I need help with my 17.5hp briggs it doesnt want to turn over i have checked the valves and it still does it when i get it to turn over it backfires and then starts run fine once started

You may want to check your flywheel key, it may be partially sheared and throwing your timing off a little.

Checked the keyway it looks fine it turns untill it gets comp then stops i noticed the gear on the starter has been staying engaged dont know if this could be the problem

The gear stays engaged once it cranks ? Or only when it fails to crank and you stop trying. Because the engine actually cranking is what causes the gear to disengage. I would check to make sure your battery is fully charged and in good condition, Also Make sure your starter motor isn't worn out. Alot of times if the drive is flopping in the bushings for the starter housing it will cause your starter to bind and wont allow you to get full power from the starter. To check it make sure your starter gear is disengaged. turn off the key and remove it to avoid any accidents, then try and wiggle the shaft of the starter to check for any play.

There is also the possibility that the compression release is not working.

There is also the possibility that the compression release is not working.

What is the compression release

the battery is new

What is the compression release
The compression release is a small mechanism on the camshaft that at cranking RPM's slightly raises the intake or exhaust valve on the compression stroke to reduce the effort required to start the engine. Once the engine is running, centrifugal force disengages the release so the engine can develop full compression. This is why it's so important for the valve clearances to be properly set.

Thanks for the help redone valve claerances like new again