I have a 94 Z-28, bought not running, installed GM opti spark, runs but spark at plugs is low, spark at coil wire great. Thought I read somewhere the rotor should point straight up at TDC. Timing chain slack is minimal, but slack to rotor shaft seems excessive, lots of miles 155k. Flintstone

 

Did you test your plug wires? I had a similar problem, ohm tested my wires and most of them were bad. The car would idle really rough, stumbed because it was so rich, and would not go over 2500 RPM even with no load. Changed wires and plugs (they were fouled out by them) and it ran fine.

Also check for bad fuel depending on how long the car has been sitting.

 

Has new plug wires and AC delco plugs, all done at same time

 

I would do an ohm test on the coil before ripping in to the opti, just because this test is easy to do. Just measure the resistance between the connector points and post (where the coil wire goes), you want the same resistance between all the connector points (should be 4 if early model, 3 if later). If you get varying resistance it means your coil could be shorting out.

If you really want to check the opti(distributor), you'll have to remove the crank pully, drain the radiator, remove the waterpump, and then you can get to the distributor.