The CT90 winter project

Intentions were to do some work over the winter but things got in the way and the little CT 90 project got shoved into the corner. I bought it last fall for $600 and it ran a bit but seemed really weak. I tried to start it a couple weeks ago with no luck. Stuck a new battery on it, had good spark and was getting fuel. Checked the compression and dammit - 60psi. It needs 150 -175 to run.

So yesterday, I popped the head and jug. Dammit again. It looks like the jug had water in it at some time. There are a lot of pits and some scoring in the bore. The piston shows a lot of abuse as well. The head is carboned up but with a cleaning, it should be good. I'll probably lap the valves and throw in new valve guides while I'm in there.

A company called Dratv can bore the jug from 50mm - 54mm and supply new piston and rings for about $100 or a new set is $200. I want to keep it as cheap as possible so will send out to see if it can be bored. Pic below:

[LakeCleElum]

Piston is Ruff......Looks like it was run low on oil......

As a kid, I tried to overheat and blow those up by over-revving.....They were indestructible........

[Scott Douglas]

If you really want to keep it cheap, sand that piston down some, re-use the rings (they don't look bad) and get your self a bottle brush hone and hone that cylinder till it shines.
What's the worst thing that can happen?
You have to take it apart again to do the re-bore etc.
What's the best case scenario?
It'll run, might use a bit of oil, but will probably be OK once all the parts get familiar with themselves after a while.

Originally Posted by peteremsley

I think this motor is still used in some foreign bikes. There may be a super cheap Chinese piston/cylinder kit you can get. Someone I work with just rebuilt one of these, and I think that it was his route. I'll check tomorrow/tues.

Cool - Thanks

Originally Posted by Scott Douglas

If you really want to keep it cheap, sand that piston down some, re-use the rings (they don't look bad) and get your self a bottle brush hone and hone that cylinder till it shines.
What's the worst thing that can happen?
You have to take it apart again to do the re-bore etc.
What's the best case scenario?
It'll run, might use a bit of oil, but will probably be OK once all the parts get familiar with themselves after a while.

Thanks for the suggestion - the pits are really deep. I think it will take 2 or 3 mm to get it smooth.

[Eric Coffey]

FYI:

Parting Out CT90 Engine

Originally Posted by Eric Coffey

FYI:

Parting Out CT90 Engine

Good catch! Thanks

[ckelly78z]

Originally Posted by Scott Douglas

If you really want to keep it cheap, sand that piston down some, re-use the rings (they don't look bad) and get your self a bottle brush hone and hone that cylinder till it shines.
What's the worst thing that can happen?
You have to take it apart again to do the re-bore etc.
What's the best case scenario?
It'll run, might use a bit of oil, but will probably be OK once all the parts get familiar with themselves after a while.

With the exception of re-using the rings, I have done this on several motors with good luck. I would reccomend a 3 stone spring loaded hone on the end of a drill....move slowly down through the bore, and slowly back up while running the drill, this will get a good cross hatch section onto the bore which is needed for re-seating new rings. After you have done this, clean the piston ring landings, and install one new ring at a time, and insert the piston into the bore upside down to measure the ring gap (file ring ends as needed).

This also a good time to polish the crank, and install a new rod bearing, and inspect your piston rod pin for wear.

[vash]

i called dibs on my elderly neighbor's bike.

i dont know squat about them, except i want one. he said his has a "low range"..for serious off roading. his is damn near in timewarp stock condition..with all the normal light wear and tear.

[motion]

They are cool bikes. I see that you have the aux fuel tank, which is pretty rare. Good luck with the rebuild!

I have a '72. Here is my FIL riding it in Montana. It has around 1200 original miles.

[unclebilly]

My dad had one of these when we were kids... it was the back up when our XR-100s were broken down. Kind of like the back up bike on the Top Gear Viet Nam special... or any of the back up vehicles on the old Top Gear for that matter...

He sold it to a friend of mine in later years... we were riding on the beach near our house on Vancouver Island. I flew through the really soft silt, my friend with the CR-125 flew through, my brother flew through, and then we all watched as Aaron (on the CT-90) got stuck half way through and buried it up to the shocks... Eventually we stopped laughing and helped extract his bike.

[LWJ]

I was fly fishing in the absolute middle of nowhere South Eastern Oregon with my FIL. A group of three on C-90's whizzed by with rod holders somehow attached to the stem.

This was 10-15 years ago.

I still think about what a perfect way that is to cover a lot of ground.

Enjoy!

[LakeCleElum]

No need to cheap out using the old piston or buying Chinese crapI've had terrible luck with China parts):

https://www.ebay.com/itm/HONDA-CT90-S90-STANDARD-PISTON-KIT-MADE-IN-JAPAN-RINGS-INSTALLED/182180857378?hash=item2a6ad34622:g:9tcAAMXQpwFRbMl c&vxp=mtr

Originally Posted by peteremsley

He says he sourced most of his parts from DrATV OPERATES HERE and was very happy!

Cool - thanks