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Hi BMF,

Check the wire from the coil down to the magneto for signs it may be fractured inside the insulation, particularly where it enters the crankcase cover.

Check the condenser resistance when it is warm (not running) should be more than 3 Mega ohms. (measure between coil small terminal and earth with points open)

If the problem appears to be heat related then its worth checking the ignition coil resistances with a decent digital multimeter:

primary winding 4.5 ohms

secondary winding 6 Kilo ohms

Check the magneto coil resistance when it is cold (coil terminal to earth with points closed), then compare to when the engine is warm and stops running. If it goes short circuit, open circuit or much higher resistance you have found the problem.

Magneto pulse coil resistance 1.6 ohms

(note: above specs are for a TY175C, sure it wont be far away for a 'B')

I can recommend the electronic ignition kit from John Cane, it was simple to fit and works well. I would also suggest changing the woodruff key/magneto spring washer/nut as mine was half way sheared through.

regards

Rich

Edited March 2, 201016 yr by richTY250E

Losing spark when the bike warms up is normally a source coil dying ie the insulation breaking down, check the resistances both hot & cold against the factory specs

Check Tony Downs blog. Somewhere in his articles he loses fire on a 175 and it ends up being the points. They come coated with a sealer/coating that must be cleaned off.

Thanks for the information. I will check it out and report back.