So I posted in the introduction forums a few days ago about this bike. There are some photos there if you would like to take a gander.

My DT100b

little background info on this bike...someone failed to get this thing running and left it a trailer park my friend got it for almost nothing and gifted it to me. Previous owners completely destroyed the coils trying to get flywheel off and made a mess of the wiring. Left crank case gasket was bad so water leaked in and completely rusted and corroded everything. I cleaned everything up and replaced everything but lighting coil ( will deal with lights once bike is running properly). After timing and setting points gap to spec I was getting a nice blue spark but I don't think I was getting it from the correct wire....


So I got this bike running by using the black NON-diode wire on the end of the connector. When hooked up to the wiring harness the ignition coil is suppose to grab power from the middle black and white diode wire (at least that's what I take from the way the 6 pin connector ends are assembled). My issue is, bike runs great when I jump the connection from NON-diode black wire to ignition coil. Bike does not run when hooked up to black/white DIODE-wire connection. Also, there are 6 female connectors on wire assembly connector end and only 5/6 on the wiring harness end. Is this suppose to be like this? What's the deal with the black ignition wire coming from the stator main wire assembly? Beats me! That's why I'm here!

I apologize if this is confusing, hopefully the photos help. I included a thumbs up of the connection that works for me.

Tested the diode with a multimeter and it seems to show .585v one way and nothing when polarity is reversed, which I know is normal. I know they work as one way current valves and offer a bit of drop in voltage. I'm not sure if the reading I got is correct either as I don't have any specs to compare it too. Maybe the splice is bad, will try to rewire tomorrow and post results. Read in a few places that the diode is suppose to keep the engine from running backwards. I was able to ride around the block a few times with no weirdness, so I'm not sure if that information is correct or not. My guess would be that it prevents current from flowing back to condenser and source coil, maybe battery. I've had a 75 dt250 and a 73 rt360 and neither had the diode, so it's a little strange to me. Anyone know what needs to be coming through the diode? Or what kind of diode this is? I can make out '42 RSJAM' written on the diode but my ebay and google searches yield no results. Any help would be appreciated greatly appreciated!

Have seen a diode on the wire to the coil but not often & is to stop it running backwards which can happen with points ignition if everything is in its favor at the right time [that being the wrong time lol] & if 'that' did happen & you accidentally caught the kickstart with your leg then 'bad things happen' & if you had a handful of revs on thinking she's running terrible so give it a good rev to clear its throat then WHAM, motor split in two--well it's possible anyway. Have seen damage done to early 175 mainly, not split in two but pretty badly damaged. Hence they added the diode. Doesn't stop it kicking back but stops it going further & running. I haven't got a diode on anything of mine 125 to 360 so i wouldn't worry about if if you think its faulty [ie remove it]. So the black from the mag is what runs it & via the main switch it ends uo at the coil. You 'could' run it straight from mag to coil if you didn't mind not having a switch. Also any black with a white tracer is kill switch or ignition off/on--kill ignition by earthing out before it gets to the plug coil. That's remembering back 45 years to the first diode i saw on an AG175 farm bike at work--thinking what the--is that thing. Others will come along & correct me if i'm wrong [getting geriatric lol]but that's my memory of it.

Thanks for replying!

Yikes, well hopefully with the timing being correct the likeliness of that happening isn't too great. I'd like to replace it if I can figure out exactly what to replace it 'with' haha.On closer inspection I noticed the wire from the mag was beginning to break off the diode connection with only 3 copper wires actually making contact. I'm going to attempt to fix the connection tomorrow and hopefully that solves my issues. I bypassed the ignition and kill switch for now.

On my end I get "you don't have permission to view" warnings in your first post and there are no pictures... Used 3rd party hosting? If you can attach pictures like you did in your intro post it would help.

I agree with RT325 about the diode. Thousands and thousands of bikes out there still running that never had a diode in the wire.

The wire without the diode is there to connect the "Yamaha points checker" or similar to check and set the timing (points opening at the right time).

And yes, without seeing it, it is pretty normal to have a multi-wire connector that has an empty slot or two.

P.S. Battery has nothing to do with ignition on this model... spark happens when a "surge" of voltage/current gets sent to the ignition coil. If engine is running backwards the polarity of this "surge" is reversed so diode prevents it from getting to coil and therefore no spark and can't run.

I have a few diodes left, if you need one.

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Ordered a new diode just in case I have a bad one. Thanks DEET!

Here are the photos.

Thumbs up showing the connection I was using to get the bike goin. Just using the 'Yamaha points checker' wire mentioned by MarkT. Connecting this straight to ignition coil works great. Will try getting a better connection to diode after work and post results. I'm just happy it's running for now

Yup, that jumper just bypasses the diode for the points checker. I'm a little surprised there isn't a short pigtail lead that would just hang there until needed on the other side of the connector to make it easy to hook up the points checker... many bikes had this... but sometimes people get confused and connect the "extra" black wire to ground which kills the spark so maybe Yamaha stopped doing that? Not sure. Nice pictures!

Updates on the bike...

Crimped metal ends on diode gave me some trouble so I just decided to wait till the new one arrives jumped the diode for now by soldering a small jump wire that pretty much skips diode. That way I can plug in the connector to wiring harness as normal. With that said


I know I mentioned the lighting coil was a bit smashed up but I noticed I was getting power to the headlight wires. I had a 2 wire Japanese 6 volt headlight (kind that grounds through mounting bolts) and some headlight mounts off a 74 dt100 that I got for $10. hooked up the headlight and BAM I had a running light. Headlight has a little on/off toggle switch on top which is convenient because I don't have hand controls yet. Also hooked up kill switch and messed with setting the air screw on carb 1 1/4 turns out. Messed with throttle screw some the Let it idle for a few minutes w no problems. Once it got dark, I went on a ride around the neighborhood. I was surprised at how well the light worked at night. When I got back my gf jumped on the back and we went for round 2. Anyway here's some pics from earlier in the day!

Congratulations! Nice work and bike looks great!