These old Yamahas are built like a tank. They are very similar to Honda XR400’s in both size, durability, and longevity. As long as you keep them in good condition they will run forever and never let you down. A single cylinder air cooled bike is in my opinion the ultimate all around off road machine.
What interests me about these bikes is their use of dual carburetors. Dual carburetors were added to single cylinder bikes during a transitional period when 4 valve cylinders began being used. Engineers needed to find a way to better control and atomize the fuel going in through the dual intake ports. At the time they had difficulty accomplishing this task using existing single carburetor technology.
What surprises me is that these motors did not use the YICS technology that Yamaha incorporated into it’s streebikes of the early 80’s. The Yamaha Induction Control System was essentially a tube which internally connected all of the cylinder intake ports to balance vacuum pressure and eliminate symptoms of slightly unsynchronized carburetors and even out combustion across 4 cylinder engines.
It seems to me that the best way to approach intake development of a single cylinder motor is to do the same thing. Think of each intake as its own cylinder and balance intake pressures internally prior to combustion. Today’s modern 4 stroke singles do this.
The funny thing is, YICS technology is no longer used on multi cylinder engines. Advancements in carburetor technology as well as fuel injection systems (obviously) counter the need for internal intake balancing. The YICS system also made carburetor adjustment difficult. However, I’ve owned lots of YICS motors and have tuned them to perfection despite the difficulty.