I take it by your name that you may have a good amount of experience working on cars? This thread brings me back to the days of points, condensers and generators...i was always taught, there are three things to get an engine to fire....fuel, spark, and timing....is there ANY way the flywheel key was broken, or timing could have changed? I know squat about 2 cycles, but R & R ing the C&P is a pretty straightforward swap, heck, even i did it..(luck i promise) But, if you were running before, and all you did was the c&p, it should run , even with all the settings unchanged.Is there any chance that the p&c aren't sealing? Air is getting into the carb/manifold junction, Is there any possibility that the spark is grounding to the cyl BEFORE reaching the spark plug ( it takes alot less voltage/amperage to fire a test light)? Was the piston ring gap at the locator pin in the groove(although i think you wouldn't be able to install it in the cyl. if it wasn't).Arrow on the piston pointing toward exhaust? Most of these questions are needless, but once in a million, asking, and checking, can come up with the answer..one last question, the choke is operating correctly? Not staying closed?....You have me stumped , but i can't wait to find out the solution...rsbhunter