20 replies to this topic

[Sisco]

That's what i've been reading, its very useful but i was wondering if the fuel level was low would the mixture screw need to be further in to compensate.

I'm going to take it off and check it over but wondered if it could cause the lean mixture unless the screw is fully in? I'm struggling to understand how the jet height controls the amount of fuel.

[KernowCooper]

When the car is at idle position 1 on the needle is the idle mixture, and if you lower the jet you expose a lower position on the needle and allow more fuel through the jet which richens the mixture

[Sisco]

Thanks kernow that really helps make sense of how it works. So the level of fuel in the chamber wonts make much difference. The mixture screw adjusts the idle position of the needle in the jet.

I guess its either the lever for the jet or a different needle that would make any difference based on that.

Thanks for your help.

[KernowCooper]

Its important to have the float level set at the correct height, and then set the mixture at idle, you'll see once you have the bottom off how the mixture screw actually works.

[Dan]

The float level makes a huge difference to the tune of the carb, the level in the float chamber is the level of fuel within the jet itself. If the fuel is lower in the jet there is a larger aperture between jet and needle than expected where the fuel is being drawn from, but also the venturi doesn't work as effectively because the fuel surface is further from the choke depression. It makes starting very poor if you have a low fuel level. All you are setting with the mixture screw is the idle mixture, from there on all enrichment is calculated by virtue of the needle. The relationship between needle position, jet height and fuel level determines the size of the aperture that fuel can be drawn through by the venturi and the physics of the thing is designed to work with the fuel level at a specific height.

[M30]

If you take the air filter off, and have a small mirror, you can see if the top of the jet moves when you adjust it without removing the carb. If you cant quite see it this way, you could remove the dash pot and see what happens then. It might just save you disturbing the manifold gasket.

Stu