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First if you cant adjust the idle mixture it may be because you have the throttle open to far and is switching over to the primary circuit. This will make the mixture uncontrollable. Make sure you have the RPMs down to less than 700 RPM. My engine wouldn't run under 950 RPM because of the ignition. Once it was fixed it was fine.
The pump diaphragm actually does two things. It shoots gas into the engine of course but what a lot of people don't realize is that it also controls the mixture. If you used to much gasket cement and plugged up the hole on the vacuum port it will run rich all the time when the throttle is open.
The older YF's work similar to the later ones.
What looks like it might be the pump actuating it's actually the vacuum in the manifold acting on the diaphragm making it go up and down depending upon it's load.
If you can visualize the pump diaphram going up raising the metering rod & pushing fuel out the discharge nozzle dumping fuel into the intake.
contributors, J.Strenk
