When u turn off the choke, it opens the butterfly completely, causing more air to flow, if the gas flow dose not, or can not increase enough to meet- the increased air flow when the choke is off it can not run. Fix, gas flow must be broth back to requirements.
A2
If an engine will not run unless the choke is on, it indicates that the carburettor is not mixing the fuel/air correctly, not enough fuel, or too much air. This is most likely caused by a blocked jet. it could also be caused by an air leak between the carb and cylinder.
It could be the slow running jet, or the emulsion tube.
The best remedial action is to strip the carb and clean it.
I'm not familiar with the Yamaha carb, but you may get away with removing the jets externally and blowing them through.
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