Owner’s manual
Is it worth repairing a lawn mower engine?
Yes, it’s worth repairing the engine on your MTD 13WJ771S031 when the fix is a tune-up or a small fuel or ignition issue; it’s usually not worth it when the repair is a major internal engine failure or a transaxle-level problem that approaches the tractor’s replacement value. Use the 13WJ771S031 owner’s manual to confirm safe service steps and maintenance intervals.
- Choose repair when it’s a maintenance item (spark plug, air filter service, fresh fuel, oil change).
- Choose repair when the engine cranks but runs rough, hesitates, or won’t start due to fuel or spark issues.
- Consider replacement when compression is low, the engine knocks, or it burns oil heavily.
- Consider replacement when the tractor also has major drive issues (slipping, no movement, loud transaxle noise).
- Stop and inspect after hitting an object; repair damage before operating.
The manual points to common causes like stale fuel (older than 30 days), incorrect choke/throttle position, dirty air cleaner, and spark plug issues. These are typically the most cost-effective repairs.
| Symptom | Most common cause | Typical fix |
|---|---|---|
| Cranks but won’t start | Stale fuel, plugged vent, dirty air cleaner | Drain/replace fuel, clean air path |
| Runs rough or hesitates | Spark plug gap/fouling, restricted airflow | Replace/clean plug, service air cleaner |
| Excessive vibration | Loose or damaged blade | Tighten hardware, replace blade |
Major failures usually mean high labor and parts cost:
- Internal engine damage (rod knock, seized engine, low compression)
- Repeated overheating damage
- Multiple major systems failing at once (engine plus drive system)
A “cheap” engine problem can be a safety problem if vibration is caused by a loose or bent blade. The manual also calls out that blades should stop within about 5 seconds after disengaging the blade control; if they don’t, the tractor needs professional service.
Last updated: February 2026
What size tire is a 13WJ771S031?
For the MTD lawn tractor model 13WJ771S031, the operator’s manual lists front tires as 15 x 6 x 6 and rear tires as 20 x 8 x 8 (with different tread/shoulder styles available). For exact tire type and pressure, use the specs in the 13WJ771S031 owner's manual.
The manual shows these tire sizes for this tractor series:
- Front tire size: 15 x 6 x 6
- Rear tire size: 20 x 8 x 8
- Tire styles may be listed as round shoulder or square shoulder
Typical tire part numbers shown in the manual:
| Position | Size | Example tire part numbers (manual) |
|---|---|---|
| Front | 15 x 6 x 6 | 734-04240A (round shoulder), 734-1731 (square shoulder) |
| Rear | 20 x 8 x 8 | 734-04241 (round shoulder), 734-1730 (square shoulder) |
Before ordering, we recommend checking these items on your current tire:
- Size molded into the sidewall (for example, 15x6x6)
- Shoulder style (round vs. square)
- Rim diameter (the last number, such as 6 or 8)
- Clearance to the deck and frame (especially if the old tire was replaced before)
- Tire pressure spec for your setup (listed in the manual)
Correct tire size on a riding mower affects deck leveling, traction, and cut quality. The manual even calls out checking tire pressure before leveling the cutting deck, because uneven pressure can make the deck cut unevenly.
Last updated: February 2026
What are the parts of a lawn mower engine?
On the MTD lawn tractor model 13WJ771S031, the engine system is built around fuel delivery, air intake, ignition, and rotating internal components that create power. Common engine-related parts you will see referenced in service and troubleshooting include the fuel tank, carburetor and choke, air filter, spark plug and ignition, flywheel, and crankcase.
- Fuel tank and fuel line: stores fuel and delivers it to the carburetor.
- Carburetor and choke plate: mixes fuel and air; the choke helps cold starting by restricting airflow.
- Air filter (air cleaner): keeps dirt out of the engine to prevent wear and rough running.
- Spark plug and ignition system: creates the spark that ignites the fuel-air mixture.
- Flywheel: helps smooth engine rotation and supports ignition timing.
- Crankcase and internal rotating parts: houses the crankshaft and lubricating oil.
Your tractor’s dash controls are part of how the engine operates day to day:
- Ignition switch and key: starts and stops the engine.
- Throttle control (FAST/slow): sets engine speed; mowing is typically done at FAST.
- Choke control: closes the choke plate on the carburetor to aid starting (some models combine choke with the throttle).
- PTO/blade engagement and parking brake: safety-related items that can prevent starting if not set correctly.
| Symptom | Most common engine area to check first |
|---|---|
| Engine cranks but will not start | Fuel freshness, choke use, spark plug connection |
| Runs rough or surges | Air filter condition, fuel quality, carburetor |
| Overheats | Oil level, cooling airflow restrictions |
| Hesitates at high RPM | Spark plug gap/condition, fuel delivery |
Knowing the main engine parts helps you diagnose faster and buy the right replacement parts. For example, stale fuel (older than about 30 days) and a dirty air cleaner are common causes of hard starting and erratic running on riding mowers.
For model-specific operating details (choke use, ignition switch type, and starting steps), use the 13WJ771S031 owner's manual.
Last updated: February 2026
