No spark... can I get some help?

#21
So guys, I've got some bad news.
I ordered a total refresh kit from dratv for the "ac generator and points/condenser." It comes with new magneto and generator windings as well as the typical points and condenser. Plus a bunch of small parts. Good buy for 50 bucks. I replaced everything today, along with replacing the coil with one I got off amazon.

No spark.

(please refer back to the wiring diagram on page one to familiarize yourself with the circuit)
The timing for the contacts is perfect, the gap is perfectly spec and the condenser is connected correctly. I did have to reuse the old points, because the new ones werent in spec and during adjustment half of the two part contact broke off. I used a flat file on the old ones, they look shiny. The new coil comes with the boot on it, so it's not a matter of having a resistor cap or anything.

Before replacing parts I had 6 volts ac on the magneto to coil wire, now I'm getting between 11 and 19 on a good kick. The new coil did have two low voltage wires going to it, where the original had a case ground. I put the green wire to ground, black/stripe wire to input. That seems correct after I ohmed each way through the new and old coils to compare.

Just like before, I'm getting roughly 1000 volts on the high side. Not enough. What's the problem? What's happening here? It's seriously killing me. I suspected that possibly the ignition switch could be faulty inside, so I pulled that and desoldered the coil signal wire to ensure it wasn't constantly grounding it. I checked the switch for correct operation on that circuit once I had it disconnected, then ran an ohm test through the coil signal wire to ground to see if it was shorted. Accounting for the resistance in my leads it was around .6 or .7 ohms to ground. That sounds very low, but in actuality the magneto winding was under an ohm and the coil wire (in parallel) is around an ohm and a half I think. Using those numbers the expected result would be .42 ohms, making it hard to actually test a short to ground. It's so close to zero.

Do you guys have any other suggestions? My stepdad just said take the flywheel off and wipe clean the magnet surface, and I agree I could clean it better than I did but I don't think that's the problem.

I'm open to any suggestions, guys. Seriously. I've never had to work on any one vehicle this long in my life. It's driving me insane. And I'm not even getting paid for this one! :p

Thanks again for any help
 
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#22
I am not familiar with your particular magneto but am very familiar with the lucas energy transfer system which is basically the same as your setup. It is critical that the points open when the magnets are at a particular position as the system works by the points closing and shorting the coil. This causes a lot of current to flow at one point when there is a flux reversal about to happen. At that point the points open and there is a big spike in voltage which causes the spark. This is different than a battery ignition where voltage is in the coil when the points are closed and then when the points open it causes a collapse in the magnetic field and a spark. The AC coil is designed to spark on the instant rise in voltage in the coil. Here is a excerpt from a tractor magneto article.

There is a critical position during rotation
of the magnetic rotor
– Magnetic field collapsing and reversing in coil
– Maximum current is flowing in the primary,
and opening the points at that point in the
rotation produces the “best” spark
• Critical rotor position is the distance
between the edge of the magnet and the
trailing pole face and is called the Edge
Gap
 
#23
I set the points per an instructional video that seemed to match the oem guidelines. Points open exactly when the flywheel lines up on the "F" mark and measures spec gap at the next mark, "T."
 
#24
The replacement coil you scored is for CDI, which is wound for 200 Volts on the primary, delivered as hefty DC pulses from a capacitor in a CDI module.

The CT70 ignition coil is wound different. The primary has more windings, and also is optimized for being powered with AC Voltage.

Electrically you have the new coil connected correctly, which is why you are measuring 1,000 Volts at the secondary. Your stator is working, but the new
ignition coil is mismatched to it.

Look at the physical size of this OEM Honda coil. The primary winding has lots more turns than a CDI coil:
"Vintage" Engine Coil for Your 1968 70 Honda Z50 Mini Trail Needs New Wire | eBay

That's an AC coil for a points and condenser magneto. The coil you bought is probably tiny by comparison.

Here's a new clone of that Honda coil:
Honda CT70 Ignition Coil KO 81' Mini Trail 70 Ct 70 | eBay

You should get that cloned coil, or an OEM Honda CT70 coil.
 
#25
This is what I picked up, and yes it is much smaller:
Amazon.com: Ignition Coil Honda CT70 CT 70 Dirtbike Trial Bike NEW: Automotive

The coil I took off of it was put in some years ago because the original coil wire melted on the exhaust. The one I just took off and the new one I linked above seem to have similar resistance on the primary and are almost the same size. That coil ran for a number of years, so I just want to make sure you have all the information I do before I spend MORE money on this little bike. My original excitement for this project has really taken a turn for the worse, as has my bank account so I've become more cautious with what to spend money on next.

Thanks again
 
#26
This is what I picked up, and yes it is much smaller:
Amazon.com: Ignition Coil Honda CT70 CT 70 Dirtbike Trial Bike NEW: Automotive

The coil I took off of it was put in some years ago because the original coil wire melted on the exhaust. The one I just took off and the new one I linked above seem to have similar resistance on the primary and are almost the same size. That coil ran for a number of years, so I just want to make sure you have all the information I do before I spend MORE money on this little bike. My original excitement for this project has really taken a turn for the worse, as has my bank account so I've become more cautious with what to spend money on next.

Thanks again
That Amazon link sure looks like a CDI coil. I know the ad says it fits the CT70 but I'd say that fits the newer models and clones with CDI ignition. The Green is ground, Black/White is 200 Volt DC pulses in.

The coil you want, has a heavier core and more turns in the primary.

Jon
 
#28
Alright, so I got my new coil in my mail last night. The sun was going down, but I was so excited to see if it would work that I rigged it up to the bike outside the frame real fast since I left the coil wires accessible by just removing the battery. I held the connections to the coil together and kicked it over-I heard the familiar crackle of ignition arc! Before even confirming it I ripped the intake, carb, air box, battery tray and fuel tank out for about the 6th time on this project. I installed the new, MUCH larger coil and piece it all back together. I kicked it over the thing ran for about 3/4 of one second and then died. Damn!!

I confirmed good spark with a gap tester and started looking around. The intake only has two bolts and one of the sets of threads into the head is WIPED out. Apparently this last time of taking it apart was the last straw and the one bolt will no longer seal the gasket on both sides. It's sucking air and making a squeak noise with each rotation. So now it's time to buy a helicoil tap and tool set. I sure hope this thing runs like a top when it's done.

On a side note, anyone wanna buy a thoroughly freshened up CT70? Less than two years from the first one ever built. New EVERYTHING on it :) It seems the issue was both a failing, mismatched coil and a failing magneto primary. Definitely a learning experience on this one.

Inventor: Thanks for the millionth time. I don't know if I'd have bought that more pricey coil without your push. It's literally the third one I've bought for this puppy! But I'm glad that it's working now. Your help made a big difference. I'll post an update thread once she's up and running-hopefully with a video if we can do that here.
 
#29
She lives!
I got some free time today after my helicoil kit came in. 28 bucks well spent. I've never put in coils before, but it wasn't bad, and thank got it brought this old girl to life! She was real cold natured, and cut off on me a whole lot with some miserable time spent trying to kick it repeatedly in 100 degree heat to get back home, but at least it's running. It straightened out a little once it warmed up.

She's still really low on power, max speed on a flat road is 25, usually I can only get it to 20. I'm gonna properly rebuild the carb and adjust the valves soon.

Thanks again to all the helpers! It feels great to ride my childhood bike around again!
 
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