SOURCE: i have a yamaha pw50, it runs slow when cold &
Most likely a combination of sparkplug heat range (type of plug) and jetting, needle or air mixture settings in the carburetor. Assuming good condition of air filter and operation of the choke plunger. Are you using the correct octane fuel and fuel/oil ratio? Give the plug a clean and emery the spark contacts, set gap at 0.7mm., ideally take the plug to a Yammy dealer and check for correct range for your bike in the area where you ride.
Best to have a range of spare clean plugs handy for wet or hot days etc.
The jetting maybe dirty or wrong size too. In the meantime move the throttle needle clip down one notch. Refer to the site below for a diagram of the carburetor parts. Cheers.
http://www.cmsnl.com/yamaha-pw50r-2003_model11163/partslist/A-09.html
SOURCE: The check engine light on
Error code 24 is a faulty oxygen sensor -- a relatively cheap fix. Incidentally, a faulty O2 sensor is the most common cause for a steady "Check Engine" light on any vehicle. Your V-Star is just courteous enough to tell you exactly what is wrong.
SOURCE: 1998 yamaha royal star
Backfiring is a sign of an air leak. Somehow air is being allowed into the exhaust system. If you or a previous owner has drilled out the baffles to increase sound, this could be the cause. A burned valve could contribute but is not likely unless the engine has been severely overheated. A burned valve will also cause poor performance and it can pop upon acceleration which was not mentioned in your original question so I doubt it is a valve causing the problem. Plus - these engines are pretty reliable and robust. I bet on the air leak.
Scott
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