Mercury 25 BigFoot FourStroke Outboard Motor 1998 Service Manual (P/N 90-854785R2)

Mercury Marine service manual 90-854785R2 covers maintenance, diagnosis, and repair for the 25 BigFoot FourStroke, serial numbers 0G590000 and above, with electric- and manual-start configurations. The cover is dated November 2001; the main service sections are dated December 2001.

Text search and diagram navigation: Service text is searchable, with embedded bookmarks for sections and procedures. Small wiring labels and connector pin numbers require zooming. Some oil- and water-flow diagram callouts are not reliably searchable.

• Powerhead repair: Cylinder-head and crankcase teardown, valve-clearance adjustment, timing-belt replacement, piston inspection, and bearing selection.

• Ignition and starting: CDI/ECM diagnosis, DVA and resistance tests, charging-system checks, electric-starter repair, and wiring diagrams.

• Fuel system: Fuel-pump overhaul, carburetor rebuilds, float and idle adjustments, and auto-enrichener testing for the documented configurations.

• Trim and mid-section: Power Trim diagnosis, hydraulic repairs and bleeding, Gas Assist Manual Tilt replacement, and clamp/swivel-bracket assembly references.

• Lower unit: Gear-housing teardown, bearing and seal replacement, water-pump service, pressure testing, and propeller selection.

• Controls and recoil starter: Throttle/shift cable setup, tiller-handle repair, rewind-spring tensioning, and starter-interlock adjustment.

Written for Mercury dealer mechanics and service personnel, this manual assumes training in outboard servicing and familiarity with the specified shop tools and special tools.

Ignition Diagnosis and Wiring Coverage

The ignition tables link no-spark, weak-spark, and restricted-RPM symptoms to component tests, helping narrow the fault before replacement. DVA adapter 91-89045, ignition-coil resistance data, and the Starting Circuit Troubleshooting Flow Chart support the electrical diagnosis.

Electrical variants: Component illustrations distinguish S/N 0G960499 and below from 0G960500 and above. Charging tests cover the 6-amp manual-start and 15-amp electric-start systems.

The Electronic Control Module (ECM) section explains overspeed, overheat, and low-oil-pressure protection, with removal and installation procedures. Ignition timing is electronically controlled and is not mechanically adjustable.

Wiring selection: Section 2D covers USA serials through 0G760299 and Belgium serials through 0P058999. Section 9 supplies revised color diagrams from USA 0G760300 and Belgium 0P059000, including remote-control electric, electric tiller, gas-assist remote-control, and manual-tiller configurations.

Fuel-Pump and Carburetor Variants

Fuel-system procedures separate USA 0T408999 and below / 0T409000 and above, and Belgium 0P153499 and below / 0P153500 and above. The later configuration has a water-cooled fuel pump and revised carburetor vent-hose routing. Jetting tables distinguish Standard and Bodensee models.

Fuel-pump pressure specifications accompany diaphragm and check-valve inspections. Carburetor checks cover float and needle-valve wear and electrothermal auto-enrichener operation. The carburetor cautions explain how steel wire used for jet cleaning can enlarge the jets and affect performance.

Powerhead, Lubrication, and Cooling

The engine repair chapters extend to valve-guide replacement, valve-seat reconditioning, balance-piston inspection, and color-coded main and connecting-rod bearing selection. Component torque sequences, clearances, and wear limits accompany the procedures.

Manual-start engines have a compression-release camshaft that makes conventional compression readings misleading. The cylinder-leakage section relates escaping air to possible valve, piston-ring, or head-gasket faults.

Trochoid oil-pump inspection and thermostat testing are supported by oil-pressure and temperature specifications, the Oil Flow Diagram, and the Water Flow Diagram. Maintenance schedules, lubrication points, fluid capacities, and storage preparation are also covered.

Power Trim Repair and Gas Assist Limits

Power Trim Flow Diagrams and the Hydraulic System Troubleshooting Flow Chart address drift-down and loss of holding force. Shock-rod, memory-piston, and valve service includes seal placement and bleeding.

Pressure checks using Test Gauge Kit 91-52915A6 verify operation after repair; they do not locate the original fault. The trim motor and trim oil pump are replacement assemblies with no internal service procedures.

Gas Assist Manual Tilt coverage includes linkage adjustment, leak checks, and unit removal and installation. Its pressurized assist unit is not serviceable.

Gearcase and Water-Pump Service

The lower-unit chapter covers the 2.42:1 gearcase, clutch-dog and bearing-carrier inspection, seal installation, and gear-housing pressure testing. Gear backlash is not shim-adjustable. A bearing-carrier fastener update applies to USA 25 models through 0G621670.

The water-pump instructions warn that reversing a used impeller's established vane set can cause vane breakage. The Propeller Information Chart lists propeller options by boat load and operating range.

Always consult the full OEM procedures and follow shop safety practices around the propeller, ignition system, and pressurized trim assembly.

PDF Contents and Included Diagram Pages

One printable English PDF contains the service manual and its Color Diagrams section. The 471-page PDF includes 1 cover page, 4 preliminary pages, 446 pages across Sections 1–8, and 20 pages in Section 9, including its contents page, blank page, diagram title pages, legends, and drawings. All of these pages are included in the total.