Honda ATC 90 & 110 Three-Wheeler 1973–1981 Shop Manual (P/N 6291806)

• Coverage and document identity: Covers the 1973-1978 Honda ATC90 and 1979-1981 ATC110, using the earlier US90/ATC90 shop material as its foundation. The Honda factory cover is marked 6291806 and copyrighted 1980.
• 136-page PDF: Includes 104 pages of front matter, the main US90/ATC90 manual, and the ATC90 '77 insert; a 12-page ATC110 '79 addendum; an 11-page ATC110 '81 addendum; and nine pages of US90 service bulletins.
• Engine repair coverage: Details engine removal and installation, in-frame piston and cylinder work, cylinder-head and valve inspection, cam-chain service, recoil starter and rotor removal, subtransmission work, clutch and oil-pump inspection, and carburetor repair and adjustment.
• Chassis repair coverage: Walks through front- and rear-wheel disassembly, hub bearing and oil-seal replacement, split-rim tubeless tire service, rear brake work, final-driven-sprocket and chain service, steering-head repair, and frame disassembly.
• Model-specific changes: Separates US90-to-ATC90 differences and ATC90K2-to-ATC90K3 updates, including the removable engine bracket, revised drive mechanism and steering assembly, chain guide and inspection window, redesigned tensioner, split tubeless wheels, and ignition and fuel-control changes.
• Electrical and diagnostic material: Provides model-specific wiring diagrams, cable and harness routing, breaker-point ignition checks, and 1981 ATC110 CDI, ignition-coil, A.C.-generator, pulser-generator, and timing tests.
• Included US90 bulletins: Documents throttle-lever interference, tire-valve leakage, the serial-numbered hollow-to-solid rear-axle change and conversion parts, plus the factory operator-label campaign.

This Honda shop manual is suited to service personnel and mechanically proficient owners who have the specified tools and already understand standard workshop practice; Honda expressly notes that basic shop procedures and general safety principles are not taught in detail.

Model Changes and Serial Breaks

The ATC90 '77 insert applies to engine number ATC90E-1600001 and subsequent and frame number ATC90-1600001 and subsequent. It revises the wheel and split-rim tire procedures and supplies the related model specifications. Earlier sections explain how the ATC90 differs from the US90, then isolate ATC90K3 changes from the ATC90K2 so the correct chain, wheel, steering, ignition, and fuel-control information can be selected before repair work begins.

The rear-axle bulletin adds another important break. US90 frames US90-100001 through US90-122815 originally used a hollow axle; US90-122816 and later received the solid axle. The required conversion hardware also differs between machines through US90-109022 and those from US90-109023 through US90-122815. A separate installation plate warns that reversing the right and left rear-wheel bosses can reduce spline engagement, spread the tread, weaken the axle, and misalign the assembly with the frame.

For the ATC110, the 1979 addendum supplies its own specifications, torque tables, service limits, maintenance schedule, breaker-point wiring, harness routing, carburetor adjustment, high-altitude setup, fuel-filter cleaning, brake adjustment, and clutch notes. The 1981 material identifies the later 12-volt CDI system and adds component resistance checks, ignition timing diagnosis, CDI tester requirements, and removal, inspection, cleaning, and lubrication coverage for the endless O-ring drive chain.

Procedures, Diagrams, and Factory Data

The repair chapters are organized around disassembly, inspection and repair, and reassembly rather than a maintenance-only overview. Factory tool references include the 90cc engine tool set 07000-02801, steering-stem nut tool 07085-91801, front and rear wheel bearing/oil-seal drivers 07054-91801 and 07054-91805, clutch outer holder 07923-0340000, and generator rotor puller 07933-2000000. Technical tables provide assembly torques, capacities, adjustment data, electrical test values, and inspection limits for valves, guides, camshaft and rocker arms, piston and cylinder, clutch plates, oil pump, brakes, bearings, and related parts.

Useful illustrations include the US90 wiring diagram, ATC90K3 wiring diagram, 1981 ATC110 CDI ignition schematic, ATC110 cable and wire-harness routing view, clutch and oil-pump exploded assembly, and rear-wheel and drive-mechanism exploded view. One manual-specific tire warning cautions that forcing the beads away with tire irons can damage the rim seals and create an air leak. The 1981 chain section likewise warns that steam cleaning, pressure washing, unsuitable solvents, or solvent-based aerosol lubricants can damage the chain O-rings.

This publication covers substantial in-frame engine repair, but it is not a stand-alone guide to every internal overhaul operation with the engine removed; Honda directs those operations to the Honda 90 Series or CT90 shop manual. Because the covered work includes lifting and supporting the three-wheeler, handling gasoline, servicing brakes, and working around the drive chain, follow the model-specific cautions and use secure shop support equipment.