The "D" and "E" models of the next two years were a little disappointing from the "punter's" point of view due to the fact that very little was altered at the factory from the "C". Most striking difference were the exhausts, with fully integrated silencers.

Yamaha TZ350D 1977

Minor porting, piston and exhaust upgrades saw the peak horsepower output up by 2 to reach a claimed 64bhp @10,500rpm. The factory also chose to fit an offset, angled plug head with the "D", in an effort to combat cylinder and piston distortion as well as slightly raising the compression ratio. New expansion chambers were now fitted from the factory with removable silencers. Small changes were also made to items like the base gasket, crank webs, small end bearings, the gasket for the side oil draining bolt on the gearbox, carby floats and choke systems etc. Tyre widths were also increased from the factory though rim sizes remained as per the previous model.

Not to say that the TZ350 became uncompetitive with the introduction of the "D", quite the contrary, with customer bikes winning 350cc races and domestic championships across the non-USA world from 1977 through 1978. (America didn't run a 350cc class but allowed 350cc bikes to run in other larger capacity classes.)

The "E" had a new frame which some say re-positioned the engine slightly more forward featuring detachable alloy engine plates and around 20 other additional bolts, nuts, washers etc. as well. The swing-arm length remained as for the "D" as did just about everything else apart from the rear lower engine mounting bolt which was lengthened by 5mm. .