ORIGINAL: Cyberwolf
but if you have not messed with the hall sensor there should be no need to check it, if the engine ran ok before.
That's why Ialmost didn't suggest checking the timing. If it ran before then it may just be a lean low needle. Hard starting and backfiring are lean low needle settings. They are alsoignition problems. What voltage are you running on the ignition? More than 4.8V packs can burn these things up.
See the bottom of the following web page for timing instructions.
http://www.ch-ignitions.com/CHinst.html
Timing degree wheel.
http://www.ch-ignitions.com/timing.html