In the early 1970s, Honda introduced the ST90 (“Sport Trail” or “Trailsport”) as a machine that could split the middle between their minibikes and full-size street-legal enduro machines. Like the models of the closely-related CT series, the ST90 had a stamped steel backbone chassis, a horizontally-mounted four-stroke engine, and road-legal classification. However, it was a more aggressive model from the regular CT mounts.
“Unlike its sibling the CT90, the ST90 was developed with more focus on off road riding. It has engine protection cradles, more ground clearance, and higher front and rear fenders.” –Silodrome
The 89cc SOHC engine was paired with a three-speed semiautomatic transmission and produced around 6 bhp. That might not sound like much, but owners loved these bikes — the little Hondas that could.
“ST90’s are very fun bikes. Mine would do around 52-53 mph with my 230 lbs on board. My nephew says he saw 60 mph on it, and I don’t doubt him. He was only about 170lbs at that time.” -Member HandKPhil, ADVRider
That’s not too shabby for a 6-hp minibike rolling on 14-inch wheels! Alas, the ST90 only survived until 1975, as poor sales caused Honda to pull the bike from the market. That makes it one of the rarer models among vintage Honda minibikes today. This hot-rodded ’74 ST90 comes from our old buddy Matt Wieckowski of Ontario’s Big Dream Motorcycles.
“A friend of mine bought his dream bike but the engine was tired so he wanted something with a little more pep and also a little more practical to ride on his farm.”
As the old motor was in poor condition, Matt and his buddy decided to swap in a 140cc Piranha electric-start engine that would increase the power output by 250% (~15 bhp). It would be no easy feat, however, as the Piranha motors were designed as a straight swap for the 70cc CT models.
“The 90 is only a two-mount motor as opposed to the 70’s three-mount. Adapter plates were made and fit to make sure engine would sit in the same spot as the original.”
Matt rewired the bike to accommodate the 12-volt electrical system and e-start, and rounded out the build with new bars, lights, performance exhaust, off-road tires, and more. Importantly, none of the modifications involved chopping or welding, so the bike can be returned to original condition in the future.
“This was an important point for both myself and the customer. The end goal was to give it a little leaner, meaner look to go along with the boost in power.”
We love this little hot-rod, and you can see it yourself this weekend at The Grand Bend Festival of Speed, which Matt puts on at the Grand Bend Motorplex in Grand Bend, Ontario. There will be classic motorcycle racing, a vintage car rally, a sidecar showdown, and tons of classic and custom bikes and cars on display.
Most of the Big Dream Motorcycles will be in attendance, as well as Lawrence Hacking’s Honda XR650R Paris-Dakar Rally bike. If you’re in or around Ontario, we highly recommend you make the trip!
Below, you’ll find our full interview with Matt about this Honda “ST140” mini-monster.
140cc Honda ST90 Custom: Builder Interview
• What’s the make, model, and year of the donor bike?
1974 Honda ST90.
• Why was this bike built?
A friend of mine bought his dream bike but the engine was tired so he wanted something with a little more pep and also a little more practical to ride on his farm.
• What was the design concept and what influenced the build?
The ST90 is one of those bikes (along with CT70’s and Z50’s) that everyone either owned or knew someone who owned one. A very iconic Honda minibike. Unfortunately, the old engine was showing its age so a new Piranha 140cc engine was fit in to give it a new lease on life. The goal was to retain as much of the original look as possible whilst also freshening it up a bit.
None of the modifications affected the original bike so at any point it can all be put back to original condition. This was an important point for both myself and the customer. The end goal was to give it a little leaner, meaner look to go along with the boost in power. I think we achieved that balance nicely
• What custom work was done to the bike?
A Piranha 140cc electric start semi-automatic was fit. This was no easy feat as although the 70 and 90 motors are very similar, the 90 is only a two-mount motor as opposed to the 70’s three-mount. Adapter plates were made and fit to make sure engine would sit in the same spot as the original. The bike was also completely rewired to accommodate the 12V system and new electric start system. New bars and headlight along with some nice and grippy off road tires.
• Does the bike have a nickname?
ST140.
• Any idea of horsepower, weight, and/or performance numbers?
Approx 15hp. Which is a big jump from the original advertised 6hp. The weight is probably in the original 180lb range. Any of the saving from the parts taken off are likely offset by the e-start engine and parts.
• Can you tell us what it’s like to ride this bike?
This bike flies lol!! It will easily pull the wheel up all the way up to third gear. Very fun bike to play around on.
• Was there anything done during this build that you are particularly proud of?
Fitting that engine in was no easy feat. I’m certainly quite proud of (and I guess you could call it my style) toeing the line between custom and original. These bikes are pretty gorgeous on their own, so it’s important to me to try to keep that original styling.
• Is there anyone you’d like to thank?
A huge thanks to PhatMX here in Canada and also to Bart Moto Co. in the States for the parts.
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Facebook: @bigdreammotorcycles
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Photo credits: Red Button Photography
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