The EGR spacer shouldn't affect idle unless the EGR valve is stuck open. Since you just installed underdrive pulleys it is a good idea to reset your computer. Disconnect your negative battery cable for 15 minutes. Reconnect it. Start your car and let it idle for 10 minutes. All accessories off. Then turn it off. Turn all accessories on. Start it and let it idle for another 10 minutes. Take it for a spin and see how it feels.
If this doesn't correct it, it might be the idle air bypass valve that needs cleaning. That's the cylindrical thing on the front of your throttle body.
Idle adjustment is a little trickier than just turning the screw. You should probably leave that alone for now. If it's 600 by the stock tach, I'd say it's probably a little low. You may want to hook up a digital tach and verify what it really is. Stock tachs aren't very accurate.
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351W 89 Mustang GT Convertible
[This message has been edited by jimberg (edited 06-08-2001).]