Speedfight 100 has just died, please help?

hi, any advise would be grateful.
i have a speedfight 100, i have just changed the engine,as my old one was knocking really bad, put engine in, connected eveything up and everything seemed fine, i started it up and it run great, i put a bit of extra oil in the petrol until the oil got through,, that was on friday, i went to it this morning and put some exhaust sealer on the exhaust and barrel, then i tried starting it, with no luck, i been at it for about 3 hours, i take the plug out, it is wet, petrol getting through, the plug sparks, i have only the electric start to start it as the kickstart is broke, and when i spin it over it is not a solid blue spark but a yellowish spark, i changed the petrol, i have took the supressor cap off and put directly on plug, no joy,, i had an old plug and tried that , still dont work, can anyone help, has this happened to anyone,

thanks