because two strokes use theunderside of the piston to pull the initial fuel charge, most bikes with conventional carb location make the fuel/air charge travel downward and make it change direction rather rapidly, it is not always a straight shot. On the bridgestones the carbs are mounted low to make the air fuel charge as straight a shot with as little distance as possible. it is cool.
Cool find.
I don't think the location of the carbs on this Bridgestone has anything to do with making the charge travel a "straight shot". The reason they're where they are is because this engine has rotary disc valves to control the intake timing of the fuel-air charge rather than conventional piston ports.
In most two-strokes, the carbs are mounted behind the cylinder and use piston ports. As the piston rises, it uncovers ports in the back of the cylinder and the vacuum of the rising piston pulls the fuel-air charge into the crankcase. After TDC, the port closes and transfer ports open on the sides of the cylinders to move the charge into the combustion chamber as the piston descends.
With rotary valves, a disc valve is hung off the end of the crankshaft and entry of the charge is controlled by when this disc uncovers ports into the crankcase. This system frees intake timing from any restrictions of the geometry of the piston ports. One specific advantage is that the port timing can be asymmetric (different times with respect to TDC for when the port opens and closes) which is not possible with a piston port. It also allows the piston to be designed without worrying about the rear intake port which makes the piston stronger.
Rotary valving was very popular in two-stroke twins in the 60's and 70's. Kawasaki used it in nearly all their two-stroke bikes except the triples (where it is impossible because there is no place to put the disc for the center cylinder). I think it has been used in the Suzuki Gamma four and perhaps some of the MotoGP two-strokes. Suzuki and Yamaha stuck to piston ports and mainly for this reason the Bridgestones and Kawasakis of this era were significantly faster, although the Yamaha (R5 and subsequent RD350) were considered the better all-arounder and consistently won comparison tests among the 350's.
Reed valves were the key technology development that allowed the simpler and more compact piston port two-strokes to retake the high ground over the rotary valves as this solved the key issue with piston ports - kickback through the carb. I'm not aware of any rotary valve two-stroke engines on street motorcycles produced today, although there is probably an outlyer out there someplace.
- Mark
