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John Deere S120 Troubleshooting
The
John Deere S120 lawn tractor from S100 Series was manufactured between 2021 and
2025. The John Deere S120 tractor is powered by Briggs and Stratton 44N677
4-cycle V-twin cylinder OHV vertical shaft air-cooled gasoline engine. A 44.2
cubic inches (724 cc) engine has a cylinder bore of 3.12" (79 mm), piston stroke
of 2.89" (73 mm), and rated power of 22 Hp (16.4 kW). This model is fitted with
a hydrostatic transmission with infinite forward and reverse gears.
The John Deere S120 is equipped with a manual steering system,
mechanical PTO clutch, disc brakes, 15x6.00-6 front tyres, and 20x8.00-8
rear tyres. The operating weight is 437 lbs (197 kg). Compatible
attachments for the tractor: 42" mid-mount 2-blade mower deck with
cutting height of 1.0-4.0" (25-102) mm, 44" front-mount one-stage snow
blower, and 46" front blade.
| Engine Troubleshooting |
| Engine will not start or hard to start |
| Empty gas tank - Add fuel |
| Wrong grade of gasoline - Change gasoline |
| Dirty fuel system - Check and clean |
| Faulty spark plug wire connection - Inspect spark plug wiring connection, change or repair |
| Spark plug is faulty - Replace spark plug |
| Fuel filter is clogged - Install new filter |
| Defective safety switch - Replace if required |
| Carburetor settings incorrect - Adjusting as required |
| Poor cylinder compression - Check cylinder block and change worn parts |
| Incorrectly adjusted or damaged throttle cable - Adjust correctly or change the cable |
| Cooling fins are clogged - Clean and wash as required |
| Ignition failure - Inspect ignition system and change failed components |
| Stuck or faulty throttle cable - Service or replace the cable |
| Dirty air filter element - Service or change air filter |
| Clogged or damaged fuel pump - Inspect fuel pump and change as required |
| Engine starts but then stalls |
| Faulty spark plug - Replace spark plug |
| Clogged or faulty carburetor - Service or replace carburetor |
| Ignition coil is faulty - Ignition coil should be replaced |
| Damaged or weak valve springs - Valve springs need to be changed |
| Stuck valves - Clean the valves |
| Low engine oil level - Fill the crankcase with engine oil |
| Engine is overloaded - Load reducing is recommended |
| Cooling fins are plugged - Wash or clean as required |
| Too lean fuel mixture in carburetor - Use proper settings |
| Ignition timing is incorrect - Ignition timing needs to be adjusted correctly |
| Valve setting is incorrect - Adjust valves correctly |
| Engine knocking sound or abnormal noise |
| Low octane fuel - Use correct gasoline |
| Engine is overloaded - Reduce load |
| Lack of engine oil - Need to add engine oil |
| Incorrectly set low idle speed - Need to adjust idle speed |
| Fuel pump is clogged or faulty - Service or change fuel pump |
| Main bearings or crankshaft wear - Repair or replace defective parts |
| Valve clearance is wrong - Check and adjust |
| Piston rings or cylinder bore are worn - Change faulty parts |
| Carburetor is improperly adjusted or clogged - Carburetor needs to be adjusted correctly or cleaned |
| Air filter is dirty - Clean or replace air filter |
| Fuel pump is defective or dirty - Service or change fuel pump |
| Defective ignition - Inspect ignition system and change failed components |
| Worn cylinder head gasket - Replace cylinder head gasket |
| Stuck or burned valves - Service or change valves |
| Push rod is damaged or worn - Install a new push rod |
| Cylinder block components are faulty - Replace defective parts as required |
| Engine uses too much fuel |
| Insufficient throttle opening - Adjust or replace throttle cable |
| Improper carburetor main jet adjustment - Adjust the carburetor main jet properly |
| Spark plug is loose or defective - Tighten or replace spark plug |
| Stuck, burned, or worn valves - Service or replace valves |
| Worn sleeve, piston, or rings - Replace faulty components |
| Valve clearance is improper - Check and adjust |
| Engine using too much oil |
| Gaskets or seals are excessively worn - Worn parts need to be replaced |
| Worn valve stems and valve guides - Valves need to be changed |
| Seized or wear piston rings - Change piston rings |
| Scored cylinder liner or piston - Inspect piston and liner, change as required |
| Clogged breather hose or pipe - Change or clean |
| Hydrostatic Transmission Troubleshooting |
| Excessive HST transmission noise |
| Bent or unadjusted speed control pedal linkage - Replace or adjust linkage |
| Excessive transmission load - Reduce loading |
| Fluid is contaminated or fluid level is insufficient - Top up to proper level or change transmission fluid |
| Worn or defective parts of the hydrostatic transmission - Change or repair defective parts |
| Shift linkage is rusty or worn - Lubricate or replace the linkage |
| Hydrostatic pump is seized or faulty - Change or repair pump |
| Misadjusted or faulty transmission shift control components - Adjust correctly or replace |
| Transmission fluid insufficient - Add fluid to the transmission housing |
| Damaged or worn drive pulley - Install a new pulley |
| Not adjusted or damaged shifter linkage - Adjust properly or replace linkage |
| Speed control linkage is worn or unadjusted - Change or adjust linkage |
| Broken transmission gaskets or seals - Replace seals or gaskets |
| Final drive shaft leaks - Shaft seals need to be changed |
| Transmission case is cracked - Replace case or transmission assembly |
| Mower Deck Troubleshooting |
| Excessive mower vibration |
| Mower deck drive pulley is worn or dirty - Change or clean pulley |
| Drive belt is damaged or loose - Belt needs to be adjusted or replaced |
| Unbalanced or warped cutting blades - Balance or change blades |
| Blade bolts are not tightened well - Bolts should be tightened |
| Discharge chute keeps clogging |
| Mowing speed is too high - Use a slower speed |
| Grass is too high - Use a lower cutting height |
| Low engine speeds - Open choke fully |
| Mower cuts too low - Use correct cutting height |
| Incorrect mower drive belt installation - Install drive belt as required |
| Discharge area is clogged - Inspect and clean cutting deck |
| Blades are not sharp - Blades need to be sharpened |
| Damaged or worn blades - Blades need to be replaced |
| Mowing speed is too high - Use a slower speed |
| Low cutting height - Must be set to high cutting height |
| Filled grass bag - Remove grass from bag |
| Blades are scalping the grass |
| Lawn is uneven or bumpy - Lawn leveling required |
| Mower deck set to low cutting - Adjust mower to higher cutting height |
| High mowing speed - Use a slower speed |
| Improper mower deck level - Adjust mower deck level properly |
| Blades are worn or bent - Blades need to be replaced |
| Tire pressure is too low - Inflate tires as required |
| Low tire pressure - Inflate tires as required |
| Deck level is not correct - Adjust the level of mower deck correctly |
| Blades are too dull - Sharpen the blades |
| Blades are worn or damaged - Replace the blades |
| Deck is plugged with grass - Mower deck needs to be cleaned |
| Deck shell is damaged - Rebuild or replace mower deck |
| Mower spindle is loose or defective - Install new spindle or repair it |
| Electrical Troubleshooting |
| Battery won’t charge |
| Electrical cable connections are corroded or loose - Service or tighten connections |
| Faulty battery terminal clamps - Change terminal clamps |
| Dead cell in battery - Battery change required |
| Low battery output voltage - Low battery charging, charge as required |
| Battery won’t hold charge - Service charging system or change battery |
| Disconnected cables or faulty battery terminals - Check cables and service or change terminals |
| Starter motor is not working |
| Battery is discharged or defective - Charge or change the battery |
| Wiring is incorrectly connected or disconnected - Inspect wiring and connect correctly |
| Low battery voltage - Charge the battery |
| Spark plug gapped improperly or faulty - Clean and gap or replace |
| Solenoid or starter motor is faulty - Repair starter or install a new solenoid |
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| Lawn and Garden Tractors Technical Specs |
