With a flat tappet cam, the first few seconds are crucial. I like to set the idle speed up a few turns before initial fire up, and run it at 2000 rpm for a few minutes. Be sure to have the engine all ready for running, topped up with coolant and oil, so it starts right up and runs good. I realise this doesn't always go as planned, but any time I've screwed up and had to monkey with one to get it to keep running, the new cam hasn't lived long.