Engine miss fire??

Hi everyone. Long time reader, first time poster. Firstly many thanks to all of you that have dedicated your time to posting answers to the truth... Without you all i would have been lost long ago. My most recent problem i can't find the answer for yet.
I have a superhawk R80 (1500km) with SBP Xchamber (200km) and shift kit (700km) running a 9 speed shimano cassette and XT derailleur Pretty sweet when it's going!
When i try and start the MB it will start but it coffs and backfires out of about everywhere it can come out of. I have noticed it backfire out of the air filter, head gasket and exhaust gasket. First i thought the exhaust was blocked so i changed to the stock one and it made no difference. Ran the hose thru the xchamber and it ran thru fine. I pulled the plug (BPR7HIX) and it was all gummed up and black where as it's always been a beautiful coffee tan. It also had fuel on it. I checked for spark. Yep i have that. I checked the magneto and yep that's all in place with the key and turns with the piston as it should.
I tore down my speed carb, cleaned and rebuilt but still the same problem. Checked all my wiring and that's all fine.
The latest thing i replaced was the head. I bought a new one cause i broke the cooling fins off the original. I lost a little compression but it still ran fine. I took it out the day before all this happened and rode 45km with no stress. Rode it to work the next day but when i finished it wouldn't start. I did notice that the tacho was spiking all over the place as it was backfiring and coffing so maybe it's a bad CDI?
I'm really lost for ideas now. If anyone can help i would appreciate it
Cheers

Thanks for your quick response. I checked the magnet. I took the nut off and i could see the key where it should be. Everything looks like it should. I got the multi meter out and tested the magneto. With it switched to 200 ohms i couldn't get a reading between the black and white wires and the blue and white wires gave an erratic reading so i'm guessing i've found my problem. I checked the CDI while i was at it and all the readings seemed fine there.
Time to order some parts and i'll post back with what i find...
Cheers again

Problem solved

After further inspection of the magneto coil i found the little wire that comes out of the coil and runs to the earth had broken off. It looked fine till i touched it and realised it wasn't connected. Figured i'd replace the part (from a handy vendor right round the corner) and bam the beast is alive again.
Thanks for all the help. Hope someone finds this thread useful in the future.