Retrospective: 1977-1981 Yamaha DT125 MX

1977 Yamaha DT125 MX
1977 Yamaha DT125 MX. Owner: Don Carver, Creston, California.

The Japanese were selling a lot of “street-scramblers” in the late 1960s, but these were merely street bikes with upswept pipes. Yamaha, in particular, was advertising three twin-cylinder “scramblers” in 1968, the same year it brought out the DT-1 Enduro  250 single, soon followed by the AT-1 Enduro 125. That enduro nomenclature made a bike a little more serious about bad roads, but still, it was a compromise, doing neither street nor dirt extremely well.

Despite its imperfections, the 125 changed the world for a lot of Americans. With a gallon of gas in the small 1.8-gallon tank this little charmer weighed in at about 220 pounds, light enough that just about anybody could pick it up. When buyers scooped up all the AT units that first year Yamaha understood it was on to something profitable.

Move along to 1974, minor improvements were made, and model codes were changed. The bike was redesignated the DT125 — the DT now denoting all of the enduros, from 125 to 400. The 125 chassis was quite conventional, with a cradle-style tubular-steel frame, dual shocks at the rear and a pair of 18-inch wheels.

For 1977 a new version of the DT125 appeared known as the DT125 MX, instantly recognizable as it came with a single shock rear end, just like the Yamaha’s YZ Monocross racers. As they liked to say those many years ago, “That looks really cool!” Image has always been important in the motorcycle world, and this had great image. Like the racers it used a cantilever-style swingarm, with a long DeCarbon hydraulic shock running all the way to the steering head, under the gas tank. The lengthy damper proved to be excellent for shock absorption, allowing the rear wheel to follow the bumps and dips rather than bounce over them. A dose of nitrogen gas made sure the shock would not bottom out.

1977 Yamaha DT125 MX

Wheels were a 21-incher on the front with a 2.75 Yokohama trials tire, and an 18-incher at the back, with a 3.50 tire. The Takasago wheels each had a rim lock, a hint as to the expectation of a goodly amount of abuse. The five-inch drum brakes on both wheels were adequate in the dirt but rather weak when used on the pavement. The tubular frame cradled the engine/transmission, with a large backbone concealing the shock absorber. The subframe elevated the saddle to some 32 inches above the ground, the suspension allowing for 10 inches of ground clearance. The center-axle 31mm fork had 30 degrees of rake, five inches of trail, providing some seven inches of travel. Almost 53 inches ran between the axles.

The engine was semi-new, still with an oversquare 56 x 50mm bore and stroke totaling 123cc, but now with radial fins on the cylinder head for better cooling. A 24mm Mikuni slide carburetor using reed-valve technology fed gas and air into the crankcase, while Yamaha’s Autolube sent oil to where it should go. An aluminum sleeve fit into the cylinder, utilizing a five-port induction system, with a compression ratio of 7.2 to 1. Power was on the discreet side, with some 10 horses at 7,000 rpm, but that might have enhanced sales, as it was not enough to get into serious trouble.

1977 Yamaha DT125 MX

The Autolube oil container, holding a little more than a quart, was discreetly concealed behind the left-side panel, and once the panel was removed the reservoir could be swung out and refilled. A little light went on in the instrument cluster when the oil got low. The oil-injection system did vary the amount going into the engine depending on throttle load, which served to reduce oil usage as well as prevent fouling the plug.

To get rid of that troublesome need to occasionally set timing, as well as check points, the DT125 was blessed with a magnetically triggered capacitor-discharge ignition system, better known by its abbreviation, CDI. This benefited the engine by offering a quicker spark, reducing the possibility that any of that oil and gas mixture in the combustion chamber would foul the plug. The magneto also served to keep the small six-volt battery charged.

The exhaust system was well designed. Enduro bikes tend to fall over on occasion, and the idea is that the rider disentangles him- or herself, gets up, lifts the bike, pulls in the clutch, gives a kick and away they go. Presuming no damage to the header pipe or muffler. The DT125 header went up and back under the right side of the tank, and then crossed over to the muffler and spark arrestor on the left side, tucked away behind frame members. Very protected, very efficient.

1977 Yamaha DT125 MX

Getting power to the rear wheel was done via helical gears running the ponies back to a five-plate wet clutch and a very good six-speed transmission, where the top two gears were actually overdrive. A minimalist chain guard covered the chain, with sprockets having 15 and 49 teeth allowing for a solo rider to exceed the 55 mph national speed limit. The relatively comfy saddle was capable of seating two friendly riders. High fenders kept mud-collection problems away, and turn signals kept the feds happy, along with a speedo and tach, indicator lights and a horn.

And to ride? Fun! Within reason. Turn the petcock, pull the choke knob if cold, turn the key and kick to start. The little engine did best, of course, when a rider weighed less than 200 pounds, but it was happy scrabbling in the dirt. With a few minor changes this model lasted through 1981, after which two-stroke street bikes became illegal in the U.S. 

1977 Yamaha DT125 MX

2 COMMENTS

  1. Wow. I had a 73 DT. I would have guessed power was in the mid-teens, but I guess not . I recall an indicated top speed of 70+ mph with taller gearing. And the 7+ hp of my YSR 50 seemed like a LOT less. I guess my butt-dyno needed recalibration.
    Thanks for the article. Some fond memories bubbled up. Good times.

  2. My 1978 DT125 would run out of steam around 60 mph on pavement. I would have guessed 15 hp also, but not really surprised by 10. Neveer lacked for adequate power except when driven on pavement.

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