Motorcycle Repair: No Spark, british twins, piston crown

Question
I have a 1969 Bsa D14 and can't get a spark at the plug. I've changed the magneto, rectifer, points, condensor, coil, lead, plug and wiring. i know theses are all working as i have tryed them on another bsa bike and they all run fine. What would be the cause of this.

Answer
Aidan,
 Be sure ALL grounds are tight and clean.  Typically a system will fire with a bad rectifier since the plugs fire on A/C voltage directly from the stator and the rectifier is only used to convert that A/C voltage to D/C in order to charge the battery.  All I can think of at this point is that you may have missed a critical setting at the points or prehaps wired them incorrectly.  You didn't mention it but the condensor should have been replaced along w/ the points.  You can have perfectly set points and a well operating system but if the condenser is bad you will have failure.  Work backwards w/ your multimeter from plugs to stator.  This way you will eventually find your weak link.
Points are tempermental and need to be set VERY presicly.  Some manufacturers give the BTDC measurement from the angle of the crank and others from the distance of the piston crown from TDC.  The gap, when seperated is also very critical.  Use your multimeter to assess when the points seperate at the exact angle and a good feeler gauge to check gap w/ magneto flywheel off.  There are aftermarket electronic ignitions made for British twins and they run about $100-$150 (65-95 lbs.).  They are well worth the price if you plan on riding the bike alot.  Set them once and forget about them, no parts to wear.

Hope this helps, Mike