1987 Chev pick-up w/350 , 4 weel drive, 3/4 ton, no AC,  automatic3 speed, TBI, HEI dist.w/coil on manifold(square type)  Am trying to time the engine but can't locate  the spark control wire to disable the computer in order to base time the engine. Someone told me that sometimes they don't have a connector plug for that year and that you have to locate the correct wire, cut it and resplice it back together when it is timed.
Also this truck has poor performance and idle is unstable at different times, was thinking maybe to look at the AIS to see if it is ok, any help as to what to do next.  Thanks for any help,

Hi Lucky,

Circuit 424 tan with a black stripe is your Bypass wire to unplug to set base timing.  This will set a code that will turn on the Check Engine Light and need to be cleared.  The connector could be in a few different places.  The most likely is underneath the plastic box on the firewall passenger side.  There are some relays for the fuel pump etc under there.  Dig around and you might find it.  There is a black disconnect on the single wire.  It could also be hanging around on the drivers side usually visible out in the open near a harness.

If it still has a problem after the timing is set correctly, there could be a number of causes.  A scan tool can be used to monitor the various inputs and outputs to get a direction, including IAC counts (idle control motor), O2 sensor and fuel trim numbers.

Here are a couple of links:

This one will help you find a qualified professoinal to take care of this.  This would be the most cost efective option.  Shops on this engine are members of the International Automotive Technicians Network:

http://www.iatn.net/dir/search.asp

If you still want to tackle this yourself, you need information.  You can get access to systems information on line.  While this may not help you to diagnose the issue, it will give you an appreciation for the complexities of your ride and serve to illustrate the value of a pro in this situation.  Here is a link to ALLDATA DIY Online:

http://www.alldata.com/vehicle_owner