Hesitation - Still

How did you set the timing? You have to disable the ECM's control of the ignition timing before adjusting it, otherwise the ECM will be advancing it and your setting will be way off. This is usually done by either jumpering the A and B terminals in the diagnostic connector, or by disconnecting a connector to the distributor, I think it should say on the emissions sticker.

Usually allowing the car to warm up doesn't require jumpering ECU. My guess is that an 87 does not require jumpering.

Based on experiences: A vaccuum advance problem could cause hesitation. Resetting the timing might not help. The problem could be the distributor, the advance mechanism or a vac leak. In theory, the vac leak can play a part with the advance mechanism. The stalling could be casued by a vac leak, and the A/C button only raises the idle. Start with the vac leak, if any. Rick

Wrong guess.

What vacuum advance?

An 87 Z-28 with a TPI 5.7 is full EST/ESC, IOWs no vacuum advance, no mechanical advance. A vacuum leak is worth looking for but not a common problem on this particular set up. Odds are better that one of the fuel injectors cashed it in (crappy Multecs etc, yadda-yadda).

I see, although on some Hondas warm up is all it needs.

Maybe on some vehicles. Thanks for the corrections, I'll be checking my facts next time. ;-)

The question posed concerns a Chevrolet.

Or, check what the OP is having problems with, hate to see someone go off on a wild goose chase.