the slower the engine turns, the more air will leak out...no little/no compression, no ignition, you gotta make sure it's spinning fast enough

 

in aircraft and now it has caught on to cars..is using a differential compression tester...more reliable

 

The jetting and needle position are very important given the fact of the non stock motor. If the plug looks fine the problem in initial throttle response could be in the air/fuel adjustment screw. I might be wrong but I believe that this is the only time that particular adjstment comes into play is throttle response.