TD-3 and Desert Rat, DING DING DING !!!!!!!! You are the winners. One intake valve shim was not seated correctly. ( OK, I admit it was on my workbench). Replaced it and bike started on 2nd kick. Thanks to all for the information !
Just installed Athena big bore in 2006 YZ450F. Bike won't start. Have spark, gas, cams lined up at top dead center of compression stroke( verified with straw in spark plug hole ), bike ran perfectly just prior to teardown. Compression seems good but not great kicking it over. Absolutely nothing but occasional backfire. Is it possible for rotor in magneto to "slip" or be mistimed ? Any info will help.