When I try starting the bike I have to turn it over for long periods and try various choke settings and quarter to half throttle after a while it will start but it has to do a continuous rolling start.  I have installed a carb kit and new air filter.  once started it runs pretty decent and has good snap.  It does seem to start a little better when I use pull cord and have throttle half open.  Odd time it backfires during start.
Will do a compresion check tomorrow in case I have a burnt valve.
Help Mark.    

try a new spark plug,it may not look bad but there cheap, also check coil resistance from the terminal to ground should be no more than 3ohms, terminal to center where spark plug wire goes no more than 5kohms, also make sure ALL wiring connections are clean and tight especially grounds, invest in a tube of dielectric grease and apply to all electrical connections/connectors to prevent furture problems. good luck  

you are on the right track, check the compression, also try checking the magneto and stator for dirt or rust as these can cause a weak spark.