The pretty much stock TY I fitted a custom air box and bigger carb to, had a 26mm OKO PWK copy. Other than those changes, it had an aftermarket WES exhaust system, and electronic ignition. Both of these things being very common mods on TYs here.
In common with other TY Yamaha's the most restrictive part of the stock air-box, is the intake boot, which is quite long, and relatively small diameter. I try to make intake hoses as short as possible, and the air box itself as large as will fit in the available space. There is a very good Kevin Cameron article, about resonant air-boxes, which I found very interesting, as I knew that custom air-boxes, worked on older bikes, but had no real idea why, until I read Kevin's piece!
Those alterations improved what was other than the ignition and exhaust, a pretty much stock TY175, very noticeably, making it good enough to win a British twin-shock championship event, being ridden by a quite heavy rider.
Another mod that is quite popular for TY175s here, is boring the cylinders, to increase capacity. This means either a 68 or 69mm bore size, and while it does increase power at lower engine speeds, the motors rattle very loudly, and as they don't seem to like to rev out, aren't nice things to ride in serious competition.
There is a thread on the TY area of this forum showing how a pro tuner, increased the transfer area of a customer engine, and while the bigger carb and resonant air-box, did make an otherwise stock bike, run a good deal better, I have found the pre mono motors, work far better with increased transfer area, along the same lines as the pro tuner has detailed.
After the first bike with custom air box and bigger carb, most of the others I have done, have porting alterations as well. Shortening the front pipe is also something that works well, and seems to help with more power at all engine speeds?
The 100mph bike sounds very impressive, I would guess it must have been making around 30hp, unless it was some sort of lay down streamliner type of thing?
