At a Robinson safety course a couple of years ago at Torrance Pat Cox was most emphatic that to prime the engine before start, when the primer is not fitted, you should raise the collective, then twist the throttle.
Most people that I have watched twist the throttle when the collective is down. I am not sure why this is ineffective but I believe it must be to do with the way the vapour distributes into the manifold when the raised collective allows the vapour to go past the butterfly.
I have also seen people on cold mornings give five quick twists of the throttle then hit the start key. No start. What you should be doing is to wait for the squirt of fuel to vapourise into the manifold, which may take ten to fifteen seconds on a cold (i.e. ISA -15) day.
One day all aircraft will have fuel injection. Aye and maybe one day man will fly to the moon...
(Irony alert
)