Mountfield SV150/RV150 Engine Servicing Guide

Servicing Your Mountfield SV150/RV150 Lawnmower Engine

The Mountfield SV150 and RV150 engines, while found on many popular lawnmowers, can sometimes present a few challenges for owners. This comprehensive guide will walk you through the essential steps to service your engine, ensuring it runs smoothly and reliably. We'll cover everything from fluid removal and pull cord replacement to carburettor cleaning and essential component checks. By following these procedures, you can significantly extend the lifespan of your lawnmower and maintain its cutting performance.

Understanding the SV150/RV150 Engine

You'll often find a sticker on your lawnmower's engine identifying it as either an SV150 or RV150. While there might be minor variations between models, the core servicing procedures for both are largely identical. These engines, while capable, are known for being a bit temperamental, particularly regarding maintaining a consistent running speed or starting issues. Addressing these common problems often comes down to diligent maintenance and a thorough understanding of their components.

Essential Preparations: Removing Fluids

Before diving into any servicing, it's crucial to remove all fuel and oil from the engine. This is a critical step, especially to prevent white smoke issues that often arise from tipping the mower incorrectly. The SV150 and RV150 are particularly susceptible to this. An oil extractor, such as a Pela 6000 Oil Pump, is highly recommended for this task. It not only efficiently removes fluids but also provides a safe container for their disposal.

• If the lawnmower is running, let it run for 5 minutes to warm the oil, making it easier to extract.
• Remove the spark plug and keep it out until all servicing is complete.
• Slightly tip the lawnmower back by placing an object under the front of the cutting deck.
• Use your oil extractor to remove all fuel and oil. Never tip the lawnmower on its side or upside down when it contains fluids.

Removing all fluids makes servicing much easier, allowing access to components like the carburettor and the underside of the cutting deck for blade maintenance.

Servicing the Pull Cord

A faulty pull cord can prevent your lawnmower from starting. Servicing or replacing it is a straightforward process that grants excellent access to other engine components.

Steps to Replace the Pull Cord:

1. Remove the three bolts securing the starter recoil to the engine.
2. Pull the starter cord out as far as it will go and clamp the spring mechanism to prevent it from rewinding.
3. Carefully remove the old pull cord from the starter.
4. Measure a new pull cord (3.5mm diameter is suitable) against the old one, allowing a little extra for knots.
5. Burn the end of the new cord with a lighter to prevent fraying and create a point for easier insertion.
6. With the clamp still in place, push the new cord through the starter recoil, aligning the inner and outer holes.
7. Tie a knot in the cord and pull it tight with pliers, then push the knot securely into its housing.
8. Do not release the clamp yet.
9. Burn the other end of the new cord and attach the starter handle from the old rope.
10. Firmly hold the new starter rope and slowly release the clamp, allowing the spring tension to take up the slack.
11. Spray the starter mechanism with WD-40 to ensure the pawls engage correctly.

Carburettor Cleaning and Servicing

A dirty or clogged carburettor is a common cause of surging, hunting, and poor running. Regular cleaning is vital.

Carburettor Service Procedure:

1. Remove the air filter cover and air filter.
2. Place a small tub beneath the carburettor and carefully disconnect the fuel line. (Note: these models lack a fuel tap, hence the importance of draining fuel first). You can temporarily block the fuel line with an old bolt if needed.
3. Take numerous photographs of the carburettor linkages and springs from various angles before proceeding.
4. Using a 10mm socket, remove the two long bolts that pass through the carburettor and air filter housing into the engine.
5. Hold the carburettor level and unhook all linkages. Remove the carburettor from the engine.
6. Drain any remaining fuel from the carburettor into the tub. Clean the exterior of the carburettor before disassembling.
7. Using a 10mm spanner, carefully remove the bowl at the bottom of the carburettor. Be cautious to avoid cross-threading.
8. Remove the bowl gasket, float, and retaining pin. Keep track of the small needle, as it's easily lost.
9. Using a thin flat-head screwdriver, remove the main jet from inside the carburettor. This jet has tiny holes that must be clear for proper engine function.
10. Spray all parts, including the jet, float, and bowl, with carburettor cleaner. An ultrasonic cleaner can be highly effective for thorough cleaning.
11. After soaking (around 30 minutes), use compressed air to blow out all components, ensuring all tiny holes are clear.
12. Reassemble the carburettor, ensuring the needle moves freely within the float. Refit the bowl and gasket, ensuring the drain bolt is accessible. Hand-tighten the bowl bolt, then lightly tighten with a spanner, being careful not to cross-thread.

Refitting the Carburettor

Refitting the carburettor can be a bit fiddly, but taking your time will ensure success.

1. With no fuel in the carburettor, align it with the engine and refit the linkages. Ensure the governor flap moves freely.
2. Reconnect the fuel line and check for any leaks.
3. Connect the breather pipe from the air filter box to the engine.
4. Align the air filter box and insert the long bolts, tightening them evenly. Avoid overtightening.
5. Replace the air filter and clean any debris from the air filter box.

Setting the Automatic Choke

Correct choke adjustment is vital for starting and optimal running.

1. With the air filter removed, locate the small butterfly valve (choke) inside the carburettor.
2. Set the throttle control to the 'choke' position. The butterfly valve should be fully closed.
3. If it's not fully closed, adjust the throttle cable. Loosen the screw holding the cable near the engine, reposition the cable so the choke closes completely, and retighten the screw.
4. Test the throttle by moving it to the fastest setting (butterfly fully open) and back to choke (butterfly fully closed).

Addressing Common Issues: Head Gaskets and Kill Switch

Head Gaskets: These engines are prone to blown head gaskets, often at the bottom, causing significant running problems. Replacement involves removing the starter recoil and spark plug, then undoing the eight bolts securing the cylinder head. Carefully remove the head, replace the gasket, and clean any carbon build-up before reassembly. Ensure the piston is at the top dead centre during this process to prevent dirt ingress.

Kill Switch Cable: The kill switch cable can sometimes interfere with the throttle cable due to their proximity. Ensure there is adequate space for both cables to move freely. Securing the kill switch cable to the lawnmower handle with a cable tie can prevent it from obstructing the throttle.

Ignition Coil and Flywheel Key Checks

Ignition Coil Gap: While not always necessary during a routine service, an incorrect ignition coil gap can cause starting issues. A simple method to set the gap is to use a piece of card (like a greetings card) between the coil and the flywheel magnets. Loosen the coil bolts, insert the card, and turn the flywheel until the magnets attract the coil, holding it against the card. Tighten the bolts, then remove the card. This ensures an even air gap.

Flywheel Key: Kickback issues after servicing can sometimes be traced to a damaged flywheel key. This key ensures the correct ignition timing. To check it, you'll need to remove the starter covers and use a flywheel puller to remove the flywheel. Inspect the key for any damage or imperfections and replace it if necessary. A damaged key can lead to incorrect timing, causing vibration and starting problems.

Blade Maintenance and Belt Check

Blade Removal and Sharpening: With the spark plug removed and fluids drained, tip the lawnmower with the carburettor facing upwards. Access the underside of the cutting deck and undo the bolts holding the blade. For best results, have the blade professionally sharpened. If sharpening yourself, use a file or bench grinder, maintaining the correct cutting edge angle. Always use a blade balancing tool to ensure the blade is evenly weighted, preventing excessive vibration.

Self-Drive Belt: With the blade removed, it's an excellent time to check the self-drive belt. Remove the belt guard (if fitted) and clear any accumulated grass or debris. Blockages can dislodge the belt, leading to drive failure. Inspect the belt for wear and replace it if in doubt.

Cleaning the Cutting Deck

A clean cutting deck is essential for preventing rust and maintaining the mower's lifespan. Use a wire brush, scraper, or pressure washer to remove grass clippings from the underside. Applying a protective paint, such as Hammerite, can further prevent corrosion. Avoid cutting wet grass where possible, as it contributes to deck rot.

Replacing the Oil

After reassembling components and ensuring everything is secure, it's time to refill the engine with oil. Most lawnmowers require just over half a litre of SAE 30 lawnmower engine oil. Use the dipstick to ensure you do not overfill the engine. A small amount of smoke for the first 5-10 minutes after starting is normal, as residual oil may burn off in the exhaust.

Re-fitting the Spark Plug

As a final step in the servicing, refit the spark plug. It's recommended to use a quality spark plug, such as an NGK BR2-LM, with a gap set between 0.6-0.8mm. Avoid cheap, unbranded spark plugs, as they can lead to poor engine performance and starting issues.

Spare Parts and Service Stickers

When sourcing spare parts, it's vital to have the correct lawnmower details. Locate the service sticker, usually found on the cutting deck or engine. This sticker contains crucial information for identifying the right parts. Taking a photograph of this sticker is a good way to keep a record, as they can fade or become damaged over time.

Final Thoughts on SV150/RV150 Engines

While the Mountfield SV150 and RV150 engines can be a bit demanding to work on, a proactive approach to servicing can keep them running efficiently. Understanding their common quirks, such as potential head gasket issues or cable routing challenges, allows for targeted maintenance. For those seeking easier-to-maintain alternatives, engines like the Honda GCV series found in the Honda IZY range are often recommended for their reliability and user-friendliness.

Frequently Asked Questions

Q1: Why is my Mountfield SV150 lawnmower smoking?

A1: White smoke, especially after servicing, is often due to oil that has seeped into the combustion chamber or exhaust during tipping. This should burn off after a few minutes of running. Persistent or dark smoke could indicate other issues like a blown head gasket or incorrect oil level.

Q2: My lawnmower won't start. What could be the problem?

A2: Common starting issues include a dirty carburettor, a faulty spark plug, an obstructed throttle cable, or a problem with the kill switch activation. Ensure you have fresh fuel and that the spark plug is correctly gapped and firing.

Q3: How often should I service my Mountfield SV150 engine?

A3: It's recommended to perform a basic service, including checking and cleaning the air filter and spark plug, at least once a year. A more thorough service, including carburettor cleaning and oil change, should be done annually or after approximately 25-50 hours of use.

Q4: Can I use a generic spark plug for my SV150 engine?

A4: While generic plugs might fit, using a recommended quality plug like an NGK BR2-LM with the correct gap will ensure optimal performance and reliability. Cheap plugs can cause misfires and starting problems.

Q5: What is the correct oil for a Mountfield SV150 engine?

A5: The recommended oil is SAE 30 4-stroke lawnmower engine oil. Always check your lawnmower's manual for the specific oil type and capacity. Overfilling or using the wrong oil can damage the engine.

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