Doubled Displacement – DT100-Powered 1988 Yamaha YSR50

In Japan, Less than 5k, Small Displacement by Tim HuberLeave a Comment

Produced from ’87 to ’92, Yamaha’s YSR50 was a mini sport bike that offered many amenities typically reserved for larger displacement models. Because of this, the full-faired 50cc scoot frequently had its engine swapped-out for something more powerful as the bike’s stock sub-40mph top-speed really left something to be desired. This particular example has had a 100cc Yamaha enduro engine dropped into its double-cradle frame. The YSR is normally powered by an air-cooled 49.3cc two-stroke engine that produced a claimed 7.1 hp and 4.27 ft-lbs of torque at 8,500 rpm. Paired with the 1/20th-liter engine was a five-speed gearbox.

Yamaha’s DT100 was a trail-riding machine built from ’74 to ’80. Powering the Japanese enduro was an air-cooled, 97cc, two-stroke single that reportedly made 10 hp at 7,500 rpm and 6.51 ft-lbs of torque at 7,000 rpm. The DT’s dry weight was only 40 lbs heavier than the YSR, so replacing the mini-sport’s 39 lb engine probably wouldn’t add very much weight though it should make a noticeable difference in power.

The added power isn’t so much that it will clash with the YSR’s short 41.4-inch wheel-base, but it will definitely add oomph to the little thing. This particular example also has its newly swapped engine registered, which is pretty rare, and even more importantly a kickstarter was added to the YSR so bump-starting it is no longer necessary. The seller claims to have been working on YSR’s for the past 20 years and over that time he has been amassing a collection of the best pieces from two-decade of YSR projects – all of which reportedly have been used on this build.

In addition to the DT engine, this YSR has been fitted with a Daytona tach, JDM tailights, an adjustable steering-damper, adjustable FOX racing shock, dual fork springs, stainless steel brake-lines, Hypercycle triple tree, Team Calamari fork brace and swing arm brace, aluminum clip-ons, 1/4-turn race throttle, Answer Racing “Roost Boost”, Toomey Racing pipe, and a new battery underneath a keyed locking battery cover. The shift linkage and kickstart lever were both custom fabricated for this YSR project, and the entire bike is sitting on Douglas Racing disk-wheels wrapped in Dunlop rubber.

The YSR was offered in a white and red livery for its entire production run, though Yamaha did produce a rarer black and yellow paint scheme in ’91 and ’92. The even rarer colors are the white and blue livery – offered only in ’89 – and the equally rare blue and yellow paint scheme which was offered only in ’88. Though it’s got dents and dings, it’s pretty cool that this example is adorned in the one-year-only blue and yellow paint digs. Also, all the lights and signals on this example reportedly work perfectly.

The odometer on this example currently reads 4,331 miles, though I’m unsure if it was reset when the DT power-plant was added, plus who knows how many miles said DT had on it prior to being dropped into the YSR’s double-cradle chassis. The bike has a license plate in the photos and the ad says the title is clean, but the plate has a 2012 sticker as it’s been registered non-op. In addition to the DT-powered YSR, the sale includes the YSR’s original numbers-matching engine. It’s not perfect, but I really dig little bikes like this.

You can find this DT100-powered 1988 Yamaha YSR50 (VIN: JYA2RRA04JA013956) for sale here on Craigslist in Salinas, California with a price of $2,600.