If it's fuel injected and been running in the last couple of hours, just turning it over normally works for me.
If it's a first start of the day, 4 secs on the fuel pump or until some pressure shows, then shout clear prop and crank it. Works for the injected travel air I fly.
For carburetted 152 and 172s on a first start of the day, I pull the primer out really slowly and give 2 squirts. Note that you want to feel that some actual fuel is being delivered by the primer. This is not always the case.
If a 152 or 172 has been used that day, then 3 pumps on the throttle (accelerator pump) is invariable enough to get it going on the first crank.
Fuel injected 172s have been my worst experience so far
