Carburretor float level

[Keith.owen]

Hi,
I know this is an old chestnut, but, I have recently purchased a concentric card for my NH project.

When I mentioned that I have had problems with an overly rich mixture with my VH using the Amal factory default values...........no 3 cutaway, needle in middle position, 106 jet, he suggested that the float position can affect the mixture. This is something I don't really understand for the following reason. The fuel system, with the exception of the idle mixture is fed via the main jet which is submerged well into the float bowl hence the main jet is always immersed in fuel. The idle jet is fed via a duct in the main body but again I guess this has always to be immersed else air would be sucked in via the tickler. I imagine that as long as I can adjust the idle speed using the needle and stop then at least it is not flooding. Hence, I don't see how the float level would make the mixture leaner, only richer if it was flooding. If it was flooding the there'd be fuel dribbling into the manifold and any number of other problems.

Running a 106 needle jet, 3 1/2 cutaway and putting the needle on the top notch seems to be going someway towards solving my mixture problems; There is some soot which rubs off to leave a lighter colour - perhaps just my idle mixture is rich?

I really am intrigued to hear what thoughts some of you out there might have on the subject.


BR,

Keith

[iansoady]

I believe that as the petrol level is below the lower level of the choke, it's the venturi effect of air passing through the choke that actually pulls the petrol up into the carb bore when the engine is running. Therefore, if the level is lower than it should be, the petrol has to be lifted more giving a weaker mixture. Vice versa for level too high.

[david.anderson]

Keith
The main jet sits near the bottom of the float bowl and above it is the needle jet and needle. The fuel rises up these jets to the level of the fuel around it in the body of the float bowl. The fuel sits just under the spray tube ( the tube projecting up into the main choke of the carb) ready to be sucked into the engine. The float height/fuel level is very important relative to the top of the spray tube. Increasing the float height will richen the mix, as will reducing the height of the spray tube. And the spray tubes in amal carbs can be different heights as supplied from the factory, depending on the machining in the body for the spray tube.
As air passes through the main choke it speeds up and as it passes over the spray tube it creates a vacuum that lifts the fuel into the main choke where it mixes with the air (to hopefully the right mix to be fed into the engine). If the fuel level is close to the top of the spray tube then it takes less vacuum to pick up the fuel. When the float level is lowered the vacuum created by the passage of air down the main choke will pick up less fuel because it has to lift the fuel further, which will lean the mix for the same vacuum.
Set the float height as per the bushmans site
http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html
then set about tuning from there. If the float height is correct then the idle screw will be about 1.5 turns out.
But back to jetting I have found that with Australian fuels (ethanol free) a 105 needle jet is necessary in my VH (needle lifted all the way). The exhaust system will also have an effect on mixture.
And you should expect to see a light soot ring around the base of the plug but not on the insulator or electrode.
David

[Keith.owen]

David,
Actually I had never thought it through to the point of considering the level of the fuel in the actual spray tube.

Question. There is an element of down draught on the hunter. Does this need to be taken into any consideration, in your opinion?

From all of this the mixture perhaps changes a bit if you go up and down sharp gradients?!

Keith

[roger.attwood]

Have a look at this web page http://amalcarb.co.uk/rebuilding-mark-1 ... arburetter. I always thought that raising the float level would richen the mixture - after all, when tickling the carb you allow more fuel into the chamber, raising the level and allowing a richer mixture for starting. Also, I read somewhere that the fuel in the chamber should be level with the small bump on the outside face of the chamber, at the bottom of, and between the M and A of AMAL - measure using a clear tube as described on the webpage.

[david.anderson]

Keith
One of the advantages of the concentric over the monoblock is the angle at which it can operate is much greater.
The angle of the carb for practical purposes does not affect the fuel level in the float bowl. As you tilt the carb the fuel level in the centre of the float bowl remains the same, but the fuel level at the edges varies. The jets and spray tube are located in the centre of the float bowl so the fuel level at that point remains constant. In reality the grades that we go up and down are less than the cornering angles that we can achieve.
The downdraft on the Ariel singles head is exaggerated because the carb face is not at right angles to the port. Why Ariel chose to tilt the carb more than necessary is a mystery to me. Under normal circumstances I would expect the carb face to be at 90 degrees to the port to give a straight flow from carb to port. Gas flow always likes to go straight.
David

[Keith.owen]

Hi David,
I have found a technical tip on the Burlen website that suggests the float should be set so that fuel is at a level of between 4.33 to 6.35mm from the lip of the bowl (i.e 5.34 mm if ever one could adjust to such a tight margin). So, I shall make a jig that I can fix the bowl to and clamp in a vice so that it is level. Should be a fairly easy matter to check and adjust a stay-up float accordingly.

If I can eliminate this, once and for all then I'll be moving in the right direction.

Keith