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