Without a doubt, the Honda ST90 is one of the most underrated models of the Honda CT series. Produced for only three years, this motorbike, fondly called the Might Dax (marketed in the U.S. as the TrailSport™), succeeded the highly acclaimed CT70 lineup. Portability, reliability, and off-road fun are just some of the outstanding qualities you will discover about the ST90 in this guide.
The Honda ST90 was an 89-cc minibike produced from 1973 to 1975. Successor to the celebrated Trail 70 series, this two-wheeler featured the same Dachshund-like chassis, retro styling, foldable handlebars, and offered a 3-speed semi-automatic transmission.
Despite its second-fiddle popularity, the Honda ST90 is well-loved for its non-discriminating size and beginner-friendly functionality more than its Dax likeness. These attributes may not have made the pocket bike a huge hit back in the day. But they have successfully earned the ST90 its loyal, rock-solid following that still raves about the bike’s simple offerings.
About the Honda ST90 TrailSport™
The Honda ST 90 is a slightly bigger version of the CT minibike series introduced by Honda in August 1969. The 89-cc Dax descendant, in particular, was produced from 1973 to 1975 and was not given a CT designation despite the lineage. Instead, it was sold exclusively in North America as the TrailSport – quite different from what most consumers assumed to be the meaning of the letters “ST” (Sport Touring) in the motorbike’s nomenclature.
Generally, the ST90 pocket bike has a bench-style saddle, Dachshund-like frame, and folding handlebars characteristic of the CT series. Likewise, it shares the same air-cooled 4-stroke OHC engine and a 3-speed semi-auto gearbox with an automatic centrifugal clutch assembly as the DAX system. The two-wheeler’s distinct T-bone frame help distinguish it from the more popular Honda minis such as the Z50 Monkey & Gorilla and the CF/CY Nautydax bikes.
Its 14-inch wheels and slightly bigger power mill make the ST90 appear like an older brother of the Trail 70. More importantly, these large-diameter wire-spoke wheels (paired with the bike’s cradle-down tube) make the Honda ST90 a properly proportioned motorbike with an aggressive look and novice-friendly mannerisms – perfect for in-training young riders and older kids approaching off-roading proficiency, and earning it the moniker the “Mighty Dax.”
1973 Honda ST90 Specs & Features
Engine
The ST90 is brought to life by a single-cylinder, 89 cm³ (5.4 in³) air-cooled OHC power mill. Bore-stroke ratio is 50 x 45.6 mm (1.969 x 1.795 inches), while compression ratio is 8.2:1. A 17-mm Keihin carb with a 40-mm air filter mount handles air-fuel mixture and can upgrade to a PZ22 aftermarket carburetor for better top-end performance.
Overall, this engine configuration lends to a bone-stock top speed rating of 35 mph or 56 km/h (low-speed pass with limiter) and 50-80 mph or 80-129 km/h (high-speed pass sans limiter).
Fuel & Lubrication
Fuel tank capacity is 2.6 L/0.7 USgal (inclusive of 0.1-USgal reserve) of regular gasoline with a minimum PON 86/RON 91 rating. Lubrication-wise, the manufacturer recommends high-detergent, premium-quality 4-stroke oil or its equivalent.
Engine oil should be an SAE 10W-30 or 10W-40 and have at least an API-grade of SJ meeting JASO T903 MA standards. Other viscosity grades such as SAE 30, 20/20W, or 10W are permitted following ambient temperature.
Drivetrain
A semi-automatic, 3-speed transmission (with a left-foot-operated return system, N-1-2-3 shift sequence) and an automatic centrifugal clutch assembly deliver power to the wheels. A 428H O-ring chain (view on Amazon) with 94 links plus joint handles wheel spin. The pocket bike’s wide-ratio transmission makes for predictable handling and stability.
For reference, here are the stock gear ratios for the 1973 ST90:
| Primary Reduction Ratio | 3.722 |
| Transmission Gear Ratio – I | 2.538 |
| Transmission Gear Ratio – II | 1.647 |
| Transmission Gear Ratio – III | 1.045 |
| Final Reduction Ratio | 2.666 (15/40T) |
Ignition
It has a Contact Breaker Point ignition system requiring an NGK D-6HS/Nippon Denso X20 FS spark plug (with a gap of 0.6-0.7 mm or 0.024-0.028 inch). It is wired for 6 volts, is fitted with a 6V 5.5 AH/(10 HR) battery, and has an A.C. generator with a rated output of 0.049 kW @ 8,000 RPM serving as its charging system. The manual doesn’t specify the exact battery format required for the ST90 to run.
Convert its wiring harness to support a 12V electrical system with a corresponding regulator/rectifier and stator upgrade. Doing so will enable you to use a 10 HR YTZ7S battery with 113 x 70 x 105 mm dimensions. A CTEK 12-Volt Battery Charger (view on Amazon) will also help extend the longevity of your new 12V battery.
Tires & Brakes
Stock tires consist of 3.00-14 (4PR) front and rear tires mounted on wire-spoke rims. Recommended cold-tire pressure for the front and rear are 124 kPa (1.27 kgf/cm2, 18 psi) and 165 kPa (1.69 kgf/cm2, 24 psi), respectively. Brake type and specs are also unspecified in the manual, but images show the ST90 uses drum brakes at the front and back to complete its tire-and-wheel assembly.
Suspension
The front suspension consists of Ceriani forks, while the rear has a swingarm, and the wheelbase is 1,170 mm (46.1 inches) – almost six (6) inches longer than the CT70s. There are no details of minimum ground clearance or turning radius in the service manual. But given that the ST70 and ST90 share the same design, it would be safe to say ground clearance is at least 165 mm (6.5 inches) while turning radius is at least 1.6 meters (5.2 feet).
Dimensions
Overall dimensions are 1,755 x 760 x 1,030 mm (69.1 x 29.9 x 40.6 inches – L x W x H), making the pocket bike longer but narrower than its counterpart, the Polaris Outlaw 90. Seat height is approximately 813 mm or 32 inches. Dry weight is 86 Kg (190 lbs.), while load limit capacity is 100 Kg (220 lbs.).
Not only can the ST90 carry heavier kids, but it can also accommodate older riders. Overall, the TrailSport is portable and more robust than most of its competitors.
Exterior
Enclosed in a cradle-down tubular steel frame (62° caster angle, 67 mm trail) are the ST90’s crankcase, internal Honda ST90 parts, and plastic body panels in a color scheme that takes after the CT70 K0-K3 versions. All models had a black saddle, 6V headlights, turn signals, and tail/brake lights.
They also come with rearview mirrors and a speedometer (view on Amazon). Factory hand grips, serrated footpegs, and up-type fenders add to the rugged appeal of the bike.
Honda ST90 Value
Here is a full rundown of resale values for all 89-cc Honda ST90 models released between 1973 and 1975:
| Year – Trim – Model Number | List Price | Retail/Trade-In Values |
|---|---|---|
| 1973 Honda ST90 | N/A | $480 – $1,990 |
| 1974 Honda ST90 | $480 – $2,500 | |
| 1975 Honda ST90 | $475 – $1,945 |
Pre-owned TrailSports keep their value quite well, with resale pricing ranging from $1,200 to $4,000. Several of these units are available on Craigslist and other well-known trader sites. Expect little to no cosmetic and mechanical damage for units priced over $1,500, and some of them may even come with a few aftermarket upgrades, a converted electrical system, and new lighting.
Machines sold for parts are rare. If you are looking for a near-mint Candy Yellow Special trim (fondly called the Big Bird), be prepared to pay a handsome price.
Pain Points of the TrailSport™
Starting Problems
The kick-starter for secondhand ST90s is reportedly problematic. It kicks with the spring pressure but does not necessarily turn the engine over. Several factors could cause this to happen:
- Fuel starvation
- Congested lines/filter/drain system
- Bone-dry or maladjusted clutch plates
- A fouled spark plug
Check the spark first by visually inspecting the adjuster and stock flywheel/stator assembly. The latter is a wet setup, so expect oil to leak out when removing the flywheel cover.
As for the plug, there are cases where even new plugs turn out filled out with soot even after just a few minutes of test riding. When this happens, you need to look into accompanying symptoms like smoke, as these indicators will make a difference in how your diagnosis turns out. You may have a weak spark attributed to bad points/condenser, worn rings/blow-by, too much oil in the system, weak coils, or an incorrect plug gap.
Other recommendations to address starting problems include a clutch adjustment and engine oil change to a variant compatible with wet clutch engines that meets JASO T903 MA standards. Never use engine oil labeled as Energy Conservation or Resource Conservation I/II. Or you may only need to balance the air-fuel mixture and use hotter plugs in your two-wheeler.
Lack of Transmission Options
Many enthusiasts felt it was a shame that the ST90 only offered a 3-speed gearbox, given that it rides smoothly and provides good balance. Good thing mechanics nowadays are becoming more adept with transmission rebuilds.
According to these experts, you can add a sub-transmission to an ST90 – one way is using the ATC90 power mill as a donor and doing a bit of machining. These steps allow the ST90 to have a sub-transmission while keeping its auto clutch setup. Utilizing a CT90 engine is another option. However, its motor mount has a different bolt pattern than the ST90’s so converting the transmission may not be straightforward.
Rarity of Parts
For the most part, the Honda ST90 has all the basic components you need to build on. The biggest challenge it poses is, perhaps, the non-interchangeability of parts with a CT70 or CT90. This makes performing restomods more complicated than it already is. Expectedly, this is one of the reasons some enthusiasts veer away from owning this classic mini bike.
About Honda
An institution in the motorcycling landscape, the Honda Motor Company Ltd. is one of today’s most successful global automotive manufacturers. The Japanese firm evolved from automotive parts and motorized bicycles with surplus engines to mass-production of automated military aircraft propellers and race cars, alongside venturing into other industries.
The maker of the Honda ST90 is recognized worldwide for spearheading innovations in electric automobiles, motorsport vehicles, and energy solutions.
Conclusion – Honda ST90 Review
The Honda ST90 may not be the creme de la creme of collectible, vintage pocket bikes, but it is getting there – and fast. Its practical features, simplified transmission, and nostalgic exterior continue to grow on both regular consumers and enthusiasts.
Sure, there are newer, more tech-advanced minibikes where youngsters can practice on. But for serious, in-training off-roaders, nothing beats learning how to ride on this classic, 3-speed two-wheeler.
Kris is an avid off-roader and outdoor enthusiast who loves to brave the elements and take on challenging terrain. He also enjoys sharing his passion and knowledge with others so that they, too, can appreciate the ride.
About Kris