Historical Context:
        
        
            Polaris was a significant player in the snowmobile market during the late 20th century, known for its performance and innovation. The 1991 models represented a refinement of their established designs.
        
     
    
    
        
            Competitors:
        
        
            Primary competitors included Ski-Doo (Bombardier Recreational Products), Arctic Cat, and Yamaha.
        
     
    
    
        
            Value Proposition:
        
        
            This manual provides OEM-level repair information, crucial for maintaining the performance and longevity of the 1991 Polaris 400 snowmobile.
        
     
    
        
            Manual Advantage:
        
        
            As an original manufacturer publication, it contains the most accurate and detailed procedures, specifications, and diagrams, unlike aftermarket manuals which may be less precise.
        
     
 
    
    
    
        
            Engine Mechanical:
        
        
            Engine disassembly, assembly, component inspection, bore, piston, ring, crankshaft, connecting rod, cylinder head, valve train
        
     
    
        
            Fuel System:
        
        
            Carburetor identification, adjustment, cleaning, rebuilding; fuel pump, fuel lines, fuel tank service
        
     
    
        
            Ignition System:
        
        
            CDI unit, ignition coil, spark plugs, wiring diagrams, timing specifications
        
     
    
        
            Cooling System:
        
        
            Liquid-cooled system components, radiator, thermostat, water pump, coolant specifications, flushing procedures
        
     
    
        
            Exhaust System:
        
        
            Muffler, exhaust pipes, gaskets, inspection and replacement
        
     
    
        
            Drive System:
        
        
            Clutch (Drive and Driven), belt inspection, adjustment, replacement, chaincase, gears, lubrication
        
     
    
        
            Suspension Front:
        
        
            IFS (Independent Front Suspension) components, shock absorbers, springs, control arms, steering system, alignment
        
     
    
        
            Suspension Rear:
        
        
            Rear suspension assembly, shock absorbers, torsion springs, sliders, bogie wheels, track tension adjustment
        
     
    
        
            Braking System:
        
        
            Hydraulic brake system, master cylinder, caliper, rotor, brake pads, fluid specifications, bleeding procedures
        
     
    
        
            Electrical System:
        
        
            Wiring diagrams, lighting, starter, battery, charging system, switches, fuses, troubleshooting
        
     
    
        
            Bodywork Chassis:
        
        
            Frame inspection, body panel removal and installation, seat, handlebars, controls, snow flap
        
     
    
    
        
            Preventative Maintenance:
        
        
            Lubrication points, fluid checks, belt inspection, track adjustment, general inspection routines
        
     
    
        
            Diagnostic Procedures:
        
        
            Troubleshooting charts for engine performance, electrical issues, starting problems, driveability concerns
        
     
    
        
            Repair Procedures:
        
        
            Step-by-step instructions for component removal, repair, and reinstallation for all major systems
        
     
    
        
            Overhaul Procedures:
        
        
            Complete engine rebuild instructions, including torque specifications and clearances
        
     
    
        
            Adjustment Procedures:
        
        
            Carburetor tuning, clutch engagement/disengagement, track tension, steering alignment
        
     
    
        
            Installation Procedures:
        
        
            Instructions for replacing wear items and major components
        
     
    
    
        
            Depth:
        
        
            Comprehensive: Covers from basic maintenance to complete engine and drivetrain rebuilds.
        
     
    
        
            Diagnostics Detail:
        
        
            Detailed diagnostic flowcharts and symptom-based troubleshooting guides.
        
     
    
        
            Specifications Detail:
        
        
            Includes precise torque values, clearances, tolerances, fluid capacities, and specifications for all components.
        
     
 
    
    
    
        
            Engine Type:
        
        
            2-stroke, Liquid-Cooled Twin Cylinder
        
     
    
        
            Displacement:
        
        
            Typically around 398cc (specific designation may vary slightly)
        
     
    
        
            Bore X Stroke:
        
        
            N/A (Specifics found in manual)
        
     
    
        
            Horsepower:
        
        
            Approximately 50-60 HP (varies by specific tuning and model variant)
        
     
    
        
            Torque:
        
        
            N/A (Specifics found in manual)
        
     
    
    
        
            Carburetion:
        
        
            Dual Mikuni carburetors (e.g., VM34 or similar)
        
     
    
        
            Fuel Type:
        
        
            Unleaded gasoline (typically 87 octane or higher)
        
     
    
        
            Oil Mixture:
        
        
            Pre-mix or Autolube system (manual will specify exact type and oil specification)
        
     
 
    
    
    
        
            Oil Change:
        
        
            N/A (2-stroke engines typically don't have oil changes in the traditional sense; refer to autolube or pre-mix ratio in manual).
        
     
    
        
            Clutch Inspection:
        
        
            Recommended every 500-1000 miles or annually, or as specified in the manual.
        
     
    
        
            Spark Plug Replacement:
        
        
            Recommended annually or every 1000-1500 miles. Manual will specify exact type (e.g., NGK BR9ES).
        
     
    
        
            Track Tension Adjustment:
        
        
            Check before each ride and adjust as needed per manual instructions.
        
     
    
    
        
            Coolant:
        
        
            Ethylene glycol-based antifreeze. Manual specifies mix ratio and type (e.g., Polaris Coolant).
        
     
    
        
            Brake Fluid:
        
        
            DOT 3 or DOT 4 hydraulic brake fluid.
        
     
    
        
            Chaincase Oil:
        
        
            Refer to manual for specific gear oil type and capacity (e.g., SAE 80W-90).
        
     
    
    
        
            Common Wear Items:
        
        
            Drive belt, sliders, bogie wheels, carbides, clutch components, ignition components.
        
     
    
        
            Engine Considerations:
        
        
            Proper pre-mix or autolube is critical to prevent engine damage. Overheating due to coolant loss or blocked cooling fins is also a concern.
        
     
    
        
            Drive System Considerations:
        
        
            Clutch alignment and belt condition are vital for optimal performance and longevity.