This is a very common problem. As a flat rate technician where money is time, first thing I do is replace the float,needle,seat and if applicable seat o ring or washer. M/C floats get heavy over time and this cannot be checked or verified in a simple manner.
Next verify proper float level and confirm all vent hoses and tank breathers are free from kinks and obstructions ( most times I have to replace all hoses due to age and wear max cost $10.00) , If there is a vacuum operated petcock older than 5 years I will rebuild or replace it as they will deteriorate after some time.
Of course if the bike is less than 1 year old there may be casting flaws in the carb body but I have only seen this one or two times in 30 years of working in the trade. In closing most problems are due to age or someone with limited experience taking floats out and re installing them bending clips and mis aligning the tabs which engage the float. Once again minor problems easily rectified.
JT
