My 88chevy 1500 has a timing issue only time it runs decent is when the timing is about 15* out ( to the left if your loking at the balancer)firing order good new cap rotor wires and plugstiming chain dead on also

Replacing the Ignition Control Module (ICM) should solve the problem.

my 88chevy 1500 has a timing issue only time it runs decent is when the timing is about 15* out ( to the left if your loking at the balancer)firing order good new cap rotor wires and plugstiming chain dead on also

Is that base timing or while running?
That seems too much for base but only slightly more advanced than I recall for idle once at operating temperature for the standard engine set-up.
I remember you've changed the cam in this, yes? Have you allowed for this with the rest of your "custom" rebuild?
Depending on exactly how you've changed things, running "decent" may no longer be obtainable.