Motorcycle Repair: Yamaha DT250 no spark, ac volts, windings

Question
Hello Terry,

My Dad recently bought a Yamaha DT250; lovely condition, running and starting well at first - often on the first kick despite being mid-February. Really pleased with it.

However, all of a sudden there is no spark. Using a multi-meter on the start coil and capacitor appears to show a dead short.

He has also purchased an electronic ignition to fit in the future, the timing still has to be set up on this but he figured if he set it up loosely he should at least get a spark, even though it wouldn't be at the right time. This produced no spark either, the only common part in both systems is the flywheel.


Can the magnets on the flywheel fail or change polarity? Or can you think of any other reason for this problem? I'm sorry if I haven't provided enough detail.

I look forward to hearing from you.

Answer
Hi Amy,
If the bike has breaker points that may explain your dead short, the points may have been closed when you tested it. The windings are low resistance, likely you won't get much of a reading without a complex 4-wire meter. The wire that goes from the condenser/points to the coil should have a considerable pulse voltage when you kick the starter over. Use your meter here, AC volts at @200v range. See if you get a pulse when kicked over. If not the ignition wire may be shorted to ground somewhere and your voltage is going there instead. It's very unlikely the magnets have changed.
Regards,
Terry.