Starting new C172s

I have flown it 6~7 times now, and still find starting the engine of
new Cessna 172S quite tricky, and often I have to try two or three
times to start it. I never had problem in older C172s.

So what's the trick, guys? How much throttle (1/8, 1/4 or 1/2 inch) do
you open? How fast do you advance the mixture when the engine starts to
fire? Anthing else?

Thanks