Small Engines (Lawn Mowers, etc.): Poulan Chainsaw, fulcrum lever, walbro carb
QuestionPoulan 2000, TypeII, SN 1H037B601, Walbro carb. My saw will not idle. When started it runs at very high speed with no throttle response. When I attempt to adjust the low speed needle the engine will die. This occurred before and after I cleaned and installed a repair kit. Can you help?
thanks, Dale
AnswerDale
Is the carburetor now primed and ready to provide fuel to the engine? Note that both the low-speed adjust circuit and the high-speed adjust circuit draws fuel from the lower chamber. When the engine is warmed up and idling, then each engine intake cycle would create a pulse of vacuum at the engine side of the carburetor. Since the throttle butterfly is nearly closed, the L1 idle fuel port would see that vacuum pulse and draw fuel from the lower fuel chamber and the warm engine receives enough fuel to continue idling. As each intake cycle draws a short slug of fuel from the lower fuel chamber, this creates a slight vacuum in the lower fuel chamber. The lower chamber diaphragm senses this slight drop in pressure of the lower fuel chamber with respect to the outside atmospheric pressure on the other side of the diaphragm. This pressure offset causes the diaphragm to move towards the center of the carburetor, and the diaphragm pin presses on the flow control fulcrum lever, which momentarily overrides the spring to open the fuel flow seat.
If you use the rigid gasket, make sure that the diaphragm is next to the carburetor body and the rigid gasket is between the diaphragm and the chamber cover. When these two are reversed, the diaphragm pin is to short to push on the lever that opens the fuel flow seat. Figure Thirteen shows a diaphragm that was installed wrong. Notice how the thick gasket is on top of the black diaphragm. The thick gasket should have been behind the diaphragm in this picture. The silver pin in the center of the diaphragm is what pushes on the fulcrum lever that overrides the spring and opens the fuel flow seat.
