Originally Posted by
DS Motorsport
The issue you're having is caused by a too lean mixture.
The 'bubbles' are caused by pre-detonation, the mixture ignites too early causing too much heat at TDC. This melts the aluminium.
Also the metallic noise you hear is caused by this.
You can sand the top of the piston and the burnroom with 400 grit sandpaper and finish with 1000 grit.
Then richen both LSN and HSN needle and see if the issue disappears.
Not only lean mixture but too tight head clearance as well.
I wouldn't be surprised if the piston side is also damaged. After trying to blend out some of the damage you need to check the head clearance is ok but you need to be prepared that the engine will never perform at 100% now - a replacement piston/sleeve/head/rod will probably restore most of the performance.
Have a go at blending out the damage, run it in again for a couple tanks and see how you go.