CT70 No spark

hi all!!

I’m working on cleaning up and fixing my inlaws barn kept 1974 Ct70. It has not been ran in some time but has good compression and is in great shape.

I tried starting it up yesterday and noticed there is no spark at the plug. The lights on the bike work so I assume the key switch is working properly.

Where would the best spot be to start? I know the ignition relay is under the tank, are those common to go bad?

If the points and condenser needs repair/rebuild, would that cause 0 spark at the plug?

A good place to start for a barn kept bike is looking under the seat/tank to see if any mice chewed up the wiring harness.

On a 74 you do have a blinker relay by/possibly under the tank and is not related to spark issue. If you are referring to the high voltage ignition coil those rarely fail.

Make sure all the connections are made especially the black wire (ignition wire). May have to look in the headlight bucket also. The 74 model has bar control kill switch.

Remove the flywheel cover, rotate the magneto counter clockwise, and as you reach the "F" mark the points should just start to open. Verify this. Next take a really narrow finger nail file or fine Emory cloth and lightly sand between the contact pads of the points, just to clean up. Check for spark at the plug. Also the plug has to be grounded to the motor to check for spark at the electrode tip.

If the points and condenser are bad that would cause 0 spark at the plug.

Great info! I was thinking the key switch is working is the lights are working properly. It has a few blown out bulbs but knowing some work eliminates the key switch.

The wiring looks good with no damage or chew marks on it. There is a bar control switch, that’s what I was wondering could be the potential issue.

I was reading the manual that for the points when the flywheel “F” mark the points just start to open; I notice on this bike that seems to be off with the engine mark as they don’t slightly open, so I assume I will need to adjust the points.

I just wondering if when I pull the flywheel, maybe it would be a good idea to replace the condenser and points anyways as maintenance?

I’ll also add, I just change the spark plug already

Without pulling the tank, there's a big section of the wire harness that cannot be seen. I'd hold off on the tank removal, for now. First thing to do is gap the points...could be as simple as that. It's a good idea to clean the point contacts...they could be dirty/rusty...it doesn't take much to foul them. Still no luck, at this point, pull the primary lead (coming from the HT coil) from the main harness, disconnect the modular plug (at the stator) then rig a jumper lead from the primary coil to the HT coil (primary) input. This removes the wire harness and keyswitch from the ignition circuit, entirely. If you get spark, in this <temporary> configuration, then you'll know that the issue is in the wiring; if not, time to throw new points & condenser at it.

Ok great idea! I’ll give that a shot and see what happens. I already pulled the tank, it was so full of rust, I needed to de rust the inside of it. So that is currently out.

If there were no fuel filters in the gas lines, the carb is most likely contaminated too.
Check the air filter. Over time they turn to crunchy dust.

Hey cjpayne!

I assumed the same. I rebuilt the carburetor and made sure everything was clean. It wasn’t bad at all. I replaced the air assembly as the old one was 80% missing lol

FYI, the main jet holder is also the emulsion tube. Those minuscule wall orifices can only be cleaned with a jet file, or thin wire, if any of them are clogged. No chemical cleaner can do the job, no matter how clean & shiny the tube may appear afterward. If you encounter weak throttle response, this is the moist likely/common cause.

Modern points don't like to be filed. They have for very many years been just plated,when you remove that they wont last. You can do it to see if it's the problem but plan on replacing them soon. It's hard to do on these engines but what I do on points is put them on the high point of the cam or hold them open with a small screw driver and spray contact cleaner on them. Then I close them and drag a piece of notebook paper thru them adjusting the drag of the paper with a screwdriver so that it doesn't tear leaving pieces in the contacts. Sometimes you will be surprised at the dirt that comes out on that piece of paper.

Would it be wise/safe to hook a lead from the Black wire (from the group of wires coming off of the flywheel). Than hold that lead to a ground on the motor and see if there is sparkage? Wouldnt that indicate that things are working properly under the flywheel cover? I understand points may need to be adjusted ect, but trying to trouble shooting simplest first.

Thanks all for the Help! I now have spark!
Well went and started testing around. I unplugged the HT lead and also unplugged the Primary Coil lead from the Harness. I than ran alligator clips from the Primary Coil output and just tapped the other end of the alligator clip to the motor and verified spark was coming from the alternator.

Ok next was the HT Coil
I ran a jumper with alligator clips from the Primary Lead coming out of the Flywheel and the HT Lead and grounded the the spark plug to the motor. Kicked it a few times and bam! SPARK!!

Now i knew both Coils were working and bypassing the main harness was working... Next was to check the kill switch on the handlebars...

For whatever reason, i thought, let me try everything together again before taking switches apart. I connected the main harness and HT coil lead plugged back in, and tested for spark... and of course SPARK! Kill switch was working as it should as i ran a few more tests.

I noticed when taking apart the main harness from the flywheel there was some debris in there.. my theory is somewhere along the line, one of the connections was not tight or the debris in the main harness was the problem.

Either way, i now have spark and am a happy camper thus far lol! If only my wife was as excited as i was when i told her i had great news... it sparks lmao

Glad you got spark on the bike.
As for spark out of the wife....that's a tough one.
I've got one that's had intermittent spark for 35 years.
Still haven't figured it out.

Spark!! Good.

X3 on that, confused by females, bandwagon.

wanrep said:

Glad you got spark on the bike.
As for spark out of the wife....that's a tough one.
I've got one that's had intermittent spark for 35 years.
Still haven't figured it out.

Click to expand...

LMAO ill join the bandwagon as well