Jetting KX125

[slowdirt]

Hi

We are building a low budget KX125 (1998) based salt racer to go for the Australian 125 speed record - we will run the OEM air box - with no filter and have no need for a muffler so will add a 168mm stinger to the pipe - how much bigger should we go on the main jet to start with?

[sandblaster]

That all depends on what has been to the engine and it's condition, type of expansion chamber, fuel type, carb size, compression ratio, elevation, humidity, temperature, and a host of other things.
Have you ran the current set up and did throttle chops to read the plug color?
And what is the current speed record?

[slowdirt]

Thanks - the engine is currently being rebuilt to standard, albeit polished up a bit internally - we have two cylinders ready to go-  one with good nikasil and one lined with a steel bore and about to be bored and honed. - We will probably used the niaksil bore and keep the other as a spare - chamber is a pro circuit platinum - although we have straightened it out a bit .. Carb is OEM - fuel will be 100 octane (the highest that is still called petrol here) - elevation is 0 (sea level) - it will be hot and very dry - up to 110 F in March.   What I am most concerned by is running the standard jetting to test it and burning something up - I know it does not take much time to do!

The current 125 record (unstreamlined) is 79mph - so we are hoping we can do a bit better than that.