Yamaha xt125x starting problems

SOURCE: 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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SOURCE: cold starting problems 1981 yamaha maxim 400

I had the same problem. I found that the ground on the wire harness under the seat was not grounded! It is worth a check.

SOURCE: My Yamaha yz250F 07 starts first kick but back

I see: you changed the plug because it was backfiring.

Does it back fire when you let OFF the throttle after revving it, or does it back fire while being revved?

If it does it when you let off then Ollie's suggeston might do it. If that doesn't do it then check for dirt in your pilot jet.

If it back fires when revved up high could be dirt in your main jet.

SOURCE: ive got 2006 yamaha xt125x my display screen is

first of all you have to see the low tension on the coil,and before that see what type of code is , because that maybe the key start

SOURCE: 2007 yamaha tw 200 with 10k would not run at all.

My bike had very similar problem. It was caused by not-correctly installed hoses- check thread:
http://www.fixya.com/ThreadAsker.aspx?thid=2298656- check solution 1.from gethro
Cheers, Stefan