Honda CT90 - CT110 1978-1982 Service Manual

This factory manual documents a genuine handoff between two related "Trail" models rather than one continuous machine — the CT90 wound down its 13-year production run in 1979, and Honda replaced it the following year with the larger-displacement CT110, a bike Wikipedia's own account describes plainly: "the same machine but bored out to 105cc and weighs slightly more." Honda's own official documentation, published in March 1982 as the definitive reference for this full model span, reflects that continuity by covering both bikes together.

The CT90, in its classic 1978-1979 form, runs an 89cc air-cooled four-stroke single with a four-speed transmission and semi-automatic clutch, paired with a 1.867:1 reduction gearbox operated via a small lever under the transmission case — genuinely useful for climbing steep grades at low speed. Top speed in high-ratio road gear reached a claimed 55 mph, with fuel economy often exceeding 100 mpg, making it a genuinely efficient commuter despite being poorly suited to sustained highway travel given its off-road-biased tires and modest power. The CT110 that succeeded it in 1980 bumped displacement to 105cc via a 52mm x 50mm bore and stroke with an 8.5:1 compression ratio, running a semi-automatic centrifugal-clutch four-speed transmission with chain final drive — notably, the very first 1980 CT110 model actually lacked the dual-range sub-transmission that had defined the CT90, though Honda brought that feature back in subsequent model years.

Both models share the fundamentally simple, step-through, nearly-horizontal-cylinder architecture that made the Trail line so approachable for decades — front and rear drum brakes, a stamped-steel frame, and telescoping front forks. Honda's own factory specification data lists cylinder head bolt torque at 2.0-2.5 kg-m (14.5-18.1 lb-ft) and cam chain tensioner bolt torque at 15 N·m (11.1 lb-ft), with exhaust valve clearance on the 89.5cc engine set at 0.05mm (0.002 in.) — genuinely precise tolerances for a bike this mechanically simple. This 1982-dated edition specifically documents emission-control updates introduced across the later production cycles within this span, making it the correct reference regardless of which of the two closely related models you're working on.

This factory service manual covers the procedures, torque specs, and diagrams needed to keep a 1978-1982 CT90/CT110's engine, chassis, and electrical system running the way Honda built it.