I have a yamaha pw50, it runs slow when cold & takes about half an hour to warm up & get up to speed. does anyone know why it would run slow when cold? thanks

Sally, check the link to see the difference of a PW50/80.
yamaha pw50 and yamaha pw80 Google Search

As long as you know where the air bleed screw is, then do the following; Screw the screw in until it goes no further, (dont force it). Wind the screw out for about 1 and a half turns. Start the motor, you may have to adjust the throttle stop screw t keep the motor running. When the motor is running (at a fairly slow tick over) adjust the air bleed screw outwards a turn or so, if the motor goes faster then your turning the right way, if not, then screw in a bit. What you aim to do is to get the motor running as fast a tickover as possible. When the moto speeds up adjust the throttle stop screw to keep the tickover at a slow speed, It's as easy as that, this setting should be finally made again when the motor has fully warmed up (as the cold start may be in action if the motor is cold).Good luck

Sounds like air in the cooling system. Try taking radiator cap off and letting the vehicle run with heater on high. As bubbles come out add coolant. This may take 20 minutes to a half hour.

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

They probably aren't running the hot water at the sink before starting the dishwasher. You know how it takes time to get hot water at the tap when you turn it on? well the new dishwashers use such little water, that they will fill with cold/warm water, and try to heat it up to 170+ degrees before continuing the cycle