2003 kx 125 just had engine rebuilt but now it wont kick start or idle. It will coast start and then seems to run fine . Changed plug and air filter is clean . Any ideas ?

check and make sure choke is opening all the way,if it is only opening part way could explain why you don't need to use choke at all to start and then at higher rpm's it is not getting enough air causing it to basically choke it self out.

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..

piston kit, wrist pin bearing, top end gasket kit and a honing tool to clean your cylinder is about it I would probably buy new air filter and spark plug if I was doing it, but not essential ?

o.k......you can try checking you have the correct plug(heat range)and nice blue spark,check your air cleaner.What about your engine oil.Is it the correct viscosety(weight)?.Are you running oil injected or premix?You could be running rich.Also make sure idling isnt set to high and have you the correct gas?These suggestions are just a start to check with.

check the carb float and the float settings. also clean all the jets out. check the joke aint sticking. check the condition of the air filter for rips or holes as that would explain why the bike is bogging when reved

first drain the carby, then set your air mixture screw 2 turns out, from the fully turned in position. The starting proceedure is as follows, when the engine is COLD, turn on fuel, pull on the choke fully, and kick the engine over. DO NOT use any throttle, when the engine is cold, and the choke is on. Remember Choke on, NO throttle.
When warm, no choke, and a little throttle when kicking.

The idle circuit is still plugged up or it would not be giving you this problem. Remove the carb and dis-assemble it then soak the parts in denatured alcohol for 30 minutes. Then use a spray carb cleaner to finish the job. Spray into all the little holes in the carb body. Especially spray into the air screw and throttle screw holes. Reassemble the carb and LIGHTLY seat the air and throttle screws then turn each of them one and one half turns outward. This is the factory setting. Before putting the slide back into the top of the carb, move the clip on the jet needle one notch lower on the needle. This will richen the low end fuel mix. Install a new spark plug and clean the air filter. Please rate my answer. Thanks.

jetting in carb

Remove the slide from the throat of the carb, move the clip on the jet needle one notch lower. This will richen the low end fuel/air mix. - - - Simple, quick, and free.

Do it, then rate this answer as "Very Helpful". Thanks.

sounds like your idle jet is clogged, remove your carb and clean idle jet cause it won't idle or kick start without it...good luck to you friend...Tim