03 ltz 400 cranks and backfires won’t run

I have a 03 Suzuki ltz 400 that ran good till the radiator let go. It backfired once and didn’t get any more hot then normally would. I changed the fuel petcock and head gasket and cleaned the carb. Put new gas in and it still won’t start. Only has a back fire but won’t pop off and run. Checked timing and valves and everything is good there. Also checked compression and it’s at 150-155. Any idea what this could be and how to get this atv running again ?