RB engines are definitely going to give you a massive headache in the beginning - I had the same problem with you.
What you have to do is get it hot like you have been doing, then once it has enough heat inside the engine, squirt a little bit of fuel down the throat of the carb, keep the glow plug lighter on for a few seconds before cranking it over and as soon as you hear it starting, just blip the throttle so that it revs a little (not full throttle blips) and try and set your idle screw to the point where the engine won't lock-up on you. If you can hear it start to pinch up - trust me, with an RB, even a little .12, you can hear it when it starts to pinch up - just give it a little rev to pick up the RPMs.
I would use Josh Cyrul's WOT method of breaking in this engine, so that you don't really have to worry about it idling and looking/listening if the engine is about to lock up again. Just a thought.