It looks like a fuel mixture/load (like detonation), high constant load and heat related failure, lubrication issue also looks likely. I am not closely familiar with the 503. Twin or multi cylinder air cooled 2 strokes often require different air/fuel mixtures for each cylinder to keep the plugs the right chocolate colour, from looking at the piston pic, I would think it's plug would show a lean condition.

Time at a different throttle setting before pulling the plug can mislead the colour analysis though. It has to be immediate after a short spell at a fixed throttle position to check colour for that throttle position mixture.

Twin carbs will allow the different settings to get each pots mixture precise for that pot, differences in carb settings can be quite extraordinary for a novice. Look at a Throttle setting Jetting chart for whichever type of carb being used to see which jets are working or combining at which throttle settings so you can select or adjust to colour the plugs correctly at a few different throttle settings through the range. This will help the output through the range as well.

A synthetic or part synthetic lube oil may also be a benefit, JASO FC or API TC or better.

It is not necessarily space around the engine that counts, rather flow INTO and through and across the fins and a clear exit path. If the cylinders are running at widely different temps after getting the mixtures right, there is a case for looking at ways to even up the cooling by tweaking airflow through the cowl. It could as simple as a small restriction across the cooler pots exit to make the hotter pot less relative air resistance and so get a higher share of the air. Extending the inlet throat of the cooling fan in a conical Air Ram fashion (or Kort's Nozzle style) may increase the air flow.

This is an afterthought.... the main fan may be depriving the cooling fan of available ambient air flow, this could be exacerbated the higher the rpm goes.

yamah

Quote Originally Posted by Al View Post
I am also looking this time to fit twin carbs. Has anyone got a good set of Bings & inlet manifold for sale at reasonable money?

Pictures below of my piston

Any ideas on how to stop this happening again, as I'm not 100% sure at the moment what caused it.

PTO side piston was damaged, mag side piston perfect.
Plenty of space around the engine for air circulation
heads & barrels the correct way round

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