How to Service Your Rotax Max Carburettor – Step-by-Step Guide

Are You Servicing Your Kart Carburettor Often Enough?

The answer is probably no!

Your carburettor is one of the most important components when it comes to engine performance. Even a small amount of dirt or debris inside the carburettor can have a noticeable effect on how your Rotax Max engine performs on track.

A partially blocked jet or fuel passage can cause poor throttle response, hesitation when accelerating out of corners, inconsistent engine performance and a loss of power towards the end of the straight.

Regular carburettor maintenance is one of the easiest ways to keep your engine performing consistently.

At Used Kart Shop, we prefer a simple preventative maintenance routine rather than waiting until problems develop. For karts used regularly, we recommend inspecting and cleaning the carburettor every couple of running days, depending on conditions.

In this guide, we'll show you how to strip, clean, inspect and reassemble your Rotax Max Dellorto carburettor.

Why Does Your Carburettor Need Cleaning?

Your carburettor contains several small jets and internal passages responsible for delivering the correct amount of fuel to your engine.

Some of these passages are extremely small and can easily become restricted by:

• Dirt and dust entering through the intake system

• Small fibres or debris from filters

• Contamination within the fuel system

• Fuel residue and deposits

• General wear and contamination from regular use

Even when your fuel and air filtration systems are working correctly, small particles can occasionally find their way into the carburettor.

Cleaning these components regularly helps maintain consistent fuel delivery and throttle response.

Tools and Equipment Required

Before starting, make sure you have the following equipment:

• Small flat-blade screwdriver

• Very small flat-blade screwdriver for the pilot jet components

• 12 mm spanner

• 4 mm Allen key or T-bar

• Small pliers

• Compressed-air line with a suitable nozzle

• Kartcare One Shot Carburettor Cleaner

• Clean workshop cloth

• Small parts tray

Workshop tip: Always work on a clean bench and lay your components out in the order you remove them. This makes reassembly much easier and reduces the risk of losing small parts.

Step 1 – Remove the Carburettor Float Bowl

Start by removing the carburettor from the engine, ensuring the fuel supply is disconnected and any remaining fuel is safely drained.

Using your 4 mm Allen key, remove the float bowl from the bottom of the carburettor.

Carefully remove the floats, taking note of their position and orientation.

Inspect the bowl for any dirt, debris or fuel deposits. If contamination is present, it's worth checking your fuel tank, fuel lines and filters as well.

Step 2 – Remove the Carburettor Jets

Inside the float bowl area, you'll find the main fuel-metering components.

On the commonly used Dellorto VHSB carburettor, there are several removable jets and related components, including the main jet, idle jet and idle emulsion tube.

Carefully remove the accessible jets using the correct screwdriver or spanner.

Important: There is a smaller idle jet positioned deeper within the idle circuit assembly. This is particularly important to inspect because a restriction here can affect low-throttle performance and acceleration out of slower corners.

Use a small, correctly fitting flat-blade screwdriver to remove it.

If the jet is tight, do not force it. Apply a suitable penetrating lubricant to the threads if necessary, allow it time to work, and try again carefully.

Avoid damaging the soft brass components.

Once removed, lay the jets out in order and record their sizes so you can reinstall them correctly.

Step 3 – Remove and Inspect the Float Needle Valve

Next, remove the float arm pivot pin using small pliers if required.

Carefully lift away the float arm and needle valve.

Inspect the needle valve for wear, dirt or damage, paying particular attention to the sealing tip.

A worn or contaminated needle valve can cause incorrect fuel levels, flooding or inconsistent fuel delivery.

Check that the float mechanism moves freely and that nothing is bent or damaged.

If you're removing the needle valve seat, use the appropriate tool and inspect its sealing washer or gasket. Replace any damaged sealing components.

Workshop tip: Check the float height against the correct Rotax/Dellorto specification before reassembly. Incorrect float height can significantly affect how the engine performs.

Step 4 – Remove and Clean the Fuel Filter

Locate the fuel inlet connection on the side of the carburettor.

Using your 12 mm spanner, carefully remove the inlet fitting and inspect the small internal fuel filter.

Depending on the carburettor configuration, there may be additional filtration components.

Remove any accessible filter carefully and inspect it for contamination.

Clean or replace the filter as required, ensuring it is completely free from debris before refitting.

If the filter is particularly dirty, investigate the source of the contamination rather than simply cleaning the carburettor.

Step 5 – Remove the Air Mixture Screw

Locate the air mixture screw on the carburettor.

Before removing it, gently turn it clockwise until it just seats, counting the number of turns. Record this setting so you can return to your original adjustment.

Never tighten the screw hard against its seat, as this can damage the carburettor.

Now remove the screw completely, along with its spring and any associated sealing components fitted to your particular carburettor.

Keep these small parts together.

Step 6 – Thoroughly Clean the Carburettor

This is the most important part of the service.

Using a suitable carburettor cleaner, thoroughly clean the accessible internal fuel passages, jet housings, float bowl and removable jets.

We recommend Kartcare One Shot Carburettor Cleaner.

Pay particular attention to the smaller idle circuit passages, as these can become restricted by very small particles of dirt.

Once the cleaner has loosened any deposits, use controlled compressed air to blow through the passages and jets.

Continue until the components are visibly clean and the passages are clear.

Important cleaning precautions:

• Never push wire, drill bits or other hard objects through the jets, as this can enlarge or damage the calibrated openings.

• Wear eye protection when using compressed air or cleaning chemicals.

• Keep the compressed-air nozzle away from your skin and eyes.

• Remove or protect rubber seals, diaphragms and other solvent-sensitive components before applying cleaner.

• Follow the cleaning product manufacturer's instructions and avoid excessive air pressure.

Make sure all components are completely dry and free from cleaning residue before reassembly.

Step 7 – Clean the Carburettor Slide and Needle

While the carburettor is apart, it's worth inspecting the throttle slide and jet needle.

Remove the carburettor top cap and carefully withdraw the slide assembly.

Remove the needle and retaining clip, taking note of the clip position before disassembly.

Clean the slide and needle using a suitable cleaner and a soft cloth.

Check the needle for wear, damage or visible marks.

Inspect the inside of the carburettor body and ensure the slide moves freely without sticking.

A sticking slide or damaged needle can affect throttle response and fuel metering.

Reassemble the needle and clip in their original positions unless you're deliberately making a jetting adjustment.

Step 8 – Inspect the Throttle Cable Seal

One component that's often overlooked is the rubber seal around the throttle cable entry at the top of the carburettor.

Over time, this seal can become worn, hardened or damaged.

Inspect it carefully and replace it if necessary.

An ineffective seal may allow unwanted air or contamination to enter, depending on the carburettor design, potentially contributing to inconsistent running.

Also check that the throttle cable moves smoothly and that the slide returns fully when the accelerator is released.

Step 9 – Reassemble the Carburettor

Once everything is clean and dry, carefully reassemble the carburettor in the reverse order of removal.

Refit the jets in their correct locations, ensuring they are properly seated without overtightening.

Reinstall the needle valve, float arm and pivot pin, checking that the float mechanism operates freely.

Refit the air mixture screw and return it to the setting recorded earlier.

Finally, reinstall the floats and float bowl, making sure all seals and gaskets are correctly positioned.

Before refitting the carburettor to the engine, double-check that every component has been installed correctly.

Step 10 – Check Your Carburettor Settings

Once the carburettor is reassembled, check the idle mixture and idle speed adjustments.

As a general workshop reference, some Rotax Max Dellorto setups use an air mixture screw setting of approximately 2 turns out from gently seated.

However, the correct setting depends on the carburettor version, engine configuration and operating conditions. Always refer to the current Rotax documentation for your particular setup.

The idle speed screw is adjusted separately to achieve the correct idle speed. Unlike the air mixture screw, it should not be set simply by assuming a universal number of turns.

Check the current Rotax regulations before making changes if you're racing in a regulated championship.

Step 11 – Final Checks Before Going on Track

With the carburettor refitted, reconnect the fuel line and check for leaks.

Ensure the throttle opens smoothly and returns fully when released.

Start the engine and check for consistent idling and throttle response.

If the engine still hesitates, bogs down or struggles to accelerate, investigate the jetting, fuel supply, float height and other engine-related components before assuming the carburettor is clean and correctly adjusted.

How Often Should You Service Your Rotax Carburettor?

At Used Kart Shop, we recommend a quick inspection and routine clean every couple of running days for regularly used competition karts.

If you're racing in particularly dusty conditions, using older fuel-system components or experiencing inconsistent engine performance, more frequent checks may be worthwhile.

You don't necessarily need to completely strip the carburettor after every session. The aim is to prevent contamination from building up and identify potential problems before they affect performance.

Final Thoughts

A clean, correctly assembled carburettor is essential for consistent engine performance.

Regular maintenance can help prevent frustrating issues such as poor acceleration, hesitation out of corners and inconsistent running.

Taking a little time to inspect and clean your carburettor between race meetings can save a lot of unnecessary troubleshooting at the circuit.

Need replacement carburettor jets, gaskets, floats or servicing products?

Visit our Rotax Max Carburettors & Parts Collection for replacement components and maintenance products.

Used Kart Shop – Kart Parts, Preparation & Racing Support

Happy racing!

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