timing problems

[oLemurs]

basically before i took off my intake manifold and re gasketed it, i also got a new wiring harness but before i took it off i timed the engine to 0 degrees (i know now its supposed to be 15) im pretty sure after i timed the engine i dropped my timing light but didnt think anything about it. now once i put everything back together and put it in timing mode it reads about 40 degrees advanced but the problem is when i retard it all the way to get it actually near the 20 degrees mark it starts popping like the timing is way retarded. what im wondering is can a timing light be delayed? im using an inductive pickup and strapping it around the 1st coil pack. i also dont hear any pinging which is odd but i havnt really driven on the road much.