For old timers: 1975 Honda ST90 question

[Aviator]

I have a 1975 Honda ST90 with a broken crank. I need to buy a new engine. What motors will fit on a ST90 frame? ct70,ct90???
I am having trouble finding a ST90 motor..

 

[karlp]

I think the only big difference between the ST and CT 9os' was the routing of the exhaust pipe. The engines were the same. I wouldn't be surprised if an xr series cases weren't the same. You might find that an xr 70 motor would bolt right in! Sounds like a fun project. I had a lot of fun with those bikes.
My second bike was an CT 70 that I broke the head on. If memory serves, I put an st 90 cylinder and head on it and rode it a few more years.

 

[woodsy]

HEY WHO YOU CALLIN OLD? :)
Yea those ST/CT90 cases match up with lots of stuff but I am not sure about the 70 series. If remember correctly the 50 and 70's matched. The 90's - 110 and 125's matched (ATC, ATV series - NOT the uprights).
A crank for that motor should be REAL easy to find. Want me to see what I have laying around?
Woodsy

 

[junkjeeps]

I think woodsy is correct. The atc 90, 110, 125 were same bottom end. Only difference was cylinder bore and carb. I may have an old atc 90 bottom end laying around. I'll look and let you know.

 

[OnAnySunday]

I agree -w- woodsy also.
Same basic design on all (or most anywhos) Honda's semi-horizonal thumpers.
In fact i saw a ATC70 on ebay a few months ago that was retrofitted -w- an ATC125M engine bored out to like 150cc's or sumpthin.
Supposedly geared to hit almost 65mph! :laugh:
Too kewl, a mini-hotrod! :thumb:

 

[Aviator]

Thanks for the help. I have to start searching EBAY for a motor so I can get the ST90 running. It has been in the family for over 25years so for sentimental reasons I am going to get it running. If anybody has any leads on parts or a motor please email me.
John
Schust5@wideopenwest.com
630-416-6882

 

[Tony Williams]

I had the '74 ST-90 that I bought new as an almost 12 year old. $495.

The CT-90 has a dual range (hi/low) transmission with a four speed. The ST-90 has only a 3 speed, no hi/low.

Sorry, can't help find a crank, unless you want to go up to the junk yard at my parent's house in Montana and get it. Also, I know of several CT-90 wrecks that my uncle has, also in Montana.

My dad still rides a '70 CT-90, but don't think he's ready to part with it (about 1600 total miles since new).

 

[burtcs]

If anyone is watching this thread I have a clutch question. I have a k2 ST90.

I installed new clutch pressure plates and after adjusting the clutch per Honda 1/8 to 1/4 turn, about all that happens is the engine stalls.

Is there a trick?

regards - Steve B.

 

[whenfoxforks-ruled]

Aren't the sl series also the same.