Motorcycle Repair: wont start, honda z50, plug fire

Question
 mitch,
        honda z50- replaced int and exh valves, lapped
  in good,sealing very good.strong compression. new spark plug.timing good.
   strong spark with plug out of engine. i put engine back together and fired right up. ran good, rode bike a few miles
  get home try to start and ocasionally  backfire from
  exh and at times from carb.  i then pull head back off
  check everything but do nothing to engine then put back
 together and fires right up ride a few miles get home wont
 start. pull back apart put back together without doing anything and starts. done this 5 times now. timing is checked before i pull apart. whats up? this is puzzling to me. can a spark plug fire outside an engine and not when installed and under compression? i have heard of new spark
plugs being bad. what could be bad with a brand new plug?
i know a plug can foul out from oil, but can a brand new
plug foul out from fuel saturating it?
    im stumped! any help will be greatly appreciated!
       gary

Answer
gary,

do me a favor go and buy a little bit hotter plug for the bike then re-crank it if it doesnt change contact me again. i am doing research for u right now im at my cycle shop.ok.
p,s please leave feedback for me and you may also nominate if you choose o.k

                                 mitch kruszynsky