A question we often get asked is "I cannot adjust the idle mixture on my carburettor. The idle mixture screw has no affect and the carb is running really rich - Do I need smaller jets?" and the short answer is "No", but to understand why, we need to first understand what the carburettor is doing when it is at idle.
Weber carburettors have a number of different circuits, and the one we are interested in here, is the Idle Circuit.
This typically controls the engine from idle, up until around 2500 to 3000 rpm, at which point the main circuit pulls in and you start to run on the main jets, and the idle mixture is discharged via a small discharge port on the engine side of the throttle plate, and into which is screwed the idle mixture screw. The idle mixture is metered through the idle jet, and fine tuned by screwing the mixture screw in, which progressively closes the idle port, so leaning the mixture, or screwing the mixture screw out, which opens the idle port, and richens the idle mixture.
With the throttle plate closed, and the engine at idle, this is the only place that the engine should be getting its mixture from, so if you gently screw the idle mixture all the way in, the engine should stall. If it doesnt stall, then the carb is almost certainly flooding, fuel is dripping from the auxilliary venturi, working its way around the closed throttle plate, and that is what the engine is running on.
So what is flooding?
Flooding is a term used to describe a condition where the level of fuel in the float chamber is too high. This leads to neat fuel finding its way into the main circuit, and this leads to the fuel dripping from the auxilliary venturi.
And what causes flooding?
Flooding can be caused by a number of things, but by far the most common is excessive fuel pressure. This overpowers the needle valve, and continues to fill the float chamber with fuel. Next most likely is incorrect float height. If the float level is set too high, flooding can occur. Next to check, is the Needle Valve, as a worn needle valve will not shut off the fuel supply correctly, again, leading to flooding. And lastly, a damaged or defective float can also lead to flooding.
So how do I cure flooding?
You will need to work through the potential problem areas above. Start with the fuel pressure. Weber carburettors need around 2.5-3psi. You may read online that various experts recommend 4psi or higher, but leave that to the "Experts". Set the pressure at 2.5-3.0 psi and thats one thing you know for sure will not be the cause of flooding. Its also worth noting, that checking the fuel pressure involves getting a fuel pressure gauge, such as our WFR150, in between the fuel pump and the carburettor, and checking exactly what the pressure is. We talk to a lot of people who, when asked about pressure, simply state "Its fine" or "Its OK, its a mechanical pump". You need to check exactly what the pressure is, and adjust if required.
The next step involves removing the carburettor top cover, and removing the float. This needs to be accurately weighed on scales that measure grams, and then checked against our website. All of our floats have the correct weight stated, and if yours is heavier, then it needs to be replaced. With the float out, the needle and seat can be inspected for wear, and if a shiny mark is noted around the needle, then it should be replaced as a needle and seat assembly. As with the float, we recommend only using genuine Weber or Webcon parts. Lastly, install the float and ensure the level and drop are correct. If you are unsure what these measurements should be, please contact us on [email protected]
The most important thing, is to assume nothing, and check everything, and if you do this, then flooding should be a thing of the past for you.