1974 Yamaha MX125 No Spark

1966g10
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Eden, WI, USA
1966g10
Wondering if anyone can help me out here. Working on a 74 MX 125 and I can't get it to spark. I replaced the coil and put new points and condenser in. But I still can't get it to do anything. I got the bike all incomplete so Im just winging it right now. Not sure about the wiring. These are the wires, Light blue, green, black, yellow and Teal with red stipe.

Not sure if I should try replacing the coils on the magneto. They were very rusty and caked up. Previous owner said it would keep losin spark.
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Chayzed Pilot
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Chayzed Pilot 9/29/2016 11:05am
9/29/2016 11:05am
The black wire should go to the upper coil. That is the "hot" wire from the ignition. Did you time it correctly?
project racer 9/29/2016 12:53pm
9/29/2016 12:53pm
If it ran for the previous owner, but kept losing spark, it could have an "open" in the windings of the mag coils. They usually grow when things get hot, making for no spark. Then when cooled down, the winding contracts, making fire again.

Best thing to do is figure out the taper of the crank, and upgrade to either an RT100 or RT180 ignition.
1966g10
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1966g10 9/29/2016 4:33pm
9/29/2016 4:33pm
That's what I was leaning to. That the coil windings must be bad or something. Can I just measure the taper to figure it out? And would the RT stators some what bolt in? Then just run the one wire to the coil and one to ground and should be alright?

Sorry for all these basic questions. I'm used to street bikes, just got done building a Honda CB360
project racer 10/1/2016 2:52am
10/1/2016 2:52am
I used an RT100 ignition system on a '74 MX100A. Taper was the same, bolted right on. Used a single wire coil. The little moldex plug from the CDI box plugged right in to the stator wire lead plug. The coil just grounded to the frame mount, I believe.

Was fortunate enough to have some spare ignitions, an old Yamaha dealer, and a good salvage yard at the time. Would basically pop a few flywheels off of different models, and see if they fit the crank taper I wanted to use them on.

I know the MX125 / 175 top ends of that era would interchange. Want to say if I had to guess, lean towards the RT180 stuff . The 100 may have a bit different taper, but not sure.

I will say that in trying to find an external flywheel system for my 1979 YZ250 , much to my surprise and pleasure, discovered that the 85-88 YZ 80 shared the same crank taper as the 79 YZ250. Just a few minor mods and it went right on, and worked great.

Used a 79 YZ400 system on my 74 MX360. Bolt on and go. In a pinch, even used an old 71 DT250 point ignition on a 75 MX400B. Had to turn the flywheel ever so slightly to clear the ignition cover.

Ignitions don't know or care what they are mounted on. If the taper matches on crank / flywheel, most anything can be made to work.

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1966g10
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1966g10 10/1/2016 9:31am
10/1/2016 9:31am
All right I see what you mean I think I'll start looking for some RT 180 parts to change The ignition over. I know on the MX 125 like I have it does not have a CDI box I assume I will have to run to CDI if I use that updated ignition?

I found a couple eBay listings I assume that these are what I'm looking for and all I would need. I just purchased a brand-new coil so I have that

231226404183 and 371730033864
project racer 10/1/2016 3:31pm
10/1/2016 3:31pm
Yes ,you need the complete, matching system. Stator, flywheel, and CDI unit. If your coil is a single wire unit, it should work .

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