Most common cause of a lean running condition is a float level drop. You have a 17 year old engine. The Viton needle tip or seat will expand over time, causing the float to close off the fuel flow to the float bowl too soon. This will reduce the amount of fuel available in the float bowl, resulting in a lean running condition. When you change the choke position, you are making a richer mxture, which will smooth out the engine running condition. Many times it has nothing to do with the jets.