Murray MP550RM21 Lawn Mower Manual

The Murray MP550RM21 is a 21-inch, 5.5 HP Briggs & Stratton-powered rear bagging push lawn mower. It features a durable steel deck, 2-in-1 side discharge/bagging capability, single-lever height adjustment (1.5" to 4"), easy-start engine, and a 2.5-bushel grass bag. Below are key sections for safety, features, assembly, operation, maintenance, and troubleshooting.

1. Features at a Glance 2. Assembly Instructions 3. Safety Precautions 4. Specifications 5. Controls and Operation 6. Cutting Height Adjustment 7. Maintenance Schedule 8. Storage Guidelines 10. Regulatory Information 11. Troubleshooting

Features at a Glance

Key components: 5.5 HP Briggs & Stratton engine, 21-inch steel cutting deck, rear grass bag, height adjustment lever, recoil starter, and mulching plug (optional).

Feature	Description
Engine	5.5 HP Briggs & Stratton 4-cycle, 127cc
Cutting Width	21 inches
Cutting Height	Single lever adjusts 6 positions (1.5" to 4")
Deck Material	Durable powder-coated steel
Grass Bag	2.5 bushel rear bag with easy-empty handle
Discharge Options	2-in-1: Side discharge or bagging
Wheels	8-inch rear, 7-inch front plastic wheels
Starting System	Recoil pull-start
Fuel Tank	0.95 gallon capacity
Weight	Approximately 55 lbs

Assembly Instructions

Unpack and inspect all parts. Assembly requires basic tools (included).

Attach handle: Align lower handle with deck brackets, secure with bolts and wing nuts.
Install upper handle: Connect to lower handle, secure with provided hardware.
Attach grass bag: Hook bag frame onto rear deck brackets; ensure bag is secure.
Install wheels: Slide axles through deck brackets, secure wheels with washers and retaining clips.
Add engine oil: Fill with SAE 30 detergent oil (approx. 20 oz) as per engine manual.
Add gasoline: Use fresh unleaded gasoline (87 octane minimum).

WARNING! Do not add gasoline during assembly or in enclosed area. Keep away from sparks or flames.

Safety Precautions

Always operate mower with caution to prevent injury.

Read manual thoroughly before use.
Clear mowing area of stones, sticks, and debris.
Wear sturdy shoes, long pants, and eye protection.
Do not mow when grass is wet or during rain.
Never remove grass bag or unclog chute with engine running.
Keep children and pets away from mowing area.
Stop engine and wait for blade to stop before crossing gravel drives or roads.

CAUTION! Engine and muffler become extremely hot; allow to cool before refueling or storing.

Specifications

Model: Murray MP550RM21
Engine: Briggs & Stratton 5.5 HP, 4-cycle, 127cc
Cutting Width: 21 inches
Cutting Height Range: 1.5" to 4" (6 positions)
Deck: 21-inch steel, powder-coated
Bag Capacity: 2.5 bushels
Wheels: 8-inch rear, 7-inch front
Fuel Capacity: 0.95 gallons
Oil Capacity: 20 oz (SAE 30 detergent)
Weight: 55 lbs (approx.)

Controls and Operation

Handle Controls: Throttle lever, drive control bar (if self-propelled model), height adjustment lever.

Starting Engine:

Check oil and fuel levels.
Move throttle lever to FAST position.
Prime engine (if equipped) by pressing primer bulb 3 times.
Pull recoil starter handle briskly.

Stopping Engine: Move throttle lever to STOP position.

Mowing Tips: Mow in straight lines, overlap passes slightly. Empty bag when full to maintain performance.

Cutting Height Adjustment

Single lever adjusts all four wheels simultaneously.

Stop engine and wait for blade to stop.
Pull height adjustment lever outward.
Move lever to desired position (1-6, with 1 being lowest).
Release lever to lock into place.

Tip: For first mow of season, set to highest setting. Never remove more than 1/3 of grass blade length at once.

Maintenance Schedule

Regular maintenance ensures long life and optimal performance.

Task	Frequency
Check engine oil level	Before each use
Clean under deck	After each use
Check blade sharpness/balance	Every 25 hours
Replace air filter	Every season or 25 hours
Change engine oil	First 5 hours, then every 50 hours
Replace spark plug	Every season or 100 hours
Lubricate wheel bearings	Every season

Blade Sharpening: Disconnect spark plug wire. Remove blade, sharpen at original angle (40°), balance before reinstalling.

Storage Guidelines

Prepare mower for off-season storage.

Run engine to burn remaining fuel, or add fuel stabilizer.
Change engine oil.
Clean deck thoroughly; apply light oil to prevent rust.
Remove spark plug, add 1 tablespoon oil to cylinder, pull starter slowly to coat.
Store in clean, dry place, covered.

CAUTION! Never store mower with gasoline in tank indoors or in enclosed area.

Regulatory Information

This lawn mower complies with ANSI/OPEI B71.1-2012 safety standards. Engine meets EPA Phase 3 emissions requirements. Copyright © Murray LLC. For support, visit www.murray.com or contact 1-800-800-7310. Keep this manual for future reference.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine won't start	No fuel, old gasoline, clogged air filter	Add fresh gasoline; clean/replace air filter; check spark plug.
Engine runs poorly	Dirty air filter, old gasoline	Replace air filter; drain and refill with fresh gasoline.
Uneven cutting	Dull blade, uneven tire pressure	Sharpen or replace blade; check deck level.
Excessive vibration	Loose blade, bent blade, debris	Tighten blade bolt; replace bent blade; clean deck.
Bag not filling	Wet grass, clogged chute	Mow when grass is dry; stop engine and clear chute.
Wheels not turning	Debris around axles	Clean wheel axles and bearings.

Service: For engine service, refer to Briggs & Stratton manual. For mower service, contact authorized Murray dealer.

Support: 1-800-800-7310; www.murray.com