Kawi2strokes.com Forum

I'll hate you worse latter tonight Hoping to get more done
It Is really not one step closer to finished...its one TIG start closer to a burn through

In two stroke racing circles, reversion is called "stand off", and variations in it can be very beneficial, or very adverse, depending on the system used, piston port, reed, rotary valve.

Piston port and rotary valve usually have the worst conditions, a correctly designed reed is the best of propositions. With a piston port, as John said, the piston skirt is the valve closing. In a rotary valve, there is a slight bit of help in that the valve itself opens and closes the port off, but still, not as good as a reed. But, reeds can have a bit if adversity, d3epending on how close the fuel delivery orifii in the throttle bore of the carb are to them.

If you read Noburu and Itoh's Yamaha tech papers on "time/Wave Areas" from the early 1970's (It is an SAE document, as they all were back then) and knew Dr. Gordon Blair, as all of us from Team Kawasaki Road Racing, and Kevin Cameron did, and use a bit of thinking on what happens between whatever "valve" there is, vs slide placement and positioning, one comes to a conclusion that length between the valve and slide, depending on slide position, makes all the difference in the world in getting stand off contained as much as possible. Their work also related to stand off in transfer ports, port velocities and volumes, and other flow delivery concerns.

Time/Wave Areas refer to the areas where stand off can be increased, or decreased, such as inlet, transfer port operation, with variances in area of the port, and the time length between them in reference to the RPM's of operation. More RPM's, less time to alter the wave to adversity. When our engines "hit", this is usually when reversion/stand off becomes a moot point in the inlet tract, jetting becomes more stabile, and not adversely effected by too slow a time wave area.

Why is stand off bad? Well, any reverse pressure out a two stroke inlet port only destroys the vacuum pull on the jet delivery orifii, and signal to the fuel being pulled up and into the throttle bore for any given slide position and volume movement direction. Obviously, a steady pull twords the piston is good for jetting accuracy, but stand off does all that in. With a reed valve in place, stand off can be greatly reduced, depending on the length between the reed petal and slide. too little length, reversion pressure defeats the jet orifii pull, longer usually helps dampen reverse pressure vacuum signal at the fuel pull.

Gordon did a few inlet stand off studies at Queen's University, Belfast on one of our H2R's, literally used a time frame camera, shot the mixture puking back out the inlet vs RPM's, very interesting, lots of mixture standing off when off the power band, almost no stand off, to literally no stand off when in the power band.

As far as adverse effects on carbs not sitting flat on an engine, the only thing would be the liquid fuel level in the carb. Most big carbs have a bowl volume that won't be adverse if they are mounted off straight, shouldn't be a problem, but the float/liquid level might have to be lowered a touch.

I used to lengthen the triple, and Yamaha TZ piston port and reed valve inlet paths, helped all the way through the rev range, helped work the jetting better.