OK, old school time - does it have fuel and/or spark? So you have fresh fuel - check. When it back fires are you getting a flame or just a gas mist? A flame and you have spark and are probably very close to having it running - the timing is off some - *** around with the dizzy, I usually get the hold down tight-ish so it'll stay where I put it, but I can still move it as needed... no spark and you are either WAY off with the position of the dizzy or it is mounted in 180 degree off. From the conversation so far it is obviously the ignition.. you need to go back to the books and redo the ignition system under the hood.. go slow - check it twice! You took it apart, you CAN put it back together... patience my friend, patience. It ran before, it'll run again. Crazy thought/stupid question here - the rotor is in there right? Just for "fun" - how about a pic or two of under the hood so we can see...??