First timer with a CT90

L

LaVern

New Member
Hi all, First time here. I am retired and do a little restoring and reconditioning of motorcycles. I've been given a basket case CT 90 with vin # CT90-1475487. This makes it between a 66and 68 I believe. The wiring was all tore out and my hope was just to get it running. No lights, just ignition. AS I said it was pretty much just tore apart, no ignition switch, no headlight, etc. Anyway as I put it up on the rack I noticed that it looked like someone took a right angle grinder or some kind of tool and made a cut into the cylinder. In the pictures it is just right and a little lower of the valve cap. In the second picture I am pointing to it with a pen.I've worked on several bikes over the years but never a Honda with this style of engine. I am going to post some pictures of the cut and am wondering has this cut destroyed this engine? Thanks for looking and thanks for any insight you can give.
LaVern
 

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Hey LaVern,
sweet bike, I love those early CT90's, they are the closest thing to a "Cub 90 for the trail" like Honda envisioned.

A couple things to keep in mind for the bare bones wiring:
-You will still need a battery as these bikes use it for the ignition system.
-The white wire coming off the stator was a supplementary charging coil that was switched on via the headlight switch. This would increase the charging capabilities of the bike to compensate for the increased load of the headlight and tail light. Since you will have not headlight or taillight, you can either ignore the white wire and use the stock rectifier. Or upgrade to a regulator/rectifier like the ones from common motor collective and connect white and yellow wires together permanently.
-Once you have the charging system worked out it's an easy wiring job from there, positive of the battery to your ignition or kill switch, then ignition switch or kill switch to the ignition coil.
-Here is a wiring diagram for your bike in case it helps
CT90 K0 Wiring Diagram.png


As for the cylinder head, I think the K0s might have just been like that. At least that's what the pictures from these ebay listing indicate:
www.ebay.com

1966-68 HONDA CT90 TRAIL (KO) OEM CYLINDER HEAD VIN #CT90E-1025613 | eBay

Lots more parts for many different bikes.
www.ebay.com
www.ebay.com

1966-68 Honda CT90 CT 90 Trail OEM Engine Cylinder Head Camshaft | eBay

Good OEM Cylinder Head. No signs of excessive wear. Fins in good shape. In intake and exhaust port. Mount holes, threads and mating surfaces in good shape. Good lobes on camshaft. Rockers and valves in good shape.
www.ebay.com

Keep us updated on the progress with the bike! -Kyle
 
As an eBay Associate we earn from qualifying purchases.
Thanks for your reply. As for the wiring, I started this project a while ago, and I did some research and asked a few questions about the wiring I would need for the bare necessities and in particular answer I got back I was given a wiring basic diagram which I still have and I think I understand what is needed. Unfortunately my laptop that I had then quit on me and could not be fixed. So when that died, I lost my contact. So your explanation will come in very handy. I don't know why previously on searching this Forum didn't come up. I can see this forum will be a valuable asset to have.
As for the cut I did see one of the pictures that appeared to have a similar cut so maybe I'm alright in that regard. I guess maybe it was there to properly clean out the casting when manufacturing. I have this site in my favorites now so I will be in touch at least thru my postings.
LaVern
 
Use only a motor oil intended for a wet clutch motorcycle engine - any oil that is rated as "JASO-MA" meets the wet clutch requirement - Honda "GN4" oil is perfect. Modern automotive oils that are "Energy Conserving" or "Resource Conserving" on the bottles are too slippery for our clutch pack friction disks.

Rick
 
The hole is always there, all years.

I keep a spare coil around wired up and ready to use in situations like you describe:
IMG_20250310_181817996.jpg

Bare.minimum needed. Hopefully its visible enough to decipher.

Need a fully charger batt aince there is no charging circuit here. Red goes from battery + to coil power. Green wire taped up to red goes from battery - to metal mounting bracket of coil. Another green wire with clamp comes off the metal mounting bracket of coil and clips to head of engine. Last wire runs to the points wire. The little black box is a volt meter but optional.

If youre leaving your existing coil in place and grounded to the bike you dont need to run a separate ground clamp from the coil that clips to the head, but you do need to make sure you have a good reliable ground somewhere from your battery - to the bike.

Hopefully that makes some sense. Basically you just need power to coil, ground everything (points, coil), and points wire.
 
Joel,
Thank you. It is very clear and able to see all the detail. That hole is odd, but as long as it isn't an issue, I'm good. So using the harness you have pictured, you don't need to connect into the wires of stator, just need a good battery? But it would need to be charged occasionally? I just ordered, a new battery, an ignition switch and a new rectifier. The wires coming off of the old rectifier, were pretty bad with a bare wires coming off of it. The existing wiring harness shows sign of a short I think. The ground wire throughout the harness had melted insulation.

Next question. Since I just bought a new battery and rectifier, I should be able to use that to keep the battery charged, correct? The problem with my bike is the color code of the wires on the wiring schematics I have don't correspond with my stator. I was going to peal back a little bit more of the covering to see if closer to the stator the colors are clearer, but as they are now I can't determine what the colors are. Maybe by pealing the outer cover a bit the wires may be cleaner and will be clearer to see.
Thanks for you info and picture. I probably won't be working on it again for a week or so. Winter is leaving and I have a bunch of Spring work to get done. I will be checking my email to see if any more replies come in though.
LaVern
 
So using the harness you have pictured, you don't need to connect into the wires of stator, just need a good battery? But it would need to be charged occasionally?
Click to expand...
If your goal is just to get the engine running, as you'd stated initially, then yes. Stator and rectifier are only needed for charging.

I should also note that there is no on/off switch with the device that I pictured. Hook to battery power when you want to start the bike. Unhook when you're done testing. Don't leave it sitting and powering the coil all the time. Maybe that's stating the obvious, maybe not.
 
joel1234567 said:
If your goal is just to get the engine running, as you'd stated initially, then yes. Stator and rectifier are only needed for charging.

I should also note that there is no on/off switch with the device that I pictured. Hook to battery power when you want to start the bike. Unhook when you're done testing. Don't leave it sitting and powering the coil all the time. Maybe that's stating the obvious, maybe not.
Click to expand...
Thanks for that info. I do hope to get it running but also get the charging system fully operational with a fully functional key.
LaVern
 
LaVern said:
Joel,
Thank you. It is very clear and able to see all the detail. That hole is odd, but as long as it isn't an issue, I'm good. So using the harness you have pictured, you don't need to connect into the wires of stator, just need a good battery? But it would need to be charged occasionally? I just ordered, a new battery, an ignition switch and a new rectifier. The wires coming off of the old rectifier, were pretty bad with a bare wires coming off of it. The existing wiring harness shows sign of a short I think. The ground wire throughout the harness had melted insulation.

Next question. Since I just bought a new battery and rectifier, I should be able to use that to keep the battery charged, correct? The problem with my bike is the color code of the wires on the wiring schematics I have don't correspond with my stator. I was going to peal back a little bit more of the covering to see if closer to the stator the colors are clearer, but as they are now I can't determine what the colors are. Maybe by pealing the outer cover a bit the wires may be cleaner and will be clearer to see.
Thanks for you info and picture. I probably won't be working on it again for a week or so. Winter is leaving and I have a bunch of Spring work to get done. I will be checking my email to see if any more replies come in though.
LaVern
Click to expand...
Hey Lavern,

If you can't tell the color of the wires, you can use a multimeter to measure the resistance of the coils and figure it out that way.

Essentially just probe every combination of wires coming off the stator and match it up to this:

Highest resistance (around 1.8 ohms on my bikes): yellow-white

Second highest resistance(around 1.5 ohms on my bike): pink-white

Lowest resistance(about 0.3 ohms on my bikes): pink- yellow.

Depending on your multimeter and stator the readings might be a bit different, but the order should be the same.

Also, as a side note, the bike will run with no charging system, but if it's like mine, it won't Rev past 6k rpm due to low voltage. Just food for thought.

Hope this helps, - Kyle
 
LaVern said:
Thanks Kyle.
To check them do I just touch one wire, and to other end to ground?
LaVern
Click to expand...
No worries,

Use one probe of your multimeter on one wire, and the other probe on another wire. There should be no continuity to ground: if there is you have bigger problems.

Thanks - Kyle
 
Kyle,
I'll have to try figure that out this afternoon. When checking resistance does polarity matter? In other words, how do you know what wire you are reading on the meter? That may seem to be an eye rolling question, but I have not used a multi meter much, just checking voltage and continuity, not resistance values. Thanks for your help and information.
LaVern
 
LaVern said:
Kyle,
I'll have to try figure that out this afternoon. When checking resistance does polarity matter? In other words, how do you know what wire you are reading on the meter? That may seem to be an eye rolling question, but I have not used a multi meter much, just checking voltage and continuity, not resistance values. Thanks for your help and information.
LaVern
Click to expand...
No worries Lavern,

Polarity doesn't matter when measuring resistance of coils.

For example, you connect the positive probe to one mystery wire. Using the negative probe you measure the resistance two the other two mystery wires. If the readings are 1.5 and 0.3 (or around there.) you know the positive probe is connected to the pink wire.

Hope that helps! - Kyle
 
Hello all,
I don't know if this matters, but I started this posting with the Vin# CT90-1475487. Stating that I thought it was a between a 66 and 68. Well I got the headlight shell off today and it clearly states a Date of Manufacture as 4-72. Last evening I was searching and found the website http://www.cttrailbikes.com/modelchart.html and there was a listing of the CT90 vin # by year and it appears that earlier CT90s Had a 6 digit Vin where as in 1973 they start with a 7 digit #. I'm trying to see if the wiring and electrics are different as far as the stator goes. I don't think it matters in this regard. Please correct me if I am wrong. Below is a copy of the CT90 chart from that site and a picture of my frame tag.
CT 90 Vin # 3.jpg Thanks.
LaVern

​
 
Hey Lavern,

The stator wiring should be the same. The numbers i gave you were for a 71' CT90, so they should be accurate.

I think the only wiring that changed was for blinkers and maybe a horn.

I like the post 69 bikes the best, telescoping forks and selectable low range are a huge upgrade imo.

Best of luck! - Kyle
 
Thanks Kyle. I found a posting you put up about the stators on another section here and I think if I use that and what you have given me in this posting, I ought to get it figured out.
LaVern
 
Kyle,
As I said earlier I cannot make out the colors of my wires coming out of the stator. I have 4 coming out, one of the wires id just a plastic coating and it is Light green and a red stripe. I assume this is a ground?
I did the resistance checking like you explained, labeling the other 3 wires A,B ,C. My findings were:

A to B .4, A to C 1.8, and B to C 1.5. These numbers correspond to what you wrote on the other posting. My question is with these specs, what are the colors of A, B, and C? If I understood you these readings should tell me the colors, or at least tell me something.😁 But what? I just can't get my head wrapped around this. Thanks.
LaVern
 
The light green/red stripe is the neutral indicator wire, you should be able to see where it attaches to the engine case over the drive sprocket. Its sole purpose is to light the neutral light (in the speedo) when you put the trans in neutral.
 
dirtbkr188 said:
The light green/red stripe is the neutral indicator wire, you should be able to see where it attaches to the engine case over the drive sprocket. Its sole purpose is to light the neutral light (in the speedo) when you put the trans in neutral.
Click to expand...
dirtbiker188,
Thanks for the info.
LaVern
 
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