jet sizes

[markwr35]

Hi All
I have a Tec gy125 and Ive just put a 150cc big bore kit on. Now am after re jetting it as ave put twin exhausts on it. The problem Ive got now is the main jet that's in it now as no number on it or on the idle jet so i don't no which size it is.which size main jet would be the best to use

many thanks mark

[forchetto]

Strange it has no markings on the jets, they're measured in mm, as in a number 100 jet is 1.00 mm hole diameter, 135 is 1.35 mm and so on.

If you have a set of small drills you can use the plain part to get a reasonable idea of the size, or, better still use a jet gauge of the type used by plumbers and gas boiler fitters to determine the size:



Your capacity boost from 125 to 150 cc amounts to a 17% increase. I would at least increase the main jet size by that amount.

The size increase has to be calculated by area not diameter, this is worked out later. For example a 100 main jet has an area of 0.785 mm2. Add the 17% desired increase and this gives us a desired jet area of 0.945 mm2. (Area of a circle =Pixr2).

Now calculate what the diameter of the new jet should be:

Square root of area x Pi and multiply this by 2 = Diameter

In the example above this would be: SqrRt (0.945x3.14) x 2= 1.1 approx

You would then fit a 110 main jet. Note that if you just increased the diameter rather than the area by 17% it would give you a 120 jet, (100+17%=120).

You can add another 5% if you've fitted a freer flowing exhaust and 10% if you've discarded the air filter for a pod-type filter.