Vintage Dirt Bike Q & A

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 Post subject: 1974 MX250 ignition woes
PostPosted: Fri Aug 05, 2016 4:56 am 
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Joined: Sat Jan 04, 2014 1:07 am
Posts: 17
Hello,

The ignition on my recently acquired 1974 MX250 stopped producing spark. Stator resistance checks were beyond 10% tolerance on the low side. Another forum suggested replacing the ignition from a 1979 YZ400, so I bought and fitted a stator, rotor, CDI box and coil from a salvage '79 YZ400. Still no spark.

The manual for the 79 YZ400 lists the CDI box as Mitsubishi F008T01472, but the salvage yard shipped an Hitachi TIA01-28 instead.

The YZ400 stator has one resistance measurement (brown to black) that is 224.8 ohms. The spec is 250 ohms +/- 10%, so 224.8 is just beyond allowed tolerance. Would this coil measurement prevent spark?

The YZ400 rotor also has a crack completely through one of the four magnets. Tests were conducted with and without this broken magnet installed, but no spark in either case.

Kill switch has been ruled out through additional tests.

Is the hitachi TIA01-28 a suitable substitute for the Mitsubishi unit? I bought a second yz400 CDI box (hitachi TIA01-28) from an eBay seller who insisted the donor bike had spark with this CDI box installed. But no spark when installed with the other yz400 ignition parts in my MX250.

I am thinking of returning the YZ400 parts, buying a used 74 MX250 stator from eBay (maybe with pre-purchase resistance measurements) and a $165 modern FIREPLUG CDI box from CDIbox.com.

Thank you for any help,
Ben


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