Sorry, missed that it's an OEM gurtner. Try cleaning the original carbs jet and putting that in. At least then you'll know it's not gonna seize when it does run, as it's the same carb.
The gurtners can be very finicky. If you hadn't just bought an OEM one is advise you to get a dellorto...but I digress, at least get an in line fuel filter from an auto parts store. They hate any particulates.
You could have fouled the plug too, when it ran. Was it idling? Or with throttle?