7 Reasons a Spartan Mower Deck Belt Keeps Coming Off

It is frustrating to have to continue to shut down your zero-turn to install the mower deck belt after it keeps coming off the pulleys.

A Spartan zero-turn mower deck belt that keeps coming off your mower may be due to a worn or shredded belt, bad spindle or pulley bearings, insufficient belt tension, debris lodged in the deck, or oil or rust on the belt.

Remove the ignition key and the spark plug boots before you start working on your mower deck. Follow additional safety procedures found in your operator’s manual.

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Follow all safety instructions provided in your equipment operator’s manual before diagnosing, repairing, or operating. Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.

Your Spartan Lawn Mower Belt Keeps Coming Off Your Deck

1. Worn Mower Deck Belt

The first thing to look at when determining why your belt keeps coming off the pulleys is the condition of the deck belt. A worn belt can become loose on the pulleys and roll off of them.

Look for signs of wearing. This includes cracks forming on the belt or a belt that is stretched. A worn belt may also have a glazed shiny appearance or it may sit deep in the pulleys.

If you find the belt is worn, replace it with a new one. Even if a worn belt isn’t the main cause it keeps falling off the pulleys, it must be replaced for a good quality cut.

A worn belt will begin to slip on the pulleys and no longer turn the blades at high speeds.

While some zero-turn mower owners try to find a comparable belt at the local hardware store, I highly advise against doing this. The belt you get at the hardware store may look the same, but it isn’t always the right type, size, or quality of the belt.

I prefer to purchase an OEM (Original Equipment Manufacturer) belt to ensure I’m getting a quality belt of the right material designed for my zero-turn. The mower deck belt does make a difference to the quality of cut you receive from your Spartan mower.

2. Shredded Mower Deck Belt

A belt can become shredded causing it to become weak and more likely to break. The most common reasons a belt will shred is from incorrect installation or from rubbing on a bracket that has moved out of place.

If you find your belt is shredding along its edges, first check to make sure you are using the right belt and it is correctly routed around your deck pulleys. You may have a decal on your mower with a diagram that shows the belt layout.

If you can’t find it, don’t worry. Refer to your Spartan owner’s manual or a parts diagram to get this information.

Once you have confirmed your belt has been installed correctly, check for any brackets that your belt may be rubbing against once your mower deck is engaged.

You may find a shiny wear spot on a bracket where the belt continues to rub. Adjust any brackets you find causing your belt to rub so they no longer rub.

If the bracket you are adjusting has a bushing in it, the bushing may need to be replaced. You must replace a bracket you are unable to adjust to prevent the belt from rubbing.

Lastly, replace the mower deck belt with a new one once you have confirmed there are no more points on your mower deck the belt is rubbing against.

3. Bad Spindle Housing Bearing

A bearing may be bad in your Spartan spindle housing assembly. This is the piece on your deck where the mower blade attaches.

A failed bearing can cause your blade to wobble. At high speeds, the wobble can cause an excessive vibration that can knock the belt off your mower.

To check for a bad bearing, safely access the underside of your mower deck. Protect your hands with a good pair of work gloves and grab a hold of each end of your mower blade.

Rock the blade up and down feeling for movement. You may also hear a knocking noise.

If you find your blade is secure to the cutter housing, but there is vertical movement, remove the blade and disassemble the spindle housing assembly (also known as cutter housing).

Chances are highly likely you have a bad bearing in the housing that must be replaced. Inspect the spindle for damage.

Replace the bad parts and reinstall. You can purchase the complete spindle assembly. You may also be able to get parts to rebuild the spindle housings on some models.

4. Bad Pulley Bearing

A bad bearing in a deck pulley can cause the belt to slip off of the pulley. This is because the pulley is designed to sit flat and parallel to the deck.

A pulley with a bad bearing will result in a pulley developing a wobble so it no longer is securely attached. It may now sit angled to the deck which increases the possibility the belt will slip off the pulley from the vibration from a bad bearing.

Check each pulley by slowly spinning the pulley by hand. When you feel resistance, feel a wobble, or hear a noise coming from the bearing, the bearing is bad. The pulley must be replaced.

5. Insufficient Belt Tension

Your Spartan has a tensioner bracket and spring on the mower deck to keep proper tension on the deck belt so it doesn’t come off the pulleys and fall off your deck.

• Too much tension on the mower deck belt can cause damage to your deck components.
• Too little tension on the belt can cause the belt to vibrate, slip and come off your Spartan mower.

Debris that collects on top of your mower deck can interfere with the movement of your tensioner pulley.

Check the area around the tensioner bracket and spring and make sure the tensioner, bushing, and spring are in good condition. Remove any debris restricting the tensioner from working properly.

It isn’t only important to keep the belt area of your mower deck clean and free of debris for performance reasons, it is also important for safety reasons as well.

Dry debris collected on the top side of your mower deck can catch on fire from the heat of the belt.  

Read more about cleaning your mower deck in my guide on finding and fixing mower deck problems. A clean mower will not only improve performance, but it can also extend your mower’s lifespan.

6. Debris

Dirt and debris can accumulate in the pulley grooves. When the belt runs around pulleys with debris packed in them, the belt is stretched. This can cause the belt to roll off the pulleys and come off.

The extra stress put on the belt can also cause the belt to snap. To correct this, inspect the pulleys and remove any dirt, stones, grass, and debris you find packed in the grooves of the pulleys or under the pulleys.

7. Oil or Rust on the Deck Belt

Rust and oil can ruin a deck belt and be the reason it keeps coming off your Spartan mower. Let’s talk about oil first. The oil that gets spilled or leaks on the mower deck can lubricate the belt causing it to slip off the pulleys.

When oil sits on your belt for an extended time, the belt can swell. Look for the cause of the oil leak and repair it. Clean the oil to remove it from the deck and mower components. Replace the mower deck belt.

The other item that can get on the belt and cause it to come off your mower is rust. Rust forms when metal components are exposed to moisture.

When the rust transfers from the components to your Spartan deck belt, the belt is likely to dry out, crack and break.

Remove rust from the deck and mower deck components. Replace pulleys and components where you are unable to remove all of the rust. Install a new mower deck belt once your deck is cleaned up and free of rust.

Clean Your Spartan Mower Deck After Each Mowing & Before Storing

Going forward, once you finish mowing the lawn, take time to clean your zero turn removing grass clippings and dirt from your mower. These items hold moisture and can accelerate corrosion on your Spartan mower.

Ensure your mower is completely dry before placing your mower back in storage. Leaving it outside on a sunny day will help remove moisture. You can also try to speed up the process using a leaf blower.