Hi Roger - I have a 2003 912 ULS without a slipper clutch with 530 hours since new. I've replaced the battery, it has a gold starter, new plugs with gaps checked, and carbs properly balanced. Since I've owned it (about 20 hours ago) it always has a "rough" start for a few seconds and then clears to run smoothly all of a sudden exactly as you described earlier in this chain of messages.
What do you think - could it be that the electronic ignition control module(s) - and if so can you talk me through what you recommend to resolve them?
Thanks! Kevin