I have a 3.5hp briggs on an old victa outside on the bench at the moment which made me think of this thread, problem is:
Will not start unless a bit of fuel is dropped down the carb it would run for a few seconds before quitting.
Carby is fine and ignition is good the actual problem is there was no exhaust valve clearance at all, I opened that gap up a little to around about 10thou or so and now its perfect, I may post a video later on it, depends how many upload mbs' I have left on my internet plan.