All have 14mm thread diameter & 3/4" reach
Gap is 0.025" (0.64mm)
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The left plug problem indicates where the fault is
if you have duel carby , check for flooding in that carby
Check your kill switch, get a hair dryer and blow dry where the switch is, its simple but worth a try.
Hello and welcome to FixYa!
Have you tried replacing the spark plug wires and checking the spark plugs? I suspect that you have fuel flooding that's why it backfires. Good current to the spark plugs will result to better fuel burning. Check the spark plugs if they have big gaps, covered with carbon or maybe it's just faulty.
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Did you use a Typanium Recitifier? There should be no problems unless it is a used one. All the new ones I havew used just "bolt on" .. there are 5 wires. Check to see if you don't have a grounded wire some where.. try it with the head light off.. I usually wire my bikes to start with no head light with a 3 way switch.. once running I can then turn the light on. Go through an check all your wires again, you might just have a bad connection somewhere.. here is an excellent site .. Jim http://www.raskcycle.com/techtip/webdoc16.html
Does your tacho work? could be the ignition power module. left hand side of engine bay.