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Old 02-22-2014, 07:43 PM   #1
RVeenster OP
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CT90 Engine and Beyond

So I was working on an AT1 about a week back (thread is called “AT1 Revival”) but after taking a good look a the head, crankcase and cylinder walls it just looked too bad to continue without turning it into a money pit.
The good news is that I got in touch with a good friend of mine who did an engine swap for his fiancee’s CT90. I asked him if I could buy it off of him and work on that since my AT1 was a bust. So now I’m working on this beut.
Engine has spark and compression. My friend told me that there was something wrong with the ignition timing so I’ll be taking a look at that as I take the engine apart. Other than fixing that my plans are to:

Replace all gaskets
Bore out to 104cc (54mm piston)
Racing cam swap
Replace valvetrain

My plans are then to find a frame and all the other bits (wheels, seat, electrical, etc) and have a running CT90 for the summer! See pics below for current progress. Just taking enough parts out (mostly timing related) to get the bottom end out for boring out the cylinder.















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Old 02-23-2014, 10:34 AM   #2
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Ready to bore out

So here's the engine with all the bits laid out and what I left on the block. Had to pull out all the timing stuff to get the bottom end out. You can see from the last set of pics that the cylinder wall is in great condition with plenty of wall left for a good bore out. I'll be hitting up a shop tomorrow to get that work done. Decided to leave the old piston and connecting rod on the crankshaft--less parts to keep track of!







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Old 02-23-2014, 02:33 PM   #3
NorthernTraveler
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Timing chain

That looks like the same size timing chain they use on the 200's.

On the 200's they are known to stretch the cam chain and the timing gets off resulting in loss of power.

Might be the same here, and a new cam chain is very cheap - so replace it.
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Old 02-24-2014, 04:58 PM   #4
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Originally Posted by NorthernTraveler View Post
That looks like the same size timing chain they use on the 200's.

On the 200's they are known to stretch the cam chain and the timing gets off resulting in loss of power.

Might be the same here, and a new cam chain is very cheap - so replace it.
I agree wholeheartedly. Cam chain and tensioner are both on their way along with a completely new valvetrain!
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Old 02-24-2014, 10:30 PM   #5
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The valve train in these little engines is pretty robust and rarely fails. it only has one valve seal. Even so you can easily check the specs on the valves and springs. The timing chains take a lot of miles before showing any real wear. The timing issue could be a sticking advance mechanism, a cracked plug wire, or even a bad battery. I've experienced all these issues while sorting out one of these bikes.

I built a 104cc longer duration cam engine last summer and I wish I had split the cases and balanced the crank. It was never an issue before the rebuild but she spins a bit faster now and has a bed vibration from about 45 up to 50 then it quiets back down. I kept the stock k0 carb and it works fine with the original jets. I also did a mild port and polish.
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Old 02-26-2014, 10:20 PM   #6
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The valve train in these little engines is pretty robust and rarely fails. it only has one valve seal. Even so you can easily check the specs on the valves and springs. The timing chains take a lot of miles before showing any real wear. The timing issue could be a sticking advance mechanism, a cracked plug wire, or even a bad battery. I've experienced all these issues while sorting out one of these bikes.

I built a 104cc longer duration cam engine last summer and I wish I had split the cases and balanced the crank. It was never an issue before the rebuild but she spins a bit faster now and has a bed vibration from about 45 up to 50 then it quiets back down. I kept the stock k0 carb and it works fine with the original jets. I also did a mild port and polish.
Yeah, I'm sure the valvetrain is solid in these bikes. I just know that it's all stock and so I'm starting over just in case.

You were able to keep the stock carb even with the bore out? I would think you would need a bigger carb to compensate for the increased displacement. That's why I got a bigger carb to go with it. Sad cuz I'd really like to keep the altitude adjustment being a mile above sea level. :(
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Old 02-26-2014, 10:30 PM   #7
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So just a small update before I put some real work in this weekend. Put some want ads up for a new chassis. Got in touch with a nice guy who has a lot of cool little vintage bikes that he's been working on. His garage was quite impressive to say in the least.

Anyways got the chassis in for $325. Condition looks pretty good. It's actually one of the older CT200s, but everything should fit in alright (hopefully!). The front and rear wheels don't quite spin true so I'll probably need a new cush hub and clean out and re-grease the wheel bearings. Just little things here and there to get this chassis in good working order before I mount the engine to it.

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Old 02-27-2014, 03:33 AM   #8
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Found a CT-90 at a garage sale a few years ago ( $100 ) one of the things that was preventing it from running was that some one had put the advance together 180 degrees off. Once I changed it and went with a 10 thou over piston and new rings it ran great. The grandkids love it when they visit. You can get parts at Beatrice cycle ( the ATV doctor.com ) pretty cheap.
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Old 02-27-2014, 04:53 AM   #9
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That year CT90 frame didn't have a hi/low transmission. Notice the rear sprockets. C200 is push rod. Your engine is OHC. That engine may not be a direct bolt on to that frame. Some of those frames had OHC/non hi/lo engines(like a S or CL90)I think the way you are going about this will be a huge money pit. I've bought non running models for $100 to $400. They are more complete.Even going through Beatrice, everything will add up fast.
I bought 2 non running CT90's that were complete in good shape for $700 total. I spent $300.00 on battery,cables,tires, chains,brakes, (front sprocket is around $60.00),etc. It ran great. I gave it to my brother for his birthday. He has a few 90's & 70's.
You will have $1000-1500 in your bike going the piecemeal route
If stay on this course, split the cases. Trash can accumulate in shifting plate(?). The engine wont shift. I had an engine sit 20+ years. cranked on the 2nd or 3rd kick. It would not shift. Warmed up the oil. Changed it twice. Still no shift. Had to split the cases.





Good Luck,
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Old 02-27-2014, 10:18 AM   #10
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I've got a 77 and a 78; both running. Wanna buy one or both ?
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Old 02-27-2014, 10:44 AM   #11
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Originally Posted by oldxrider View Post
Found a CT-90 at a garage sale a few years ago ( $100 ) one of the things that was preventing it from running was that some one had put the advance together 180 degrees off. Once I changed it and went with a 10 thou over piston and new rings it ran great. The grandkids love it when they visit. You can get parts at Beatrice cycle ( the ATV doctor.com ) pretty cheap.
Absolutely, Dr. ATV all the way. Been getting all my parts from him.
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Old 02-27-2014, 08:46 PM   #12
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Originally Posted by lesman View Post
That year CT90 frame didn't have a hi/low transmission. Notice the rear sprockets. C200 is push rod. Your engine is OHC. That engine may not be a direct bolt on to that frame. Some of those frames had OHC/non hi/lo engines(like a S or CL90)I think the way you are going about this will be a huge money pit. I've bought non running models for $100 to $400. They are more complete.Even going through Beatrice, everything will add up fast.
I bought 2 non running CT90's that were complete in good shape for $700 total. I spent $300.00 on battery,cables,tires, chains,brakes, (front sprocket is around $60.00),etc. It ran great. I gave it to my brother for his birthday. He has a few 90's & 70's.
You will have $1000-1500 in your bike going the piecemeal route
If stay on this course, split the cases. Trash can accumulate in shifting plate(?). The engine wont shift. I had an engine sit 20+ years. cranked on the 2nd or 3rd kick. It would not shift. Warmed up the oil. Changed it twice. Still no shift. Had to split the cases.





Good Luck,
Les
Yeah, I'm beginning to see some differences that may provide some future entanglements. It has been pretty steep financially to be on the "a la carte" method of putting a bike together. But I'm optimistic: the engine I bought was from a good friend and it seems like there will be little to no issues once I finish the overhaul and rebuild *fingers crossed*. Worst case, the engine won't mount to the frame and I'll just sell the engine to cut losses and look for another engine to better fit that frame. There are options with these bikes!
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Old 02-27-2014, 08:47 PM   #13
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Originally Posted by acejones View Post
I've got a 77 and a 78; both running. Wanna buy one or both ?
Man, I'll let you know if I can't mount this puppy up! I'm in CO though, are you on the coast?
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Old 03-01-2014, 07:56 PM   #14
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Originally Posted by RVeenster View Post
Man, I'll let you know if I can't mount this puppy up! I'm in CO though, are you on the coast?
We're pretty far apart. I'm on the Mississippi Gulf Coast.
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Old 03-03-2014, 05:52 PM   #15
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Not much work done these past few days. Bad weather and just no time. Was able to relocate the frame, engine, and all other parts to my buddy's garage--it's super full of stuff so we'll be clearing it out for more work space.




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