It was run too lean and possibly on fuel with inadequate oil content. I always break my engines in on oily fuel - 22-25% oil for any airplane engine. I run all castor oil, but a castor blend works fine provided you don’t get too lean. Read your glow plug; if the element is still silver and shiny, it’s not overly lean. If the element looks frosty or hazed over and not shiny, you’re at the edge of over-lean. Breaking engines in IMVHO should always be done on the bench for at least half an hour. Others disagree, but with the right fuel, prop, and break-in rpm, you shouldn’t have any problems. Break-in should leave the head and piston crown light brown at the MOST. Dark brown or black = lean and mean.