Sometimes it will crank but no turn over

Could be lots of things:

• Replace your spark plugs - be sure to get the exact ones specified in your owner's manual, and gap them correctly.
• Replace your air filter and your fuel filter.
• Make sure your timing is set correctly.
• Make sure all of your spark plug wires are securely plugged in on both ends.
• Put some gas treatment in your gas tank next time you fill the tank. A friend of mine who is a heavy equipment mechanic recommends Seafoam:

https://seafoamsales.com/auto/

Seafoam got my lawn mower running like new.

One or two , thing I have ran into my self , Fuel injectors & vacuuming water out of the thermostat