Port Timing

This shows the conversion between standard KDX 200 port timing and the Vespa KDX port timing.

The Variables

1997 KDX 200 Specs (Note: This was information given to me and I am not sure how accurate it is)

Exhaust:

Transfer:

ATDC (After Top Dead Centre)

Here are the differences:

KDX 200 Stroke = 58mm

KDX 200 Conrod Length = 109mm

To calculate the port timing, we can treat the problem as a triangle and use a calculator such as Irregular Triangle Calculator

Here is what the diagram below represents:

a = conrod length

To do the calculations we first need to work out the unknown for the KDX 200, which is "c" the distance from the port to the crank centre.

Exhaust Open ATDC: 86 deg

A = 86 deg

Transfer Open ATDC: 117 deg

Transfer Close ABDC: 62 deg

A = 118 deg (i.e. 180 deg - 62 deg)

Then for the Vespa KDX we can adjust the parameters and calculate stuff...

Open ATDC

a = 110mm (Vespa conrod length)

Close ABDC

The exhaust port is symetrical, so the close will be a mirror image of the open and (in this case) 4 degrees above the horizontal.

A = 94 deg

Open ATDC

a = 110mm (Vespa conrod length)

Closing ABDC

a = 110mm (Vespa conrod length)

Exhaust:

Open ATDC: 86 deg

So for the exhaust port timing the Vespa KDX is the same as standard KDX 200.

Transfer:

Open ATDC: 115 deg

So for the transfer port timing the Vespa KDX is 4 degrees more