Hey everyone!
Quick introduction. My name's Daniel and last spring I purchased '73 DT3 as my first motorcycle to travel in the city. Before this, I had bought a Yamaha TT350 as my first ever bike, and I instantly fell in love. I rode ATV's at my friends beach house as a kid and having a bike again was an awesome feeling. Eventually I left the carb out and it seized, and so I ended up taking the engine apart above the transmission, reboring and honing the cylinder, and reassembling it myself. That is what kindled my love for working on engines in general. The dirt bike never fully got working again though as I got busy with school, but my passion was re-ignited when I was allowed to get a real motorcycle, and so I picked the DT. I was so excited to finally get to ride again and then it totally stopped working. This bike has represented a means to get around for me and without it I've been stuck unable to get places, so I am absolutely DYING to fix it.

Now about the bike. I bought it from a guy on facebook marketplace in NY and he said it was running fine and had been rebuilt. I took it down to where I live in PA and rode it a couple of times before it stalled and died on me. I met a couple of the infamous Philly dirt bikers and they helped me to pull start it, and it actually started again and ran for about a block before it stalled and died for good. While it was running, it was very choppy and skipping ignitions. I take it to a shop because I'm busy with school and don't have access to my tools, and they said that whoever installed the rings on the engine messed it up really bad and so they had to rebore it. This was annoying, but I got it back from them and rode it home, for about 15 minutes or so. After this on my way back, I encountered a new issue which was when I hit high RPMs the engine would start to skip ignitions again. I made it all the way home and parked it.

I suspected a fouled plug, as is common with 2 strokes I hear, and so I pulled it and gave it the ol' spit and shine. This didn't seem to do much, so then I tried getting a new plug. Still no luck. At this point, I'm very frustrated because I've hardly been able to ride this bike at all and I really just want to have something just work for once. I actually rode the bike 3/4 of the way to the shop at a blistering 20 MPH with the choppy ignition before finally the ignitions became less and less frequent and it broke down. I took it to the shop with a car and they first said it was a spark plug issue, but they hadn't actually ridden the bike so they didn't realize that the issue was still happening and that this wasn't the issue. 

Now we are at the current state. The guy at the shop said that the source coil was out of spec and it had to be replaced. I don't know what reading or readings he took and what the original spec was, but I took his word for it. Finding a new one of these was absolutely impossible, but I did manage to find a DT3 stator on ebay including the coil I needed so I just bought it because rewinding the one I had was going to cost $300. The coils on the stator they sent were busted and so the shop said that I needed a new one. At this point I am DESPARATE to get this bike working and so I said fuck it and I rewound the coil myself. I used calipers to measure the gauge of the wire on the original and unwound the original coil and counted every turn. I then ordered the 22 AWG magnet wire (correct size) and rewound all 440 turns on the coil after 3D printing a new sheave to keep the winds from spilling over the edge. I checked the resistance and it seemed to be good, reading the .5 ohms that I heard was the spec from another user's post.

I give it to the shop and they install it and it seems to be working, i.e. there is some spark, but not a hot enough spark apparently. The guy at the shop then bought a couple of ignition coils online and tried them to no effect. He then took a working ignition coil off of HIS DT that was RUNNING back home and it still didn't work, leading him to believe something else must be the issue. He then said that it wasn't worth it for either of us to work on the bike because the number of hours would make the cost astronomical. Dismayed, I picked the bike up and took it home.

Now is the real current stage. I have just taken the cover off the magneto and I tried to test and see if when I cranked it with a drill I could get any voltage as is shown in this  video . I ran into the issue that it doesn't look like there's any connector like the one he uses and so I couldn't determine anything. I then pulled the flywheel with a tool I made and took off the stator. I then measured its resistance just as he did in his video (between stator ground and black wire) and read a clean .5 ohms. So now I am trying to find a diagram explaining the ignition circuit and I simply cannot. I don't really know what to do at this point and so I'm turning here for some help.

That's the whole issue, thank you for reading. If any of you guys have ANY input I would greatly appreciate it because I feel like I am at a total loss and I paid $2800 for this bike. Last semester in school was tough because I had no way to get places I had to go and I'm about to start my spring semester, plus I got my first road jacket for Christmas and so now more than ever I am DESPARATE to get this thing to work. I feel like I'm totally out of options and it sucks because really I just want to get on the road again and enjoy this thing. Thanks again for any help, I really appreciate it.

TL;DR: My bike was experiencing choppy ignition, now won't run. Plug is clean. Shop said with rewound coil and new CDI capacitor, it was making spark, but not hot enough. New ignition coils including a known working one from another DT did not fix anything. Stator is determined to be in spec with respect to resistance (.5 ohms). I'm at a loss, please help, I have no other options left, yada yada.

Welcome from NZ. I'll read it all tonight so i'm likely well off the case, but when someone says its been rebuilt then has ignition problems i go straight to the flywheel crank key to be sure its ok as the eat them for breakfast if the flywheel nut isn't super tight. Common problem. Ignore me if ya way ahead of "that possibility". If--changing ideas a bit--you want to try a simple setup, just run the black from the mag directly to the plug coil under the tank which of course bypasses everything in the loom & switch. Black sticking out the end of the mag loom. Like i said, i'll read later tonight.

Hey! Thanks for the reply and sorry for my late response. I haven't checked the key but I can give that a look in a moment. Is your proposed wiring loom bypass a permanent solution or more of a diagnostic tool? So the arrangement would be everything disconnected except black wire to ignition coil and then ignition coil to spark plug? Thanks again for your help, I eagerly await your response!

Yes as you say black to coil & coil to plug. For safety you ideally need a kill switch but of course i haven't got one on mine but might regret that one day lol. Crank key you really need to pull the flywheel to be sure as removing the nut & washers to look can still trick you as even with a broken key you would still see whats above the shaft in the keyway of the flywheel & possibly think its all ok. Would still spark with a shorn off key but spark in the wrong place for timing. There's a timing tab viewed in the flywheel slot & if the points are breaking about then--then crank key is likely A-ok. Nut must be kept very tight. I must admit i still haven't read the full story--tonight job i hope. So might be a bit off the case.

Got it. Gonna see if I can manage to give that a try, I will follow up with an update. Wish me luck!

Right okay first things first. On the taskbar at top of the header of this site is a Tech page. Open it and search your bike out and download the manual ,it's free.
Once you've checked that the key is okay like Rt says, then check the plug gap,points gap and ignition timing.
Get all the basics checked and read the manual.
Did you lose the spark?
Why did the workshop you took the bike to go to the rings? 
What's the ignition components like?Do they look old ?Can you read the words "Made in China" anyplace . I had a running problem on my RT1B 360. It would cut out for no reason and wouldn't start. The new plug I put in was faulty so I replaced it and away it went then cut out again but this time the spark was still there,just. I regapped the points and checked the timing . I set it to the widest gap.
Started and ran from there on .

Hello again everyone!

I have done a bit of work and I have some information for you all. I successfully shorted the black wire to the secondary coil like RT325 suggested. I went to look for the key and I noticed that it wasn't there, It is certainly possible that I just dropped it though, as I'm sure the shop that worked on it would have noticed that. I went and bought a new key today and cut it to size and remounted the magneto. I also wired the ignition circuit from black wire to 2ndary coil to spark plug with each component grounded to the frame. I cranked the flywheel with a drill and here's the interesting bit: I got a nice looking spark at higher RPMs but as the speed dropped the spark became less consistent and eventually stopped. I took a video, look at the link to see it.


Now a reply specifically for shyted's comment but also for everyone else. I now have the service manual which I don't think I had specifically, very big help. I haven't checked the plug gap as I don't have the tool to do it, I assumed the shop would have checked that, and it's a brand new plug, but I could maybe give it a try. I watched a video online about points, what they are, and how they work last night but it wasn't specific to the bike and I don't know how to do it on this bike because the flywheel gets in the way. I also can't see any obvious cam lobe that would lift up the points on the flywheel or shaft, so if you know about how that works then I'd love to hear it. I plan to google that soon though and check it myself. Same exact situation with ignition timing, but I don't think that should cause a weak spark, only running issues. I don't know what going to the rings means, but if you're asking why the shop refused the job, it's because its an old bike and the parts availability is really bad. I have called three other shops and have been refused by all 3 for the same reason, one calling it a "nightmare job," thus my despair of never getting it to work. As far as the ignition components and their condition, I currently have two complete stators. I had an issue finding a source coil and so I bought a stator that included one, but then the coils turned out to be in worse shape than the one I have. I believe the shop replaced the condenser and possibly even the points as they all look to be in much better shape than the second stator, if you ignore that soldering job. I ended up unwinding the source coil while counting and then rewinding the exact number of turns with the same magnet wire (440 turns and I forget wire gauge but it says above) and the coils resistance out of the stator was .5 ohms, resistance between black wire and ground is also .5 ohms. I have both the OEM secondary coil as well as two Chinese coils that the shop tried, the owner of the shop even brought in his own working coil for the bike and tried it but had no luck. You can see their condition in the attached images. Thanks for the detailed reply and I hope you enjoy this even more detailed response.

photos.app.goo.gl/6jgCeySzinjXLGHRA

So hard to tell in video how fast it's spinning... it is a magneto and does need the spinning magnets to generate electricity. It won't spark at really low rpm....

But more likely is when you slow down the RPM the plug is "bouncing"... it's probably loosing good ground.

These old bikes are reliable as rocks... especially the ones with points... and if you have basic tools and understanding, super easy to work on. Most shops are just parts replacers these days and have no understanding how things work. Ignition not working on a new bike? Hook up a computer to tell you what's wrong or just replace parts until it sparks again.

Listening to video again you might just be slowing down the rpm too much...  use the kick start like you're trying to start the bike...  make sure plug stays solidly grounded...  and you'll probably have a nice strong spark.

I don't know what minimum RPM for spark is...  but guessing around 300.  Some early CDI racing magnetos needed to spin closer to 1000 rpm!