The Yamaha Bobby is the youngest of the three Yamaha leisure bike brothers that appeared in the 1970s. I purchased its 80cc Hara2 model with a "pop" and started to enjoy tinkering with Yamaha leisure motorcycles. It is too underpowered and non-powerful. The lack of torque was also a concern. So, I challenged myself to make my own & diverted expansion chamber to see if I could make it as enjoyable to ride as possible.
Sold from a recycling store and won the bid
The mileage is just over 24,000 km. It seems to be almost actual riding. After maintenance, there is no bad atmosphere at all in terms of starting and blowing up. However, when I run it, it is really slow! I would like to do something about this. The standard exhaust system was thoroughly burned with a handy burner, and the carbon that was stuck in the internal bulkheads was completely burned off and cleaned up.
No change after tinkering with the standard exhaust system
The baffles were also pulled out and the carbon that had accumulated inside was completely burned off. After it cooled down, I checked the inside of the exhaust system with a fiberscope just to be sure, but the inside was clean and crisp. The diffuser was also burned thoroughly, and the bike blows nice and dry and the exhaust sound is good, but anyway, it is a slow bike when running. The meter reading was limited to 65km/h. Even with a tailwind and getting down, the speed was only a little more than 70 km/h. Many 2-stroke old cars have baffles that can be pulled out. The straight pipe is so loud that it is embarrassing. In many cases, carbon is clogged, so owners of 2-stroke old cars should remove carbon (burner burn) from time to time.
Diversion of commercially available passol chambers
Naturally, there were no special parts for Bobby. So, I purchased an expansion chamber that was sold for the Passol.
I bought it because I liked the 70's style design of the shotgun type, not the modern aluminum silencer structure.
The exhaust sound is said to be a good sound like a chamber. I decided to cut off the exhaust pipe part of the chamber and use a genuine part for the same engine, Bobby 50, as a new exhaust pipe.
Welding integration is possible with 100V welding machines.
For the exhaust pipe, I used the one for Bobby 50 that I had as a spare part. The bend is the same. However, the pipe for 80 seems to be slightly thicker. The idea of this project was to combine this exhaust pipe and the Passol chamber.
The commercially available exhaust pipe for the Passol chamber was cut off from the welded part, and the insertion part was slightly widened to match the thickness of the exhaust pipe for the Bobby. It can be easily expanded by fixing the solid bar in a vise and hitting it with a sheet metal hammer. After the thickness matching was completed, it was temporarily fixed, point welded, set on the body, and imaged. The bracket that hangs from the swing arm pivot is welded on since it is not bad from a design standpoint.
Can be attached to Chappy depending on the stay.
I installed it on a Chappy owned by my colleague Kurita. The position of the exhaust system stay hole and the interference with the brake pedal all seemed to be fine. I think this design can be shared by both Chappie and Bobby. I don't think that this chamber has made the car more powerful, but compared to the standard exhaust system, it has made the motorcycle more powerful. With the standard exhaust system, I could not reach 70km/h on the meter, but with the chambers, I can easily reach the full scale of the meter.
POINT
Point 2 - 2-stroke models can be driven differently by replacing the exhaust system.
Point 3 - The world of welding expands from a 100 VAC household power outlet.
The motorcycle was disassembled. After disassembling the parts, polishing and waxing them, and then reassembling and restoring the car, the used car, which had been dingy and dirty, is now cleaner than expected. The motorcycle now looks quite nice, having been transformed from the dingy condition it was in at the time of purchase. It looks like an old 2-stroke car that has been carefully ridden over.
With the polishing and finishing largely complete, I went out for a test ride. The slow exhaust note was completely wet, and I had the impression that the exhaust passages in the exhaust system were clogged with carbon sludge left over from the burn. The removable baffle was burned with a handy burner and scrubbed clean with a wire brush. The carbon sludge that had accumulated inside the exhaust system was seared thoroughly from the outside with a gas burner, and the exhaust system was tapped with a plastic hammer and vibrated repeatedly. After several hours of work, the carbon that had accumulated inside the exhaust system was almost completely removed. When the engine started, the exhaust sound had clearly changed to a dry one. However, there was virtually no change in the driving experience, which was far from powerful. Although the exhaust sound was better, the engine performance did not seem to show any change.
The idea was to install an expansion chamber. I had previously tuned a Kawasaki AR80 and enjoyed it, but the chamber made it run brilliantly and combined with the big carb, it transformed it into a surprisingly powerful engine.
I decided to aim for the second loach with this Yamaha Bobby.
The chamber purchased at an online auction was for a Yamaha Passol, a shotgun design favored by modified cars back in the 1970s. The chamber seems to be readily available even today for the popular used Passol. Using the parts as a base, he welded in an OEM Bobby 50 exhaust pipe and installed an exhaust system stay to create a semi-homemade chamber that looks like it was made for a Bobby. The sound was very crisp and not at all noisy, but that was my personal impression. When we went out for a test run, we found that the engine ran well and the RPM range increased.
Looking at the burnt plugs, I saw that they were a golden brown color, so I judged that they were OK.
While riding at full throttle with a good feeling, I looked at the meter and saw the needle at a place completely out of the full scale. I was convinced that the power was definitely increased.
