Retrospective: Yamaha YG-T Trailmaster 80: 1964-1969

Lest we forget, it was the advent of rather small Japanese motorcycles that affected the two-wheeled revolution in the United States, quadrupling registrations between 1960 and 1969. And this Trailmaster 80 was part of that success, a very popular little item. Remember the old ad, “You meet the nicest people on a Honda”? The same could be said of Yamaha, as Americans were in the mood to buy small, non-threatening, inexpensive motorcycles on which to ride in town, around college campuses, through the woods—just about anywhere.

The Trailmaster 80 had a peppy little 73cc engine…which was advertised as an 80, as that number sounded better when up against the Honda 90. It had four speeds, mildly knobby tires and chrome panels on the tank…and all the bodywork was metal, not plastic. With a 1964 sticker price of $365, this little critter appealed to the economically minded as well as those who enjoyed tearing along the fire trails in the millions of acres of state and national forests that cover this nation.
The major news concerning this YG1 Rotary Jet 80, as the two-stroke engine was officially known, was that it used a rotary disc-inlet valve, with the 15mm carburetor bolted on to the right side of the crankcase, not the cylinder. It was also advertised as being fully enclosed, not exposed to those nasty outside elements. Simply put, the great advantage to rotary valving was that there was no overlap, which happened with port timing when a small portion of the fuel mixture escaped unburned through the exhaust port. With rotary valving, the full charge was compressed and ignited. This new valving was not a secret, having been used by racing motors in the ’50s, but the YG1 may have been the first street bike to enjoy its use. The rotary technique created a more responsive engine, with more linear power, most noticeable at low revs. Although response from 73cc was never very great, no matter what.

The little engine had a bore of 47mm, stroke, 42mm, with a compression ratio of 6.8:1 and a claimed power output of 6.6 horses at 7,000 rpm…which translated to 90 theoretical horsepower per liter. Not shabby at all. Ignition was by battery, with Mitsubishi flywheel dynamo lighting up head- and taillights.

The Trailmaster sported abbreviated fenders, 16-inch wheels, and lacked turn signals—they were not mandatory at the time and the company presumed they would just get knocked off. Primary drive was by gears, then through a wet clutch and the 4-speed gearbox. For the rear chain, the Trail had a short chainguard on the top, and a mud-scraping chain guide on the bottom. And it sported a low-level exhaust, which indicated that the bike was not intended for the really rough stuff.

The spine frame was of pressed steel, called “monocoque” by one misguided magazine, with a single backbone looping down and the rear of the very light engine bolted to the bottom of the frame. The Trailmaster protected the engine and header pipe with a lightweight bolt-on skidplate. Wheelbase was a shade over 43 inches, and the weight, with 1.7 gallons filling the tank, was a mere 150 pounds. Top speed for the T was at best 50 mph, not quite what one wants on an Interstate today, but adequate for the times.

More surprises were to come, as Yamaha introduced its Autolube System on the YG2 the next year. This was a real blessing to riders, who no longer had to fiddle with measuring the correct amount of oil to add. With the premix, the bike was running the same 20:1 ratio whether at idle or full power, and riders often erred on the side of too much oil rather than too little, as the ramifications were less severe. With the Autolube pump driven by the crankshaft, the mixture was determined by the engine speed and by the throttle opening, meaning that it could be sucking down a 20:1 ratio when wound out to the max, and at idle it could be 200 to 1. This certainly changed the lazy public’s attitude toward two-strokes, which now did not have to belch out clouds of noxious fumes.

The 1968 YG5-T now had an upswept pipe, both for looks and practicality, and more serious engine protection, looking much like an extension to the frame. This had two skinny tubes coming down from close to the steering head and running in front of the cylinder and then beneath the motor, incorporating a more rugged skid plate. The wheels got upgraded to 17 inches, with the rear having two sprockets, just to show the world that the rider was serious about climbing hills. The smaller sprocket had 41 teeth, the bigger, 55, and the trick was to unbolt the spacers that kept the sprockets apart, slide the bigger one over the smaller, which had been carefully engineered to fit, and bolt things back up. It took a bit of work, as an extension of a few extra links on the chain (provided in the toolkit) was necessitated to move from small to large sprocket, so the concept was probably little used.

There was also an electric starter on the latest model, which added a bit of weight to the bike, although kickstarting the little tyke was certainly not a problem.

After ’69, Yamaha dropped the YG series, as the American market was fast losing interest in those under-100cc tiddlers. For friendly fun in the woods, we wanted 250s, like the DT1.

(This Retrospective article was published in the August 2014 issue of Rider magazine.)