How To Service A Mountfield Petrol Lawnmower SV150 RV150 Engine

How To Service A Mountfield Petrol Lawnmower SV150 RV150 Engine

In this article, I’m going to show you all the steps I use to service a Mountfield petrol lawnmower with an SV150 or RV150 Engine.

You will see a sticker on the engine of your lawnmower that tells you which engine your lawnmower has.

The servicing for both is almost identical.

If you are looking to service a Mountfield lawnmower with a Briggs And Stratton Engine please check out this article. 

One of the most common questions I’m regularly asked on my Youtube channel is how to service the carburettor on a Mountfield SV 150 or RV150 lawnmower.

The SV150 and Rv150 Mountfield models are not popular with many people in the comments section. They tend to be quite troublesome especially when maintaining an even running speed or not starting.

I have a full playlist of videos on SV150 and RV150 Engines over on my youtube channel

I have repaired many of these Mountfield lawnmowers and in this article, I’m going to show you the exact procedure I use to service all my SV150/ Rv150 Mountfield Petrol Lawnmowers and make them run correctly again.

You can find lots more information and start repairing lawnmowers for profit by checking out my online training here

Remove All Fluids From The Lawnmower

Many white smoke problems are caused when a lawnmower is tipped up or over.

The Mountfield Sv150/ RV150 are particularly prone to this.

For this reason, I highly recommend using an oil extractor to remove all fuel and oil.

I use a Pela 6000 Oil Pump – Oil Extractor/Oil Extraction Kit. Oil Change Sump Pump! 6 litre and it works very well.

It also stores the old fluids until you can dispose of them in an environmentally friendly way.

If your lawnmower still runs I suggest letting it run for 5 minutes to warm up the oil.

Remove the spark plug after running the lawnmower and do not re-install it until all servicing is complete.

Tip the lawnmower back slightly by placing an object under the front of the cutting deck and remove all the old fuel and oil. Never tip a lawnmower on its side or especially upside down when its full of fuel and oil. (I have seen this done!)

Removing all the fluids makes servicing a lawnmower much easier and allows us to remove the carburettor and tip the lawnmower up to remove the blade for sharpening and check the blade adapter and keyway.

It also allows us to clean out the self-drive belt guard (where fitted)

Leave keep the spark plug removed completely from the lawnmower until all servicing is complete.

When servicing is complete I suggest fitting an 1x 5798 BR2-LM BR2LM NGK SPARK PLUG the spark plug gap should be set to 0.6-0.8 mm

Please note many parts are available here

Take a look at my Youtube video covering tips and tricks on SV150 Engines.

Please subscribe to my Youtube channel here

Servicing The Pull Cord

Removing the pull cord mechanism gives us access to almost everything we need to carry out a service.

Whilst the starter recoil is removed it’s a good idea to check the starter rope for signs of wear.

To remove the starter recoil remove the 3 bolts that attach the starter to the engine itself.

To replace the pull cord you will need to use a clamp or ask a handy helper to spare 5 minutes to help.

Remove the starter completely from the lawnmower and place it on a bench.

Pull out the starter cord as far as it will go and clamp the spring mechanism to stop the rope from being rewound.

Pull out the pull cord from the recoil and cut off the knot.

Remove the old pull cord from the starter.

Take a new length of pull cord Pull Starter Cord Rope 3.5mm x 3 Metres Lawnmower Hayter Briggs Honda Engine and measure it against the old one to get the correct length.

Allow a little extra to tie a couple of knots. 3.5 mm rope is fine.

Using a lighter or match, burn the end of the new pull cord to stop it fraying and make the end into a point. This helps push the new starter rope back through.

  • Do not remove the clamp at this point!

Push the new starter rope into the starter recoil.

You will need to make sure the inner and outer holes line up to do this. Once the rope is through, tie a knot and pull it tight with some pliers.

Push the knot safely into the starter recoil where the old one was.

  • Again do NOT release the clamp.

Take the other end of the new pull cord, burn the end and fit the starter handle from the old rope onto the new one.

Next, hold the new starter rope firmly and slowly release the clamp allowing the spring tension to take up the new rope.

Turn over the starter and give it a spray up with the WD-40 to make sure the pawls activate when the pull cord is initially pulled. The servicing of the starter recoil and rope is now complete.

Servicing The Carburettor

If you have a lawnmower that is hunting and surging it is highly likely the carburettor needs cleaning and servicing.

To service the carburettor first remove the air filter cover and air filter.

From here you will see two bolts. These two bolts run right through the air filter housing, through the carburettor and into the engine itself.

Here’s a link to over 45 lawnmower repair videos in a playlist that feature the sv150 or RV 150 Engine.

Click the triangle to watch.

Please subscribe to the channel

The first thing to do is take a small tub and place it near the carburettor. Pull off the fuel line and let the old fuel drain into the tub.

Unfortunately, this model of lawnmower does not have a fuel tap which is another reason I extract all the fuel at the beginning! You can take an old bolt and carefully screw it into the fuel line to stop any further leaks.

With the starter recoil removed you will be able to see all the springs and linkages on top of the carburettor. I strongly suggest taking lots of photographs from different angles at this point.

Luckily I have recorded a video on my Youtube channel in case you forget

Take a 10mm socket and remove one of the long bolts that runs through the carburettor. Undo the second long bolt whilst holding the carburettor with your other hand.

When the bolt is removed the air filter housing comes free. Keep the carburettor as level as possible and unhook all the linkages. You will now be able to completely remove the carburettor from the lawnmower.

Place the carburettor in a tin or tub and turn it over to remove any fuel. Clean the tub or tin before servicing the carburettor. If the carburettor is dirty on the outside be sure to clean it off before removing any parts.

Take a look at my video on how to service the carburettor

Please subscribe to channel here

You do not want any dirt being transferred to the inside of the carburettor. To service the carburettor we first need to remove the bowl on the bottom using a 10mm spanner. Be very careful with this bolt as it’s really easy to cross thread when re-installing.

The bowl should now be removed giving you access the float, retaining pin and needle. Keep the bowl gasket in a safe place. Remove the retaining pin and lift off the float. Keep a very close eye on the needle as they love to fall off and go missing!!

Place all these parts in a container, including the bowl gasket. Next, you need to remove the main jet from inside the carburettor. Take a thin flat head screwdriver and undo the main jet from inside the carburettor.

The main jet has tiny holes in and if they aren’t completely clean and clear it will cause poor running, surging and idling issues. The main jet should fall out in two pieces. If it doesn’t, take a small Allen key and poke it down from the top.

Use some carburettor spray and spray absolutely everything including all the holes, main jet, float and bowl.

If you have an ultrasonic cleaner be sure to use it as this removes dirt from internal parts that cant be reached. Leave everything to soak for around half an hour then blow everything through using an air compressor or a can of compressed air.

Make sure everything is clean and dry and all the tiny holes in the main jet are completely clear. Take your time and make a thorough job of this part of the service. Refit the main jet with the thinner part first then the smaller part last making sure the side with the cut out is visible for re-tightening.

Re-tighten using a flat head screwdriver. Take the float with the needle still in position and drop it into position and re-insert the retaining pin. Shake the carburettor and look a the needle.

Make sure the needle moves up and down freely or the carburettor will overflow with fuel when re-fitted. Re-fit the bowl and gasket and make sure the drain bolt faces the front so it’s accessible when re-fitted on the lawnmower.

Hand tighten the bowl bolt and make absolutely certain not to cross-thread it.

Tighten lightly with a spanner after hand tightening.

Refit The Carburettor On SV150 RV150 Lawnmower

Here comes the fun part! This can be a little tricky the first time you do it.

This time we have no fuel in the carburettor which makes it a little less frustrating. Offer the carburettor up to the engine and refit the back linkage (see video) 24 mins 30 seconds.

Next, rehook the front linkage and hold everything in that position! Make sure all the linkages are completely through and the governor flap moves freely.

The governor flap is the long semi-circular plastic flap that runs around the flywheel. At this point, the carburettor should “hang” on the linkages. Re-connect the fuel line and make sure there are no leaks.

It’s much easier to connect the fuel line before the air filter box is back in position and more sensible to check for fuel leaks at this point. You may need to drop a small amount of fuel into the tank to check this.

If you do have a leak it’s because the needle is not seated correctly inside the carburettor or the retaining pin isn’t in correctly.

Take the air filter box in one hand while holding the carburettor in place with the other, connect the breather pipe from the back of the air filter box to the engine and when everything is lined up insert one long bolt and lightly tighten it.

At this point make sure all the linkages are still in place and re-insert the second long bolt. Re-tighten the bolts evenly and don’t overtighten to the point the air filter box bends!

Replace the air filter at this point and clean any oil or debris from the air filter box. The carburettor service is now complete.

Setting Up The Automatic Choke

With the air filter removed you can see through the air filter box in into the carburettor.

You will see a small butterfly valve. This is the choke and depending on where your throttle lever is set, it will be open or closed.

It is important that this choke is set up correctly to enable starting the lawnmower and running at optimum speed.

Set the throttle control to choke and look inside. The butterfly valve should be completely closed to enable maximum choke to start the engine. If it isn’t you need to adjust the throttle cable.

To do this follow the cable down from the throttle to where it attaches to the engine. Be sure to not be mistaken for the wrong cable! There will be a screw holding the cable in position.

Undo the screw and move the cable to a position where the choke is fully closed when the throttle is set to choke and re-tighten the screw.

Test the throttle cable by setting the throttle to the fastest setting and making sure the butterfly valve is fully open and also fully closed when set to choke.

Bad Design

The SV150 engines have a self drive fitted and also a cable that runs to operate the kill switch. Many times I have seen the kill switch cable interfering with the throttle cable as they run very near each other.

Be absolutely sure everything has space to move, and one tip I recommend is to cable tie the upper cable to the lawnmower handle to stop it moving.

Make absolutely sure the throttle cable is not obstructed.

It’s not the best design I’ve ever seen and I have many comments on my youtube videos on RV /Sv150 engines not running evenly. Often its due to this.

Head Gaskets

These engines have more blown head gaskets than any other engine I come across.

They usually go at the bottom and obviously cause serious running issues.

To replace this gasket you need to remove the starter recoil and spark plug.

Next, you need to remove the head of the engine by removing the 8 bolts that attach it.

Some of the bolts run through the control arm. With the bolts removed prise off the head and remove the head gasket.

It’s a good idea at this point to move the piston to the top to stop any dirt entering.

Take the time to decarbonise the head and areas built up with carbon. Replace the gasket and carefully make sure everything is clean and aligned correctly before re-tightening.

Replace The Air Filter

Now the carburettor has been serviced I recommend fitting a new air filter. The last thing you want is dirt being transferred to the carburettor through a dirty air filter.

Air filters are very cheap to replace.

You can find the correct air filter here Mountfield Genuine 18550011/0 Foam Air Filter

At the very least take the air filter and clean it using some old fuel then allow it to dry completely and blow out any dirt with compressed air.

Checking The Kill Switch

The RV/SV150 engines require a microswitch to be activated to allow spark from the ignition to the spark plug.

This is a built-in safety feature that means the safety lever must be in use at all times when mowing and if the operator is not present the engine will stop.

Never try and bypass this device

To operate the microswitch, the safety lever must be pulled on the operating handle, this, in turn, pulls on a cable that runs to an activation lever near the microswitch.

Often as the cable stretches or through neglect, the cable fails to operate correctly and activate the microswitch.

If you have a lawnmower that shows absolutely no signs of starting and has no spark it’s highly likely the microswitch isn’t being activated.

Setting The Ignition Coil

It’s not really necessary to adjust the ignition coil when doing a service, however, I pick up all sorts of lawnmowers where someone has had a mess around and loosened parts unnecessarily.

I’m often asked what the correct air gap should be between the flywheel and the ignition coil.

Generally, I use an old greetings card to set the gap and I know many people use a playing card!

To set the ignition coil gap, turn the flywheel so the magnets on the flywheel are facing away from the ignition coil.

Back off both bolts holding the ignition coil in place. Next, take the greetings card or something of similar thickness and place it in the gap between the ignition coil and the flywheel.

Make sure the magnets are not grabbing the ignition coil.

Turn the flywheel until the magnets pick up the coil and are separated only by the card. Lock up the two bolts and turn the flywheel to remove the card.

This is an easy way to set an even gap between the ignition coil and the flywheel. I use this method all the time and never have a problem.

Checking The Flywheel Key

If you have lawnmower kickback problems after all servicing is complete you will need to check the flywheel key.

If a flywheel key is even slightly damaged it massively changes the timing of the lawnmower. Remove the starter recoil covers and you will see the flywheel. Use a flywheel puller to pull the flywheel from the crankshaft and inspect the flywheel key.

It should be a perfect shape. If there are any knocks or imperfections replace the keyway.

The flywheel key sets the position of the flywheel on the crankshaft and the timing of the spark.

Check out this video on how to check a petrol lawnmower flywheel key.

If the flywheel is slightly damaged you will get an incorrectly timed lawnmower with the spark plug sparking at the wrong time causing vibration and starting problems. You can read more about lawnmower kickback problems here

Split Lawnmower Petrol Tank

I have never come across a good way to repair a split petrol tank.

Unfortunately, this is a common occurrence on the SV150 engines. Take a look at this video to see an example of this.

The problem with this design on many lawnmowers is that the petrol tank and recoil come as one unit.

This is a problem because if the tank splits you need to replace both the starter recoil and the tank. Fortunately, there is a modification kit that includes a separate gas tank and starter recoil.

The above video will show you how to fit the modified parts.

The part number you will need is modification kit number 18550000/2 or 118550509/0

This procedure is a little time consuming but well worth doing and the petrol/gas tanks are a lot better built.

These problems that are fairly common on these engines are exactly the reason I suggest learning to repair lawnmowers for profit on a simple Briggs And Stratton 35 Classic Engine.

They use an aluminium gas tank and the starter recoils very rarely, if ever break.

How To Remove And Sharpen A Lawnmower Blade

With the spark plug removed and the fuel and oil extracted, tip the lawnmower with the carburettor facing up and the exhaust facing down towards the floor.

Lean the lawnmower against a wall or fence so you can access the underside. Never tip the lawnmower completely over or you will have massive white smoke problems when/if it re-starts.

Use a spanner or socket to undo the two bolts that hold the blade in place and remove the blade.

I use an electric impact driver for this.

I always suggest taking the blade to a machine shop for sharpening for best results.

It is also worth checking out the price of a new blade! If you wish to sharpen the blade yourself you can use a file to follow the natural angle of the cutting edge.

I personally just use a small bench grinder.

I always recommend using a blade balancing tool to ensure the blade is evenly weighted too. These are very simple handy tools. Take a look here Arnold 6011-X1-0195 Blade Sharpening and Balancing Kit for Lawnmower Blades

One thing to remember is that the correct angle is needed to provide the correct back lift for the cut grass to be transferred correctly to the grass box.

If the blade isn’t balanced evenly you will experience more vibration throughout the lawnmower when cutting.

For these reasons, it’s often better to use a local lawnmower repair shop or replace the blade.

Check The Self Drive Belt.

With the blade removed it is always a good idea to remove the belt guard (if fitted)  and clear out any old grass.

I have picked up many lawnmowers where the self-drives don’t work simply because the grass had built up in the belt guard and pulled the belt off!

Many times when this happens the drive belt gets chopped into pieces by the blade so it’s well worth taking the time to do this part of the service with the blade removed.

Check the belt for signs of wear at this point. If in doubt be sure to replace it.

Clean The Cutting Deck

I have seen so many lawnmowers with perfectly running engines and rotten decks.

Many SV150 engines are fitted to Mountfield cutting decks that are generally good quality.

This is one of the main reasons people have to part with or sell their lawnmowers.

To avoid a cutting deck becoming rotten take a wire brush, scraper or pressure washer and remove any build up of grass from the underside of the cutting deck. If possible take the time to paint the underside of the deck.

I usually use Hammerite black paint.

Doing this with the blade removed is very quick and easy to do and will massively extend the life of the cutting deck.

This is why it’s a great idea to extract all oil and fuel at the beginning of a service! Also, avoid where possible cutting wet or damp grass. If your lawnmower has a self-cleaning cutting deck feature be sure to make use of it.

If cutting wet grass is unavoidable be sure to check out my tips here

Replacing The Oil

With the blade re-fitted and the drive belt guard cleared it is time to replace the oil.

Most lawnmowers use just over half a litre of SAE 30 Lawnmower engine oil.

I recommend Universal SAE30 4 Stroke Oil for All Lawnmowers and 4 Stroke Engines – 0.6 L

Be sure not to overfill the engine.

You may need to thread in the engine oil dipstick to get the correct measurement!

  • Note. Your lawnmower may smoke for 5 to 10 minutes after replacing the oil. This is simply because the lawnmower has been tipped and any remaining oil runs into the exhaust area. It will burn off and clear eventually.

Re-Fit The Lawnmower Spark Plug

When servicing is complete I suggest fitting a 1x 5798 BR2-LM BR2LM NGK SPARK PLUG the spark plug gap should be set to 0.6-0.8 mm

Never buy cheap spark plugs. They rarely work.

Believe me, I have had hundreds of lawnmowers in the workshop to repair for profit, that wouldn’t run because of a poor spark plug. (I love it when that happens) I always use NGK spark plugs and have never had one single problem.

Purchasing Spare Lawnmower Parts

Many parts are available from the recommended products section of this website.

Beware of all aftermarket parts particularly carburettors and started recoils.

In my experience, it never pays to use aftermarket parts as they tend to cause more problems in the long run.

The only one I do recommend is the conversion kit for replacing the petrol tank.

Service Sticker

To be able to purchase spares for any lawnmower you will need all the lawnmower details.

To find the correct parts for your lawnmower you will need to find the service sticker.

This is usually a silver sticker located either on the side of the cutting deck or on the top of the cutting deck.

On an SV150 it may be on the engine also.

These are the details your local lawnmower service centre will need to source the correct parts for your lawnmower or if you are looking to buy spare parts online.

I suggest taking a photograph on a mobile phone (or film camera!) to keep a record of this using the free google photos app to keep a backup.

At the very least write the details down and store them somewhere near the lawnmower, this way you will always have access to the information as long as you usually carry a mobile phone.

Service stickers tend to get damaged with fuel and fade over time and without this information, it can often be difficult to locate correct spare parts

Final Thoughts

Sv and RV 150 engines are not the greatest to work on. As mentioned it’s a sore subject in the comments section on my youtube videos.

You can read more about my favourite lawnmowers in this article

There are many reasons I recommend certain lawnmowers in many of my articles, the main reasons are because they are incredibly easy to start when set up correctly, very easy to manoeuvre have super solid cutting decks and reliable powerful yet quiet engines.

I like throttles that work well and the roto stop clutch feature is a must when cutting larger areas as it saves having to re-start the mower after emptying the grass bag.

Be sure to check out my recommended products page for a full list of all lawnmowers I’m happy to recommend.


My personal favourite is The Honda IZY range of lawnmowers fitted with the GCV 135 and 160 engines. Take a look at the design here honda Izy HRG 416 SK Petrol Self Propelled Rotary Lawnmower – 4 Wheeled

I have picked up plenty of Honda lawnmowers to repair for profit and the Honda IZY range always runs like new again after a good service.

The clever design of the cutting deck and wheels combined with the handle set up makes these lawnmowers very easy to use and they produce an excellent finish. Spare parts are widely available and sensibly priced.

I have recently included the Honda IZY range of lawnmower in my list of lawnmowers that could potentially last a lifetime in my article

Be sure to check out all my free Honda lawnmower repair videos available here on my youtube channel.

Please feel free to subscribe to my channel on Youtube it’s free!

I hope this article helps with the servicing of your SV 150 lawnmower engine.

Feel free to look in the recommended products section of this website for any service parts you may need.

You can find lots more information and start repairing lawnmowers for profit by checking out my online training here

Happy Mowing.


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