Why does my ttr 125 not start then backfires?

The motor might have jumped timing or bad spark plug or both mabe even the float is jammed

KICK HARDER TRY PULL STARTING IT..IN SECOND GEAR..DOES IT HAVE ENOUGH COMPRESSION TO START?IT SHOULD START PULLING IT...START OFF WITH NO THROTTLE THAN IF NOT STARTING CRACK THE THROTTLE SO LIGHTLY AND SHE SHOULD FIRE OFF WHILE IN GEAR BEING PULLED OF COURSE BE CARE FUL NOT TO RUN INTO. WHO EVERS PULLING YOU..USE A LONG TOW ROPE..IF YOUR NOT A SEASONDED RIDER AND KNOW YOUR BIKE VERY WELL I WOULD NOT ATTEMPT THE PULL START...IF NOT CHECK YOUR COMPRESSION AND TIMING..MAJOR VACUUM LEAK MAYBE AT BASE GASKET..IS ENGINE SUCKING TO MUCH AIR? GOOD LUCK..

intake valves are leaking

The TTR-125 Carburetor's Run Lean by design. Mine had the same issues till I found a post on Thumpertalk.com on Carb and air box Mod. This is a good starting point on gets; 110 and 112.5 main and 17.5 pilot. But you must check spark plug coloring to make sure the jetting is correct. Check spark plug after the bike has been ridden for a wile. I also took out the stock baffle and replaced it with exhaust tip and now it fires up after a few kicks and worms up in a few minutes.

If the plug is firing OK then the Kill switch has no connection with the problem.
The problem seems to be the gas supply. There are three things required to start a gasoline engine. The gas, the spark and the minimum RPM. Two of these are available and the third gas is seemingly missing. So, please check if it is getting the required gas. Since gas is gravity fed in bikes, make sure it is reaching the carb in sufficient quantity.

take the plug out and see if theres a spark buddy get back to me

Ignition Coil Problem or the CDI. Check for both. Replace the spark plug with a new one for diagnostic purposes.. check also for the wire. I recommend using split fire wires as they're heavy duty. Check that the wires are making contact with the spark plug up to the starter relay to the battery.

2 stroke bike are very dependent on compression. and being a bike that is 12 years old my best educated guess is that the top end needs to be rebuilt. i usually have to have the top end done on my honda about every three years.