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Home / Lawn and Garden Tractors / John Deere Lawn Tractors / John Deere LA120

John Deere LA120 Troubleshooting

The John Deere LA120 lawn tractor was manufactured from 2007 to 2009. The John Deere LA120 driveline consists of Briggs and Stratton 441677 4-cycle V-twin cylinder OHV vertical-shaft air-cooled gasoline engine and Tuff Torq T40 belt-driven hydrostatic transmission with infinite forward and reverse gears.

A 44.2 cu.in (724 cc) engine has a cylinder bore of 3.12" (79 mm), piston stroke of 2.89" (73 mm), and rated power of 21 hp (15.7 kW). Max. forward speed - 5.5 mph (8.8 kph), maximum reverse speed - 3.2 mph (5.1 kph).

The tractor was equipped with a manual steering system, disc brakes, 15x6.00-6 front tires, and 20x10.00-8 rear tires. The wheel base is 49 in (1240 mm). The John Deere LA120 is compatible with a 42" mid-mount 2-blade mower deck with cutting height of 1.0-4.0" (25-102 mm) and mechanical lifting system.

Engine Troubleshooting
Engine will not start or difficult to start
Fuel tank is empty - Fill it up
Wrong fuel grade - Use quality fuel
Air in fuel system - Bleed fuel system
Disconnected or loose spark plug wire - Inspect spark plug wiring connection, change or repair
Faulty spark plug - Replace spark plug
Fuel filter is dirty - Install new filter
Safety contact for brake pedal defective - Install new part
Carburetor is not adjusted - Check carburetor float adjustment
Bad engine compression - Inspect cylinder block and replace worn components
Throttle cable is adjusted incorrectly or damaged - Replace cable or adjust correctly
Unstable engine running
Air intake grill is clogged - Wash or clean as required
Ignition system failure - Check ignition and replace faulty components
Stuck or defective throttle cable - Replace or service the cable
Air filter element is plugged - Clean or replace air filter
Fuel pump is dirty or defective - Service or replace fuel pump
Engine starts but stalls immediately
Damaged spark plug - Change spark plug as required
Carburetor is faulty or dirty - Clean or change carburetor
Defective ignition coil - Replace ignition coil as necessary
Valve springs are bad or broken - New valve springs need to be installed
Valves are stuck - Clean the valves
Engine keeps overheating
Not enough engine oil - Fill up the engine oil
Engine overload - Reduce load
Air intake screen is blocked - Clean and wash as required
Too lean carburetor mixture - Use proper settings
Improper ignition timing - Adjust ignition timing as required
Improper valve setting - Adjust valves as required
Engine knocking or noise
Low octane fuel - Use proper gasoline
Engine is running overloaded - Reduce engine load
Not enough engine oil - Fill up the engine oil
Low idle speed setting is incorrect - Correct idle adjustment
Clogged or defective fuel pump - Service or replace fuel pump
Main bearings or crankshaft failure - Have bearings or crankshaft replaced
Incorrect valve clearance - Check and adjust
Damaged cylinder bore or piston rings - Replace faulty parts
Loss of engine power
Incorrectly adjusted or clogged carburetor - Clean or correctly adjust carburetor
Air filter element is clogged - Service or replace air filter
Fuel pump is defective or dirty - Check fuel pump and replace if necessary
Ignition failure - Check ignition and replace faulty components
Blown cylinder head gasket - Cylinder-head gasket replacement required
Stuck or burned valves - Service or replace valves
Bent or worn push rod - Push rod needs to be changed
Cylinder block components are faulty - Replace defective parts as required
High fuel consumption
Throttle did not open fully - Replace or adjust throttle cable
Improper carburetor main jet adjustment - Adjust it correctly
Loose or faulty spark plug - Replace or tighten spark plug
Valves are stuck, burned, or worn - Change or clean valves
Worn sleeve, rings, or piston - Replace defective components as required
Valve clearance is not correct - Adjustment procedure required
Engine consuming too much oil
Damaged gaskets or seals - Replace to repair leaks
Valve stems or guides are damaged - Install new valves
Seized or wear piston rings - Replace piston rings
Scored piston or cylinder liner - Check liner and piston, change as required
Breather pipe or hose is clogged - Replace or service


Hydrostatic Transmission Troubleshooting
HST transmission makes excessive noise
Speed control linkage is worn or unadjusted - Adjust or repair linkage
Excessive transmission load - The load should be reduced
Insufficient oil level or contaminated oil - Change transmission oil or top up to proper level
Transmission components are damaged or worn - Change or repair defective parts
Hard to shift
Rusty or worn shift linkage - Install a new gear linkage
Hydrostatic pump is worn or stuck - Repair or replace pump
Faulty or misadjusted transmission shift control parts - Adjust properly or change
Low power
Transmission oil insufficient - Check and refill the transmission oil
Drive sheave is worn or faulty - Replace sheave
Shifting linkage is misadjusted or broken - Replace or adjust linkage
Speed control pedal linkage is damaged or out of adjustment - Adjust or change linkage
Transmission leaking
Transmission seals or gaskets are broken - Seals and gaskets need to be replaced
Final drive shaft leaks - Change shaft seals
Transmission housing is cracked - Install a new transmission housing

Mower Deck Troubleshooting
Mower vibrates excessively
Mower deck drive pulley is worn or dirty - Change or clean pulley
Drive belt is worn or loose - Change belt or adjust belt tension
Mower blades are not balanced or warped - Balance or change blades
Loose blade mounting bolts - Bolts need to be tightened
Discharge chute is constantly plugged
High mowing speed - Operate at slower speed
Grass is too long - Use a lower cutting height
Engine is running at too low speed - Throttle is not fully open
Low cutting height - Adjust mower to higher cutting height
Drive belt installed improperly - Check drive belt for correct installation
Uncut grass when mowing
Mower deck is clogged - Mower deck needs to be cleaned
Not sharp blades - Sharpen the blades
Worn or defective blades - Blades must be changed
Mowing speed is too high - Use a lower speed
Low cutting height - Must be set to high cutting height
Grass bag is full - Clean grass bag
Blades are scalping the grass
Lawn is not level or bumpy - Level the lawn
Mower deck set to low cutting - Must be set to high cutting height
High ground speed - Need to slow down
Incorrect level of mower deck - Adjust mower deck level properly
Damaged or bent blades - Install new blades
Tires are not properly inflated - Inflate tires as required
Uneven cutting
Tires are not correctly inflated - Check and inflate the tires
Improperly leveled mower deck - Level the deck properly
Not sharp blades - Sharpen the blades
Blades are worn or damaged - Install new blades
Cutting deck clogging - Mower deck needs to be cleaned
Damaged mower deck shell - Replace or repair the deck
Loose or defective mower spindle - Repair or replace the spindle

Electrical Troubleshooting
Battery cannot be charged
Electrical wire connections are loose or corroded - Clean or tighten wiring connections
Electrical terminal connectors are defective - Replace terminal connectors
Dead cell in battery - Battery change required
Starter cranks slowly
Low battery capacity - Charge the battery
Battery won’t hold charge - Battery needs to be serviced or replaced
Battery terminals are corroded or wires are disconnected - Check cables and replace or service terminals
Starter motor is not working
Battery is low or faulty - Replace or charge as required
Battery wiring is disconnected or improperly connected - Inspect wiring and connect as required
Low battery capacity - Recharge the battery
Improperly gapped or dirty spark plug - Clean, gap or change spark plug
Starter motor or solenoid is defective - Change solenoid or repair starter

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