something is obviously broken; reassembling that snap ring with all the weights in place can be difficult.

At this point, you should have:

• Outer basketInner auto-clutch core with weights

• Four large coil springs and four small carrier springs

• Four mounting screws (to be replaced)

• Clutch steels, friction discs, and small springs

6) Cleaning and Inspecting Components

Thorough cleaning is critical as old friction material packs everywhere inside the basket.

1. Wash the clutch basket, core, steels, and small parts in solvent or a dedicated parts washer.

2. Scrub all pockets and oil passages until all friction debris is removed.

3. Inspect steel plates:

   • Light scoring can usually be dressed with fine grit sand paper.

   • Check for warping per the service manual and replace any plates beyond spec.

   • Scuff the steel plates lightly with sandpaper to remove any glazing and create a fresh surface for the new discs.

4. Inspect all springs for collapse, distortion, or damage and replace as needed.

7) Reassembling the Centrifugal Auto Clutch

1. Place the auto-clutch core (with weights still installed) on the bench, weights facing up.

2. Install the four large coil springs onto the threaded posts.

3. Position the clutch basket over the core, aligning:

   • Round holes with the threaded posts.

   • Keyhole-shaped holes with the smaller spring posts.

   • Any indexing notches on the rear of the basket.

4. Install new bolts with the original washers in place of the old JIS screws.

5. Press down on the basket to compress the springs and start each bolt a few threads.

6. Snug the bolts evenly in a cross pattern until they are fully seated and tighten then slightly past that. They do not require heavy torque; the spring pressure helps keep them secure.

Reinstall the four larger post springs first:

1. Hook the last coil of a spring onto the tip of a small flat-blade screwdriver.

2. Place the spring over the post, compress it with your finger, and use the screwdriver to guide the coil into the notch on the post. Repeat for all four posts so the springs sit neatly in their grooves.

Stacking steels, springs, and frictions in the correct order

Before final assembly, pre-soak the new friction discs. Place the three friction discs in a small tub and pour a little clean engine oil over them. Let them sit until fully saturated.

Now stack the clutch pack in the following order:

1. Base steel with pins

   • Install the two short springs on the two center pins.

   • Install the four long springs on the four outer pins. The long springs should be almost flush with the tops of the pins; the short springs sit lower.

2. Friction disc #1 – drop into the basket, engaging with the inner hub.

3. Steel #2 – all tabs notched

   • Orientation does not matter; all notches are identical.

4. Friction disc #2

5. Steel #3 – alternating notches and round holes

   • Align the round holes over the two short springs on the center pins.

   • Align the notched tabs over the four long springs.

   • This plate is the most critical for correct spring alignment.

6. Friction disc #3

7. Steel #4 – round holes in every tab

   • Orientation is not critical; all holes are identical.

   • If one side shows wear from the snap ring, flip it to use the cleaner surface.

Installing the snap ring

1. Verify that all steels and frictions are fully seated and that the springs sit correctly in their holes/notches.

2. Position the large snap ring over the stack, aligning the finger tab with one of the steel tabs.

3. Starting at one point, press the snap ring into its groove by hand and walk it around the circumference until it fully seats.

4. Confirm the snap ring is fully in the groove all the way around; it should not ride on the edge of the basket. The auto clutch assembly is now ready to reinstall on the engine.

8) Reinstalling the Clutch Basket on the Engine

1. Lightly oil the inner bearing surface in the clutch core and both crankshaft bushings. Install the inner bushing into the clutch core if it was removed.

2. Slide the clutch basket onto the crankshaft, ensuring it seats fully against the shoulder.

3. Install the multi-tab washer over the shaft, aligning the inner tabs with the splines inside the clutch basket.

4. Thread the four-prong oil filter nut onto the shaft by hand with the rounded side facing in.

5. To torque the nut, you must hold the basket from rotating:

   • Temporarily reinstall two of the clutch cover screw holes with longer M5 bolts and nuts.

   • Tighten the nuts against the backing plate on the engine so the bolts become solid “handles.”

   • Use a pry bar between the bolts to hold the basket while applying torque.

   • The CT90 uses a steel clutch post boss, so this method is acceptable. Do not use this technique on engines with aluminum posts.

6. Use a torque wrench with the oil filter nut socket to tighten the nut to 27.4 - 32.5 ft. lbs.

7. Remove the temporary bolts. Bend one or two of the outer tabs of the tab washer up against the flats of the oil filter nut with a punch and hammer. The goal is simply to prevent the nut from backing off.

8. Clean the oil filter cavity thoroughly and install the new gasket on the filter cover.

9. Reinstall the cover and secure it with the two screws. Snug the screws evenly and do not overtighten.

9) Reassembling the Right Side Cover and Shift Mechanism

Lightly coat the machined gasket surface on the crankcase and right side cover with a thin coat of grease. Install the new right side cover gasket onto the engine, aligning it with the dowel pins. Grease helps the gasket release cleanly at future services.

The CT90 auto clutch uses a small linkage, ball bearings, and springs inside the right side cover. These parts can be fiddly so grease is your friend here.

1. Place a small amount of clean engine oil on the ball bearing that sits in the pivot in the case.

2. Install the “horseshoe” shaped piece (often called the shift cam segment) over the ball in its bore.

3. Install the small return spring that works with the segment, using a dab of grease on the end so it stays in its pocket.

4. Position the external shift arm on its splined shaft:

   • The arm should sit roughly around the 2 o’clock position.

   • The notch in the “horseshoe” segment will sit around 7 o’clock when the splines are correctly aligned.

5. Prepare the ball lifter plate:

   • Apply grease to the small spring that clips to the lifter plate and stick it in place.

   • Apply a small amount of grease in the recesses where the balls will ride so the plate stays put during cover installation.

   • Orient the lifter plate so the raised bump and notch match what is shown in the factory manual and the video.

6. Carefully set the lifter plate assembly in position, ensuring it sits flat and the spring and ball remain in place.

7. Apply a light film of grease to the kickstart shaft seal lip and carefully slide the right side cover onto the engine.

8. Once the cover seats against the gasket and dowels, install the cover bolts.

9. Snug all bolts lightly, then tighten them in a crisscross pattern so the cover pulls down evenly. We recommend using a small dab of anti-seize on all bolts for easy servicing in the future.

10. Reinstall the cleaned drain plug and crush washer.

10) Adjusting the Auto Clutch and Refilling Oil

1. With the engine off, loosen the locknut on the clutch adjuster screw on the right side cover.

2. On this mechanism, turning the adjuster counterclockwise tightens the internal linkage and takes up slack. Turning clockwise adds free-play.

3. Starting from a light-contact clockwise position, slowly turn the adjuster counterclockwise while lightly moving the shifter with your other hand:

   • As you tighten the adjuster, you will feel the pedal become firmer with less slack.

4. When you feel the adjuster “bottom” and resistance increases, back the screw off about 1/8 turn.

5. Hold the adjuster in place with a screwdriver and tighten the locknut.

6. Recheck the shift pedal free-play; most of the excess movement should be gone, but the mechanism should not be preloaded hard.

Final fine-tuning of this adjustment is done after a short test ride. A little more or less free-play may be needed to suit the specific clutch.

Filling with engine oil and zinc additive

1. Verify the drain plug and side cover are tight.

2. Fill the engine with 0.95 US qt. of SAE 10w-40 mineral (non-synthetic) oil. 

3. We also recommend adding 2 oz of zinc (ZDDP) additive with every oil change.

4. Use the dipstick to confirm oil level:

   • Insert the dipstick without threading it in.

   • Remove and read the level. It should be between the marks.

Final checks and test ride

1. Reinstall the clutch adjuster rubber cap over the cup washer and nut.

2. Start the engine and allow it to warm up. Check for oil leaks around the drain plug, side cover, and clutch adjuster.

3. Test shift through the gears in a safe area, paying attention to how firmly the clutch engages and whether any slipping remains.

4. If needed, slightly adjust the clutch screw (in small increments) until you find the sweet spot between firm engagement and smooth shifting.

Troubleshooting

If you still experience clutch issues after rebuilding:

Clutch still slips:

• Confirm you are using 100% mineral oil and zinc additive, not semi-synthetic or synthetic oil.

• Re-check the clutch pack order, especially the third steel (alternating holes and notches) and spring locations.

• Verify that friction discs were pre-soaked and that the snap ring is fully seated.

Harsh or incomplete engagement:

• Fine-tune the external clutch adjuster in small increments.

• Verify the internal shift / ball lifter linkage is correctly aligned and not binding.

Noisy clutch or rattle:

• Check for missing springs or misaligned steels.

• Inspect basket fingers for severe grooves; deep notching can catch plates and may require basket replacement.

For detailed measurements and limits, always refer to your CT90 factory service manuals. Download them here if you need them.

Parts you might need

Oil Change Gasket Kit

Clutch Rebuild Kit

Engine Allen Bolt Kit

Zinc ZDDP Additive

Oil Drain Plug Replacement Crush Washers

Replacement Oil Drain Plug

Anti-Seize Lubricant