Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Honda ST90 (motorcycle), how dead is it?

So last year, I got an old 1973 honda st90 trail bike. It’s a beautiful machine, and great for running around town. I took it to the repair shop, they got it running, but when I was riding it, it died.

It felt like the drive disengaged, and the engine revved really high and slowly revved down, regardless of what I did with the throttle, till it died. Then it wouldn’t start again for anything.

I took it to a few places and they could get it running for a few days till it died again, and they were very noncommittal as to the cause.


Bikes don’t get much simpler than an ST90. You just need to find a shop able and willing to work on older bikes. They made a gazillion of these engines (I had one growing up) in various frames. If they can get it running, then it’s something as simple as a clogged up fuel system or an ignition issue.

If this was mine, I’d attach a spark check tool between the spark plug and spark plug wire. You can probably get one for $5 - $10. If it lights up when you’re trying to start the bike, then your ignition is probably ok and your fuel delivery is at fault. If it doesn’t light up on schedule, you’ve got an ignition problem.

Narrowing the search to the fuel system or ignition would be a big first step.

If you’re unlucky, the engine got damaged when it “revved really high.” You should probably do a compression test too.

If it spent a lot of time not being used with old gas in it, passages within the carb get varnished and plugged up.

Possibly a carburetor problem?