What is a tappet on my Apollo DB 125cc pit bike

TRY INCREASING THE IDLING FROM 1.5 TO 2.5 RPM DURING THE START . CHECK YOUR SPARK PLUG AS IT MIGHT CONTAIN CARBON RESIDUES .

GEARBOX SELECTOR IS WORN/DAMAGED

remove the cover that houses the kick starter and look for a failed return spring or some other reason that keeps the kick start dog engaged.

dirt in carb

clean the carb !

ignition or valve?

Try unpluging the cutouts.they are usally on 2-3 places on these bikes ,brakes,also check the fuse that is on the red wire comming off the battery it will be blown or end of fuse holder will be pushed in and not touching the fuse.

adjust ur timing chain r reduce the teeth in gear box.....