Your engine’s oil does most of the heavy lifting for performance and longevity. Using the recommended oil and at the recommended oil level is essential to keeping your CT90 in tip top shape. Before you begin, warm the engine briefly, set the motorcycle securely on the center stand, and remove any guards or foot pegs as needed to access the right-side cover and kickstarter.

How often should I change my oil and clean my oil filter

We recommend an oil change every 1,000 miles since these little engines work hard and spend a lot of time at high RPMs.

The centrifugal oil filter and oil pickup screen should be changed every 3 oil changes (3,000 miles / 5,000 KM) over the Honda recommendation of 6,000 miles / 10,000 KM.

How much engine oil do I need for my CT90 / Trail 90

0.95 US qt. | 0.9 lit. of an SAE 10W-40 mineral oil.

Tools & Supplies

17 mm socket for drain plug
Allen keys (after converting cover screws to Allen bolts; otherwise JIS driver/impact)
Needle-nose pliers, small pick, small flat screwdriver
New right-cover gasket (optional if the old one survives)
Solvent/parts cleaner, rags, light grease, anti-seize
Mineral engine oil (0.9 L / 0.95 qt), optional zinc additive (2 oz)
Drain pan, funnel, brass/plastic mallet

Note: Converting the factory JIS screws to Allen head fasteners greatly simplifies this service in the future. Use our CT90 allen bolt upgrade to switch the troublesome JIS head screws to stainless steel allen bolts.

Step-by-Step Instructions

1) Drain the oil

Crack the 17 mm drain plug loose and position the drain pan.
Open the oil filler cap to vent so the crankcase drains fully.
Inspect the crush washer; replace if damaged.

2) Remove right cover & clutch adjuster cap

Pop off the rubber clutch-adjuster cap, remove the cup washer and nut, and note the small O-ring.
Remove all right-cover fasteners (note two longer bolts at the dowel pin positions).
Lightly tap the cover to break the gasket seal. If you previously greased both faces, the gasket may release cleanly.

3) Clean the centrifugal filter cavity

Remove the two screws holding the small centrifugal filter cover on the clutch hub. We like to replace these difficult to remove JIS screws with allen bolts from our CT90 allen bolt kit.
Lift off the cover and gasket.
Scrape and wipe out the packed sludge inside the filter cavity. Use picks and rags to reach tight spots.
Clean until the cavity is free of debris and replace the gasket if damaged.

4) Clean the pickup screen

With needle-nose pliers, gently pull the pickup screen from its place near the drain.
Rinse the screen and reinstall once fully clean.

5) Rinse sludge & wipe mating surfaces

With the drain plug out, spray away remaining residue inside the case; wipe clean.
Clean gasket surfaces on the case and cover.

6) Reinstall filter cover & gasket

Fit the centrifugal filter cover with its gasket and snug the two screws evenly.

7) Reassemble auto-clutch linkage and right cover

Prepare a new right-cover gasket; lightly grease both mating faces.
Refit the small spring, ball, and “horseshoe” lifter parts of the auto-clutch/shift mechanism. Use small dabs of grease to hold pieces in place.
Align the shift arm on the splines so the lifter holes appears centered (shift arm roughly a 2 o’clock position is typical).
Ease the right cover on without disturbing the linkage; confirm dowel pins are seated.
Install all bolts finger-tight, then tighten in a criss-cross pattern until evenly seated. Apply anti-seize to bolt threads to make the next service easy.
Refit the kickstarter, guards, and foot pegs. Apply anti-seize to fastener threads and kickstarter splines so the parts come apart easy next time.

8) Reset the clutch adjuster

Back off the locknut.
Turn the adjuster counter-clockwise until you feel resistance (this takes up internal slack).
Back off about ⅛ turn to set slight free play, then hold the screw while tightening the locknut.
Confirm the shift pedal has minimal slop. Final fine-tuning is done after a short test ride.

9) Refill oil and check level

Reinstall the drain plug with the crush washer; snug, not gorilla-tight. Torque the drain bolt to 14.6 - 25.3 ft. lbs.
0.9 L of SAE 10W-40 should be added for general riding temperatures. We also recommend adding 2oz. of Zinc ZDDP additive with every oil change. Be sure that any oil you are using is still 100% mineral oil and not a semi-synthetic.
Dip, don’t thread the dipstick to check level. Top off as needed.

Final Checks

Start the engine and verify no leaks at the drain plug or right cover.
Confirm smooth engagement and shifting. Fine-tune the clutch adjuster if needed.
Reinstall the rubber clutch-adjuster dust cap.