Full throttle hesitation is a common problem with the Piaggio Zip 50cc 2 stroke scooter. Low speed response tends to be ok but when the throttle is opened beyond half way the engine dies and returns to idle. Sometimes stalling but usually resting back at idle as the throttle is released. This occurs when the engine is warm.
There are a number of potential causes to this problem. The nature of the symptoms suggest fuel starvation so this is a good place to start.
The fuel taps on these scooters have two connections. One is the outgoing fuel and one is the incoming vacuum line. This is connected to the inlet manifold. When the engine is started a vacuum is created that releases the diaphragm in the fuel tap allowing the fuel to flow.
1) Check that the fuel tap is working correctly.
2) Clean, check and thoroughly inspect your carburetor. The needle and main jet are a common cause of these problems.
3) Clean and check your air filter.
4) Check the correct operation of the choke. When started from cold after a few minutes you should notice that the revs drop as the automatic choke releases due to the engine reaching operating temperature. Poor operation of the choke can cause a number of different problems.
5) If after checking all these components check your spark plug and all components of the ignition system.
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GOOD LUCK!!