1987–1993 Mercury/Mariner 70, 75, 80, 90, 100 & 115 HP Outboard Service Manual (P/N 90-13645-2)

Mercury Marine's 1995 service manual, publication 90-13645--2, covers the listed Mercury and Mariner 70, 75, 80, 90, 100 and 115 HP outboards, including the 75 Marathon, 75XD and 75 Seapro.

Powerhead coverage: The internal overhaul chapter covers three-cylinder engines. Section 4B for four-cylinder powerheads, listed in the manual outline, is not included in this PDF.

• Powerhead repair: Three-cylinder disassembly, piston and crankshaft inspection, cylinder honing, reed-block service, thermostat testing, and reassembly with torque sequences.

• Ignition and starting: DVA testing, stator and trigger checks, switch-box diagnosis, charging-system troubleshooting, starter-motor overhaul, and engine wiring diagrams.

• Fuel and oil injection: Fuel-pump rebuilding, carburetor service, timing and synchronization, enrichener testing, oil-pump output checks, and warning-system diagnosis.

• Lower unit: Gear-housing teardown, bearing and seal replacement, pinion-depth and backlash adjustment, water-pump service, and leak testing.

• Power trim: Separate repair and troubleshooting sections for Design I side-fill, Design II aft-fill, and single-ram systems.

• Steering and controls: Ride-Guide attachment installation and adjustment, tiller-handle rebuilding, Co-Pilot installation, and throttle and shift linkage setup.

Written for Mercury Marine dealer mechanics and company service personnel, this manual assumes familiarity with outboard service and the recommended workshop tools.

Ignition, Charging and Starting Diagnosis

The Ignition System Test Chart, Rectifier Test flow diagram and Starting Circuit Troubleshooting Flow Chart support fault isolation across the ignition, charging and cranking circuits. DVA procedures specify the Multi-Meter/DVA Tester 91-99750 or Direct Voltage Adaptor 91-89045.

Stator and switch-box coverage concerns testing, removal and replacement; starter-motor coverage extends to armature tests, brushes and reassembly. The manual warns that cranking or running with an ungrounded switch box can damage it.

Engine wiring diagrams distinguish three- and four-cylinder installations, warning-module configurations and regulator types. COMMANDER and COMMANDER 2000 remote-control diagrams provide the corresponding switch and harness details.

Carburetion, Synchronization and Oil Injection

Fuel-system troubleshooting connects hard starting, flooding, lean running and high-speed surging with possible causes. Its anti-siphon-valve checks help distinguish a restricted boat-side fuel supply from a carburetor fault before a rebuild is considered.

The Standard High Speed Jet Chart and High Altitude Jet Chart identify carburetor-specific jetting. Four-cylinder coverage includes the Accelerator Pump Fuel Flow Circuit and the distinction between the upper carburetors' adjustable idle mixtures and the lower pair.

Oil-injection procedures cover pump synchronization, inlet- and outlet-hose bleeding, output testing, and low-oil sensor checks. Warning-system diagnosis distinguishes test-button installations from systems using a warning module.

Three-Cylinder Powerhead Work and Maintenance

Internal repair coverage includes piston sizing, ring-groove inspection, crankshaft checks, connecting-rod cap alignment and cylinder-block inspection. The Cylinder Head Cover Torque Sequence concerns the cover fasteners, alongside separate crankcase and exhaust-cover sequences.

Top-guided and bottom-guided piston/rod assemblies are distinguished, including limits on mixing their individual components. Bleed-check-valve designs and the associated serial changes provide another reference when identifying installed parts.

Maintenance sections address lubrication, compression checks, out-of-season storage and recovery after complete submersion, with separate considerations for saltwater exposure and submersion while running.

Gear Housing, Midsection and Power Trim

Gearcase procedures explain shim selection for pinion depth and forward-gear backlash, with different tools and specifications for the engine families. Water-pump coverage distinguishes the older aluminum base from the later plastic base, which does not fit older gear housings.

Midsection material provides exploded component views, lubrication points and fastener specifications. The single-shock-absorber section covers removal, inspection and installation; internal rebuilding refers to instructions supplied with repair kit 41760A2.

Power-trim troubleshooting separates electrical faults from hydraulic faults and includes pressure checks, trim-rod and tilt-ram work, and motor repair. The Design I and II hydraulic pumps are replaced as assemblies rather than internally rebuilt. Removing the preset port trim-rod check valve can alter operating pressure and damage the system.

Steering Attachments and Tiller Controls

Ride-Guide coverage includes parallel and opposite-side cable routing for dual engines, dual-cable installation on a single outboard, and steering-tension adjustment. Tiller procedures cover the twist grip, throttle gearing, switch harnesses and Co-Pilot friction assembly.

Marine-engine service involves high-voltage ignition, fuel and pressurized trim hydraulics. Follow the manual's safety precautions and use appropriately trained personnel for unfamiliar work.

A Preview of Crankshaft and Gearcase Test Limits

For the three-cylinder powerhead, the crankshaft inspection section gives a maximum total runout of 0.006 in. (0.152 mm), alongside checks of bearing and seal surfaces.

The gear-housing leak test specifies holding 10–12 psi for five minutes without pressure loss, with lubricant drained. These references support component assessment and checks after gearcase repair.

Searchable Text, Bookmarks and PDF Contents

Navigation and illustration detail: Text search is available across the manual, and embedded bookmarks link to sections and procedures. Some lettering within wiring illustrations is outside the searchable text layer. Small wiring labels need enlargement; some gear-housing illustrations have light linework.

One printable English PDF contains 391 pages, including the original cover, two introductory pages, 16 section title pages, 16 contents pages and four Notes pages.