Mine usually starts on the first pull as well, and was a dream to run and to start...until I had quite a bit of fuel through it.

I had the air cleaner fall off in very dusty conditions, and the card seized from the grit. I killed it immediately. I took the carb off and flushed it thoroughly in turps and blew it clean with compressed air. I also thoroughly flushed the engine with turps , runniing it completely though the engine many times to remove as much grit as possible, the soaked the innards with after run oil. I am telling you this so you know that the pullstart rope was not only soaked with burnt fuel (from a leaky exhaust gasket) but also was soaked in turps once as well. This may have prematurely weakened the cord, although I did dry it with a rag and compressed air over the entire length of the exposed cord.

it seemed that once the cord broke the first time ( many, many starts after the soaking) it was rooted and when I retied knots in it, it just kept breaking. I would LOVE to find a place to buy new cord from... I have tried most options I can think of...