Hi,
Recently rebuilt my old TM K9. Took it to the track yesterday for the first time. Delighted with how it ran.
However, I am unsure about the piston crown color I am now seeing. I am wondering if I should be concerned?
All opinions appreciated.
Hey Eoin, are you from Ireland by any chance? I remember a kart racer from there by the same name.
That piston is hard to read for me, normally you’d see a wash pattern along with a defined radius that you can use to gauge how rich it was running. What oil you’re using.
Hi James,
Great memory, that would be me. Still in Ireland but not racing unfortunately.
I am using Shell Advance Racing M, ratio 20:1. Not seeing a pattern on the crown. The surface colouring is consistent across all the piston, a glossy purply/brown colour. The petrol used was fresh. Possibly I am rich, the main jet used is what I have used in winter months. However, the plug was not oily, looked a lot closer to lean than it did to rich.
Could not running a cold enough plug be the reason?
Just throwing an idea out there, does the piston clean up? It looks a little like heat discoloration, but if it cleans up it is obviously not.
What plug do you have in it? NGK should be a 10 most days, maybe 11 if it’s hot for the one day over there 
If you’ve got the jet in from winter it’s probably rich which is my guess looking at the piston, it’s just unusual to see a consistent “tint” to the crown like that. Normally you have a carbon area, something like the shape of a heart then an area of the crown exposed around it. The less crown exposed, the leaner the engine is running.
That looks nice and rich. I think the spark plug will confirm it.
Aluminum doesn’t discolor when oxidized at high temperature, it just goes white and ashy.
