To achieve optimum and sustained tunability and engine performance, the most used carburetor in Australian Karting, the Walbro WB series, needs regular servicing and maintenance. Read on to go faster.

The job of the carburetor is to mix fuel and air in the correct proportion for optimum combustion. While combustion is influenced by a large number of factors outside of the carburetor itself, the correct fuel/air mix is critical. The correct mixture required by your engine is affected by many changing factors such as engine load, rmp (i.e. air speed), and atmospheric conditions. The build and tuning of the carburetor will allow you to adjust this air/fuel mixture to keep the motor in tune as these conditions change.

How's it work?

The Walbro carburetor is also a fuel pump. Crankcase pressure changes reach the carburetor via the pulse line and these pressure changes actuate little 'flapper' valves on the pump diaphragm. Fuel is sucked from the fuel line connected to the base of the carburetor and pumped into a chamber which travels right through the carb to the inlet valve and seat on the other side of the carburetor body. This inlet needle controls how much fuel is allowed to enter the jet circuit (which consists of a number of channels and orifices under the 'half-moon' circuit plate which feeds fuel into the various jets in the throat and venturi of the carburetor). While the tuning needles are used by the driver to adjust the high and low speed jet mixes, the combination of lever hight and pressure at which the needle valve opens is critical as these will control the amount of fuel entering the circuit and keep it to a manageable and correct level. Too much or too little fuel in the circuit will mean the high and low speed needles will not be in a manageable rang for the driver to tune.

Servicing the Walbro

The Walbro requires regular servicing to keep performance and adjustability at its peak. The carburetor will still function without regular servicing, but will not deliver optimum performance and will become difficult to tune. "The gaskets do tend to go a little harder with premium unleaded compared to leaded petrol so you do have to change them a bit more regularly, but probably not quite as exaggerated as what some people say, change them every four or five meetings and keep a check on the blow-off pressure and the lever arm hight because the arm height varies as the needle starts to wear. The arm height goes down which then raises the hight of the diaphragm."

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1. For a standard gasket change and carb check, first step is to disassemble the carburetor. Take note of how it comes apart and this will mean putting it back together will be easier.
    
2. Wash the carburetor body out with petrol. Make sure the fine mesh filter is clean on the pump side of the carb (the side where the pulse line attaches). If you ruin a in-line fuel filter there should be no foreign debris here, however it will need regular cleaning if you don't run a filter. The job of this mesh is to catch any dirt or fluff before it travels through this chamber and gets stuck under the inlet needle valve on the other side which would flood the engine. If you have the luxury of compressed air, blow the carb body out taking care not to blow the rubber seal and brass washer from the high speed tuning needles across the workshop. Alternatively a commercially available carburetor cleaner would do the job.
    
3. Re-fit a fresh gasket set. On the pump side, ensure the diaphragm goes against the body of the carb (the 'flappers' need to be able to seal on the carb body itself) followed by the gasket and finally the pump cover. Tighten the four screws.
    
4. The metering lever side is a bit more complex. Fit the 'half-moon' gasket followed by the diaphragm. Some tuners elect to fold over the little flap in this diaphragm (which feeds the high speed jet with fuel) but don't cut it off otherwise the engine is technically illegal. Fit the metal circuit plate and tighten.
    
5. Position the new inlet needle and spring in their locating holes. With steady hands slide the metering pin (9) through the metering lever (8). You need to get the end of the needle in the 'claw' of the lever and have the spring seat properly all at the same time. This can be tricky but is easier if you have the retaining screw half done up to help locate the end of the metering pin.
    
6. You might then need to make some adjustments to get the carburetor to the setting you need. 'Pop off' or 'blow off' pressure is adjusted by cutting or stretching the metering lever spring. A special air pressure gauge and hand pump is fitted to the fuel line inlet and pumped up to see what pressure the needle breaks its seal (or 'pops off'). It is important to lubricate the needle and seat with petrol, CRC or something similar. If you don't lubricate the seat in any way you'll get a real inconsistent release of the needle from the seat. It's the same as you can't check the pop-off pressure with the top on the carburetor because the diaphragm holds and distributes the needle. You can't get a consistent pop-off. It's very important, especially on a J where you're running low pop-off, that once it's popped off, the pressure actually holds. The pressure it hods at is probably not that critical, providing the thing does hold and doesn't actually leak. If it's not holding properly, when the kart's sitting on the stand, fuel will still be dripping through to the engine, so when you go to start it its got a gut full of fuel and it just becomes more complicated and becomes harder to tune. A Rookie carby is normally run at around 6 to 8 pound pop-off. 'J' are all pretty much set up the same. Cut the spring to reduce the pop-off pressure, they are normally 12 or 14 or even up to 14 pound when they're new.
    
7. Set the hight of the metering lever arm which bends very easily using a small screw driver or even your finger nail. Measure the distance between the top of the needle and a straight edge running across the top of the carb body. "For the J, set it to about 1mm below the top of the carburetor body (no gasket). It's a setting that runs well and makes it easy to start. If you go much higher in the lever it will give it a lot more fuel and it can become a little hard to start. With the weight of the diaphragm on the arm, it can actually hold the needle open, so the thing becomes very hard to start, especially on a Clubman more than a 'J' because on the 'J' the spark is a bit stronger. "Ready made gauges are commercially available to make the lever height much easier. The Kartech gauge is a good gauge with the different increments on it. The majority of guys who are doing engines have got them.
    
8. Fit a new gasket and the new metering diaphragm. When assembling make sure the lever clips into the diaphragm -this is a common mistake when first assembling a carby and it means the engine just won't go. Re-fit the diaphragm cover and tighten.
    
9. If there is evidence of play in the throttle shaft, replace it. Any leakage here will make the engine more difficult to tune as air is not flowing through the carburetor as it was originally designed.
    
10. Re-fit the tuning needles, firstly the high speed needle making sure to re-fit the rubber 'o'-ring and brass washer before the needle and tensioning spring. Set this needle to around a half turn (for a Clubman), then screw in the low speed needle to one and one quarter. These are good settings to start your on-track tuning with. It is also best to replace the original low speed tuning needle with an adjustable 'T' needle so it can be tuned while on the track.
     
11. Fit the carburetor to the engine making sure the pulse and fuel line is in good condition and that the fuel line is safely wired or clipped. Fit the throttle cable, ensure the return springs are working correctly and the throttle linkage is adjusted properly so that full throttle is achieved without stretching the cable.

Jets & Needles

To have an understanding of tuning the high and low speed needle it is important to understand what is actually happening inside the carburetor. The carburetor consists of tree jets, low speed, high speed and a transition jet. At low speeds the engine is governed by the amount of air coming through the air bleed hole which in turn blows air through the low jet and you can control the flow of fuel here with the low speed needle. With the main high speed jet circuit, fuel just goes straight into the carburetor venturi throat via the high speed jet and again the amount of fuel is governed by a tuning needle, in this case the high speed needle. The transition jet is the most critical jet - air flows past it in the throat of the carburetor and actually pulls the fuel out through the transition jet. Therefore, up to a certain speed, the low jet is quite tunable but after a certain air speed the high jet is the more critical one. That's why, mainly on Rookies and Juniors, the low jet is the jet that's mainly used to tune the engine and not the high jet. As the air is forced in at low velocities and the engine is not revving at high speed, you're not getting the air speed through the venturi to pull raw fuel from both the high jet and the main jet circuit, whereas on a Clubman the more important jet is the high jet because that's being controlled by the amount of air velocity, going through the venturi (because of the higher revs), so it's a combination of low and high.

How to tune

On the race track there are various theories and procedures people use to tune the high and low speed needles to control the jetting and mixtures of the carburetor. The right way to tune the low jet, on a Clubman for instance, is you have to get to a point at around 11,000rpm. This is the point where the transition jet takes over - it's around 10-12,000 rev mark. A Yamaha J very rarely ever reaches that mark because it just can't rev that hard, so therefore that's why most of the Juniors and Rookies and Midgets all use the low jet to tune, Clubmans will use the high jet.

To actually set the low jet, you need to get to the 11,000 rev mark and, at consistent revs and on and off the throttle, keep leaning the low jet back until you get a flat spot, then richen it up little bit by little bit as you keep hitting the throttle until it comes off clean off the low jet. Then when you get that nice and clean (for coming off the turns) then you can forget the low jet and just tune the high jet, because the high jet, at revs, controls the fuel a lot quicker than what the low jet does. If you're going down a long straight, such as Geelong, and it's four stroking, you have to turn the low jet a hell of a lot before you can stop it, whereas with the high jet you just nick it and it'll stop the four-stroking. The high air velocity going through the carburetor is making the high speed jet more significant and therefore it has more control of the tuning of the engine. Different people have got different theories about jet combinations, but jet combinations are controlled by lever height and blow-off pressure, so that's why different engine tuners have different jet settings because they don't all run the same blow-off pressure. A lot of people don't really understand carburetors and the Walbro is a very simple carburetor, but to get it to perform properly it's just getting a balance and combination of jets and blow-off pressure.

Walbro Carby - Expanded View (40 KB)