I am assuming you have already checked the battery so I'll ask if it is getting spark and compression. If you have not already done so, chase the wires, check the spark plugs, the starter, the starter solienoid. Make sure the carbs are clean and all filters are good. My best guess is that it's a matter of compression rather than spark or fuel. If there is no problem with the fuel but there is no compression than the fuel mixture will not occur and there will be no starting it. If there is no compression, head to your local motorcycle mechanic and he, or she, will help you tp pinpoint the cause.