CHOOSING A DIFFERENT PROFILE

This information is meant as a guide only to choosing an alternative needle. It is not a guide to making an original choice of needle.

The metering needle fitted as standard equipment to any vehicle can be relied upon as being the most satifactory for all normal running conditions. However, where it is desired to make a change from standard to take care of some special requirement, a suitable replacement can usually be determined by using the needle profile charts and cross-referring to the appropriate needle graph (page 6).

The carburetter specification gives the correct metering needle, as determined by the car manufacturer, for your car. For a standard engine with a standard carburetter the starting point for considerations of needle change should be the standard needle.

In the profile charts, the list of diameters for a particular needle starts with the diameter immediately under the shank or head of the needle and works down the length of the needle in increments of one eighth of an inch. The smaller the diameter at any particular point the richer the needle will be at that point.

To illustrate what the needle dimensions in the booklet actually represent we will take as an example the CE (page 9), list the dimensions and discuss which dimensions affect which driving condition.

Refering to the profile chart, the first two dimensions are given as:  (1) = 0.089"  (2 ) = 0.085"

These are the idling or datum positions and it is advisable when changing from one profile to another to choose one with the same idling dimensions.

If this is not possible then bear in mind that if the new needle has weaker or larger idling dimensions (say 0.089" to 0.0855") then the rest of the needle will give slightly richer results than suggested by the dimensions. If the idling dimensions of the new needle are richer or smaller (say 0.089" to 0.0845") the needle will give slightly weaker results than one might expect. (These results are due to the alteration in the jet position required by the new idling dimensions.)

The next four dimensions are: (3) = 0.082" (4) = 0.0795" (5) = 0.0777" (6) = 0.076"

These govern the pickup in top gear from about 20 mph to 50 mph and are also the part of the needle which meters fuel for the part throttle or cruising conditions. A cruising speed of 30 mph will lie approximately (but depending upon the size of the carburetter) between the second and third dimensions and a steady speed of 50 mph occurs around the fifth dimension.

The dimensions from the seventh (0.0745") to the ninth (0.071 ") affect the top and full throttle conditions. The last three dimensions (with a 1.25" carburettor) do not actually take part in the metering.




When going from weak to rich it is usual to try a dimension about 0.002" smaller at a point but when going from rich to weak it is advisable to go no more than 0.001 larger on the dimension unless there are indications of excessive richness. Refer to the graphs on the following page and note the effect of changes at different diameters on the needle.

Using The Graphs

The graphs, one for each jet size, show the percentage change injetareawhen addingto the diameter of a needle. The horizontal axis gives a series of needle diameters and the curves rise in steps of 0.001

The line of the lowest curve, for example, gives the percentage change in jet area at points along the length of any needle when adding 0.001 " at that point - ie adding 0.001 " to a needle diameterof 0.085" (in a 0.1 00"jet) brings about a change of jet area of 6.5% at that point.

The graphs plot the effect of adding to a needle diameter (ie going weaker) but they can also be used, with reasonable accuracy, when subtracting from a needle diameter (ie going richer). The desired change in mixture strength is, of course, related to the change in operating jet area.