St 90

Asked this over in the pit bikes forum with no luck. Thought I might have better luck over here in the vintage section.

Acquired an ST90 without an engine and assumed a trail 90 motor would fit. I assumed wrong. I attempted to use a trail 90 motor in vain. The motor mount bolts are located differently. Can anyone tell me about this machine or a site with some info about these bikes. If it doesn't use a trail 90 motor what does it take? How hard is it to get the right motor? I really like the look and feel of this little bike and would like to put it into service. Please help.

I have tried 50/70/90/110/125 with no success.


According to the Honda Motorcycle Identification Guide, Honda made the ST90 Trailsport 90 from '73 thru '75, and it had an 89cc OHC single with a 3-speed automatic clutch. It seems weird that Honda would re-tool their horizontal engine program for one model. In the illustrations, the engine looks to be the same configuration as the 50/70/90. The frame serial numbers are as follows:

1973 - ST90-1000002 and up

1974 - ST90-1100012 and up

1975 - ST90-1200004 and up

Honda also made an S90 streetbike from '64 to '69, with a 4-speed manual clutch, along with an SL90 dual-sport, in '69 only. Those engine serial numbers are as follows:

64-69 S90 - S90E-100001 and up

69 SL90 - SL90E-100001 and up

Take a minute to list the frame serial number you have on the ST90, and also the CT90 engine you were trying to install. The CT90 engines in some years had an 8-speed dual-range transmission, which might explain a different engine mount configuration, due to case size.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now