Starting Problems Yamaha TTR 125

Remove the water trap bowl at the bottom of the petcock, (gas valve). Any water or trash in the bowl? Drain a cup of gas from the tank. Any water or trash in the cup? Dump it, clean it and re-mount it, ( not all bikes have a water trap bowl ).

Drain the carburetor. There should be a screw on the lower side of the carb float bowl. Remove the screw then replace it after the fuel drains. Turn the gas back on and wait a minute for the carb to fill with gas. Install a new stock NGK spark plug and try to start the engine. If the bike doesn't start and run properly then shut off the gas and remove the carburetor from the engine.

Remove the float bowl and clean the entire carb with a spray carb cleaner from the auto parts store. Wear protective goggles to avoid getting spray in your eyes. Spray into all the little airways and fittings in the carb. Remove the two screws on the outside throat of the carb and spray into the screw holes as well.
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Be sure to put these two screws back in the same hole they came out of. IMPORTANT > do not tighten these two screws down. Only screw these in until they LIGHTLY seat. Now turn each screw one and one half turns outward. Put the rest of the carb back together, clean the air filter and install the carb. Be sure the bolts are tight. Let the float bowl fill then start the engine. Set the idle speed with the idle screw. This process should get you back on the road.

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I've had the same problem with mine. Found the "choke" o-ring had either shrunk, or was undersized, and wasn't providing a good seal. I replaced the small o-ring on the plunger, and it seemed to fix it. If I took off the air intake hose, and covered the carb intake with my hand, it would start. That indicated to me that it wasn't drawing fuel. After babying it like this for several minutes, it would be warmed up and run OK. Good luck!