1984–1989 Honda ATC90/ATC110/US90 ATV Shop Manual

With a classic Honda three-wheeler on the lift, the correct model section matters. This Honda shop manual brings together US90 and ATC90 maintenance and repair procedures, ATC90K2-to-ATC90K3 changes, and ATC110 addenda. Separate sections address contact-breaker ignition and the later C.D.I. system, along with the mechanical changes that affect service work.

• Engine access and top-end repair: Engine removal and installation, cylinder-head and valve inspection, piston service, and cam-chain removal and adjustment.

• Clutch and driveline: Automatic centrifugal clutch disassembly, trochoid oil-pump inspection, gearshift mechanism checks, and low/high-range sub-transmission service.

• Carburetor service: Disassembly, float-level inspection, idle-mixture adjustment, fuel-filter cleaning, and ATC110 high-altitude jetting instructions.

• Wheels, steering and brakes: Wheel-bearing replacement, split-rim tubeless tire service, steering-stem disassembly, and brake-drum and shoe inspection.

• Model revisions: ATC90K3 chain-guide and tensioner changes, revised wheel assemblies, and model-specific ATC110 maintenance and service information.

• Electrical diagnosis: Contact-breaker timing checks, C.D.I. inspection, ignition-coil and generator tests, wiring diagrams, and cable and harness routing drawings.

Honda wrote the base manual for personnel responsible for maintaining and repairing these machines; standard workshop procedures and basic service practices are assumed knowledge.

Engine, Clutch and Sub-Transmission Work

The engine section describes work performed with the engine in the frame, including cylinder, piston, valve and cam-chain service. Full overhaul with the engine removed requires the separate Honda 90 Series Shop Manual. The ATC110 addenda also refer to the CT90 Shop Manual for overhaul work beyond the ATC90 procedures.

Clutch coverage includes friction-disc and plate inspection, spring checks, and reassembly. The trochoid oil-pump section provides inspection clearances, while gearshift checks address the spindle, arm, drum and guide pins. The sub-transmission illustration identifies its gears, shift fork and shafts.

Recoil-starter removal is covered as part of engine access. Exploded clutch and oil-pump views and the cylinder-head tightening-sequence illustration support component identification during disassembly and reassembly.

The manual explains how uneven cylinder-head tightening can cause cracks, warpage and compression loss. It also warns that reversed piston rings can cause oil pumping.

Carburetor Adjustment and Fuel-System Service

Carburetor exploded views identify the float valve, jets, fuel strainer and throttle components. Inspection and adjustment sections address sealing, blocked passages, float height and idle operation.

The ATC110 addendum distinguishes standard and high-altitude jetting, including the conditions for returning to the standard setup. That distinction helps owners select the relevant carburetor information for the machine’s operating location.

Chassis Changes and Wheel Repairs

Separate US90 and later ATC90 chassis procedures distinguish the original tire arrangement from split-rim tubeless assemblies. Wheel drawings show the rim halves, sealing O-ring, hub and bearing arrangement; the rear-drive illustration identifies the sprocket, damper holders and axle components.

The ATC90K3 revision section explains the added chain guide, chain-case inspection window and changed tensioner lock arm. Brake coverage includes control adjustment, drum measurement and shoe inspection.

Support the three-wheeler securely during wheel and chassis work, and observe the fuel-handling warnings during carburetor service.

Contact-Breaker and C.D.I. Diagnosis

Earlier ignition procedures cover breaker-point inspection and timing checks using a test lamp or timing light. The C.D.I. addendum adds ignition-coil, lighting-coil, exciter-coil and pulser-generator tests, plus a C.D.I. unit test table.

These separate component tests help narrow an ignition fault before selecting a replacement part. The C.D.I. section specifies a Sanwa electric tester and warns that an unsuitable meter can produce misleading readings.

Model-specific wiring diagrams accompany cable and harness routing drawings. The ATC110 maintenance schedules place ignition, clutch, chain and brake checks within the scheduled service requirements.

A Preview of Bore and Ignition Test Values

The ATC90 engine section lists a standard cylinder bore of 50.0–50.01 mm and a service limit of 50.11 mm, alongside oversize piston and ring options.

ATC90K3 ignition-coil inspection specifies a condenser capacity of 0.225–0.275 μF. For the ATC110 C.D.I. configuration, the ignition-coil test lists a primary resistance of 0.2–0.8 Ω.

Text Search, Chapter Access and PDF Contents

Search and diagram detail: OCR text search is available through much of the manual, but recognition errors affect some words and diagram labels. Embedded bookmarks provide access to the main sections and addenda. Small wiring-diagram labels are best read at increased magnification.

Supplied as one printable, English-language 127-page PDF: 94 main-manual pages, a 5-page ATC90 wheel addendum, a 12-page ATC110 contact-breaker addendum, and an 11-page ATC110 C.D.I. addendum. The total also includes four introductory pages and one memo page.