2007–2010 Yamaha YFM40FB (Big Bear 400) ATV Service Manual (Model Codes 4S11/4S14)

The 2007–2010 Yamaha YFM40FB (Big Bear 400) service manual documents engine, chassis, drivetrain, and electrical repairs, with periodic maintenance and component inspection procedures.

• Engine and transmission: Engine removal, cylinder-head and valve work, crankcase separation, crankshaft inspection, clutch service, and transmission disassembly.

• Carburetor and routine service: Carburetor overhaul, fuel-level adjustment, valve-clearance checks, throttle and starter-cable adjustments, and emission-control maintenance charts.

• Drive train: Front and rear constant velocity joint service, middle-gear shim selection, differential gear-lash adjustment, and final-drive gear inspection.

• Brakes and chassis: Front brake caliper and master-cylinder service, hydraulic bleeding, rear brake friction-plate inspection, steering-knuckle repairs, and suspension checks.

• Electrical diagnosis: Ignition, starting, charging, lighting, oil-cooler fan, and 2WD/4WD troubleshooting, plus carburetor-warmer and differential-lock circuit checks.

• Diagrams and workshop data: Exploded assembly views, lubrication diagrams, cable routing, system circuit diagrams, tightening sequences, and component-specific torque and wear-limit tables.

Yamaha wrote this manual primarily for its dealers and qualified mechanics; it assumes an understanding of vehicle repair and access to the specified tools.

Engine Repair and Maintenance

Internal engine work: Cylinder-head procedures include warpage inspection, timing-chain tensioner checks, valve-guide replacement, valve-seat inspection and lapping, and camshaft installation. Clutch and transmission sections add friction-plate measurements, shift-fork inspection, shaft-runout checks, and gear engagement checks before crankcase assembly.

Special-tool references include crankcase separator YU-01135-B and flywheel puller YM-01404 for crankshaft and AC magneto rotor removal. General information also covers vehicle identification and preparation for disassembly.

Carburetor and servicing: The Mikuni BSR33 section distinguishes float-height inspection from fuel-level adjustment and covers the needle valve, jets, piston-valve diaphragm, and coasting enricher. Routine service includes compression and ignition-timing checks, tire-pressure specifications, and separate engine-oil quantities for routine changes, filter replacement, and total capacity.

Shaft Drive, Differential, and Rear Brake

The drivetrain troubleshooting chart links noise and operating symptoms to wheel bearings, constant velocity joints, rear-brake adjustment, and gear assemblies. This helps narrow down which assembly needs inspection before dismantling the shaft drive.

Middle-gear procedures cover shim calculations and backlash measurement. Final-drive work includes ring-gear thrust clearance, pinion-bearing service, gear-lash adjustment, and oil-capacity data.

Differential service limit: Housing, bearing, motor, and gear-lash procedures are included, but the factory-assembled differential gear unit itself must remain assembled. Yamaha warns that dismantling this unit can cause malfunction.

The middle driven pinion gear bearing retainer has a left-hand thread; forcing it as a conventional fastener risks thread damage. The differential motor bench test uses a separate low-voltage supply, with a specific warning against vehicle-battery voltage.

Rear-brake procedures address the friction plates, steel plates, springs, pressure plate, and cam mechanism inside the final gear case.

Chassis and Electrical Fault Finding

Steering work extends to ball-joint replacement with the specified remover/installer, wheel-bearing service, tie-rod checks, and toe-in adjustment. Suspension coverage includes arm bushings, rear knuckles, stabilizer components, and shock-absorber inspection and replacement.

Starting-circuit diagrams explain the neutral and rear-brake-lever interlocks. Ignition, charging, and 2WD/4WD circuit diagrams pair component checks with connector views and wiring tests; the separate YFM40FBW wiring diagram includes a component key and wire-color legend.

Review the manual’s support precautions before lifting the ATV for wheel, brake, or suspension work.

A Preview of Engine and Drivetrain Adjustment Data

Engine reference data include an intake valve clearance of 0.06–0.10 mm with the engine cold at room temperature, and an engine-oil drain-bolt torque of 32 Nm.

The carburetor section specifies a fuel level of 4.0–5.0 mm above the float-chamber mating surface, measured with a fuel-level gauge. Differential gear-lash specifications give a range of 0.05–0.25 mm alongside the associated measurement and shim-adjustment procedures.

Searchable PDF and Wiring-Diagram Detail

Search, bookmarks, and diagrams: Text search is available across the technical sections. Embedded subsection bookmarks link directly to topics; top-level chapter bookmarks open the contents pages. Small wiring labels require zooming, particularly on the reduced circuit diagrams.

One printable English PDF with 388 pages total: 16 preliminary pages, 370 chapter pages, and two pages for the wiring diagram and legend. The total includes the cover and three blank or header-only pages.