1996 CV Backfiring

Just replaced intake manifold and injectors on a 1996 Crown Vic. Vehicle started fine but heard fairly loud sound like ticking or tapping. Folks here thought it sounded like a clogged injector. Before swapping back to original injectors, decided to check compression. Results - front-to-back driver side: 150, 160, 160, 160; front-to-back passanger side: 181, 185, 170, 150. However, the #3 with the 170 reading was burping (laud exhausting compressed air) back into the intake manifold. This says to me that the exhaust valve is not opening (could also account for higher readings on the other two out of the three cylinders on that side), and its not the timing chain because the other three cylinders on that side would also be messed up. Again, the engine would start and rev up just fine except for the tapping sound.

Additional info: This vehicle has 170k and was well maintained - 3000mi oil/filter changes synth or synth blend oil, plugs, wires O2 sensors, DPFE, EGR. Manifold change prompted due to crack in the front of thermostat housing - no evidence/issue with coolant in the oil/oil in the coolant - again, crack was evident only in the front of thermostat housing; engine was shut off as the temperature started rising above normal. The back of throttle plate is black - usually wouldn't think anything of it, but now, in the light of this problem, it may be more so than it should be, though it just could be my imagination. Sparkplugs look fairly consistent, so no clue there. Car set for a couple months before repairs were made. Prior to that, it appearted to be running bit lean - that is, when hot, it would rattle like it marbles in the engine only when depressing gas pedal while rolling in the range of 18-30mph, or so - by backing off the throttle and depressing it again, it would accelerate without the rattle. Six months earlier had codes for the EGR insufficiency - fixed with the DPFE(sp) sensor. Prior to the manifold incident it seemed more sluggish that it should have been.

Any advice/insight would be much appreciated. thanks

Make sure you installed the distributor correctly and the timing is correct.
Use a wiring diagram and make sure the plug wires are installed correctly.
Use a wiring diagram and make sure the injectors are wired correctly.
If this doesn't work spray some carb fluid around each injector checking for a leak. Also spray around the intake manifold.
Do some basic checks before replacing a lot of parts.