Backfiring is a symptom of bad timing. Sluggishness can be a symptom of bad timing. If you take out your spark plug and stick a pen in there and look at your points at the same time, you can get a rough idea of what is going on. Turn the flywheel and look at the points through one of the holes in the flywheel. When the points _just_ start to open stick a pen in the spark plug hole. When the points just open is when your spark plug sparks. Your piston should be almost to the top of its travel. Keep turning the flywheel and watch the pen. Does it move out more than 1.5 mm before it starts to move back down? If so, your timing is too early. Does it not move out at all before it starts to move back down? If so, then your timing is too late which means that the spark is happening when the piston is already traveling back down.
"Here's a great thread about a tool you can make to assist you with setting your timing.":http://www.mopedarmy.com/forums/discuss/7/75554/75554/