CIS mixture problem

[Ameer Antar]

I bought the Bosch Fuel Inj. & Engine Mgt. book as someone recommended. 
Although it's a good book, I figured out all that pertained to my system by 
reading on the web. But the book did not go deep enough to explain how to 
troubleshoot specific components. My problem is that at or near idle, the 
mixture is very rich, but throughout the rest of the throttle positions, 
the mixture is 'normal'. This could be many different causes, but I have no 
idea how to check. I've thought about it and it could be....frequency valve 
leaking, o-ring problem in distributer, other distributer problem, or bad 
ECU, all of which are extremely expensive to replace, and I wouldn't want 
to replace anything that was ok. Does anyone know how to check or eliminate 
these components? I was thinking I could eliminate the frequency valve, by 
disconnecting the outlet side of the valve, unplugging the connector and 
jumping the fuel pump. Or should I actually have the engine running and do 
it? Also which side of the valve is the outlet??? Please help w/ any ideas, 
even if your not an expert at CIS. Thanks again guys.

-ameer antar

'84 5kT...190k miles