Like Mr. Ken says, pre-soak the engine over night. I won't argue with castor oil, but will say I plunked mine in a cup of 10 wt engine oil. The coating needs to absorb the oil prior to starting. I sat and watched TV while hand-cranking mine...for about 90 minutes.Occasionally I'd give the "cranker" a break. Yeah, my hands got tired.
I discovered the engine did still "pinch" for some time after the start/run cycles. You want the engine up to operating temperature for many short runs. Don't go slobbering rich, nor do you want dead lean. The exhaust oil needs to carry the "polished off" coating out of the engine. I used a 6x3 prop during the jnital break-in, just to get the engines loosened-up, and then moved to a 7x4 for normal operation. The engine needs to come up to normal operating temps to stabilize the P&C. Lots of two/three minute runs!
I run approx 22% oil, with half that being castor. So while breaking in, I would start rich, sneak up to string lean, and go back to rich without a significant rpms drop...ISTR this about 1/4 turn to rich from a clean, lean 2-cycle.
i also kept spinning the prop off (electric finger) as the engine broke in. One scary moment was a flood-out that did not clear out even with the plug removed. An overnight sit, and flush with alcohol sorted out the engine. Then it was "resume the break-in." This was while the tanks were mounted on top of the Mosquito wing, but after quite a bit of bench time.
Now the engines both have a good pop, with smooth build-up and snappy release over TDC.
YMMV!