Why isn't the rotary valve two-stroke used by anybody still making 2 strokes?

project racer 9/22/2014 1:05pm
9/22/2014 1:05pm
Two Stroke wrote:
Bridgestone had a screaming fast 350 twin with rotary valve around '67 as well as smaller singles that were also pretty good performers before they got...
Bridgestone had a screaming fast 350 twin with rotary valve around '67 as well as smaller singles that were also pretty good performers before they got out of the motorcycle business. They would easily outrun the old piston port Yamaha twins. I remember Parilla go kart engines that were rotary valved that ran in the mid sixties as most of the engines were McCullough reed valved engines and a few West Bend (Chrysler) engines running alcohol. A guy named Surwall used to run a pair of West Bend 820's that flew. I used to run the B stock (two Mac engines) that did pretty well.

The simpler design and broader power band of the reed valves is why we don't see the rotary valved engines any more. Even if you break a reed (as long as it's not steel) no engine damage but if you broke a rotary valve, the engine was done. I don't remember any fiber rotary valves although some one must have tried it.
The G31M 100, and most of the race type Kawasaki rotary valves were stainless. The enduros for the most part were fiber rotary valves.

T he Bridgestones were fast, no doubt. They had a 175 SR and a 100 that ran well.
BobPA
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BobPA 9/22/2014 2:15pm
9/22/2014 2:15pm
Two Stroke wrote:
Bridgestone had a screaming fast 350 twin with rotary valve around '67 as well as smaller singles that were also pretty good performers before they got...
Bridgestone had a screaming fast 350 twin with rotary valve around '67 as well as smaller singles that were also pretty good performers before they got out of the motorcycle business. They would easily outrun the old piston port Yamaha twins. I remember Parilla go kart engines that were rotary valved that ran in the mid sixties as most of the engines were McCullough reed valved engines and a few West Bend (Chrysler) engines running alcohol. A guy named Surwall used to run a pair of West Bend 820's that flew. I used to run the B stock (two Mac engines) that did pretty well.

The simpler design and broader power band of the reed valves is why we don't see the rotary valved engines any more. Even if you break a reed (as long as it's not steel) no engine damage but if you broke a rotary valve, the engine was done. I don't remember any fiber rotary valves although some one must have tried it.
A few companies make carbon rotary valves for sea doo's currently. I have been working on them for years and have never seen one come apart.

The Shop

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YZ125H1
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YZ125H1 10/5/2014 7:15am
10/5/2014 7:15am
I think you bring up a good question. I'm not really sure anyone would know but the engineers of motors. I know rotary valve is used in much higher rpm applications because reeds simply cannot keep up. I'm thinking the introduction of power valves also took away the need for the rotary valve. Plus reed technology is much better today than it was back then. Most(all?) of the older rotary valve motors the carbs are on the side sticking out not really ideal for an mx bike. Still you think rotary valve with power valve would be the best power setup for a 2 stroke. You can tailor a rotary valve to widen or shorten the power also. I'm not sure how much of an advantage rotary valve has still vs reed but I think with the technology advances its probably minute.
jaz_240
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jaz_240 1/2/2021 5:04am
1/2/2021 5:04am
Several motorcycle makers were mentioned as having rotary valve induction engines. I will add Yamaha street and trail YG-1, 80cc and the 125cc Santa Barbara. I rode the YG-1 from 1964 to 1966. The fiber disc shattered somewhere around 8000 miles and was replaced with the same stock part.
MyBobbym
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MyBobbym 1/2/2021 5:39am
1/2/2021 5:39am
CR250Rider wrote:
[img]https://p.vitalmx.com/photos/forums/2014/09/27/66265/s1200_jawaengine1971.jpg[/img]


Trick scooter
JustMX
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JustMX 1/2/2021 6:09am
1/2/2021 6:09am
If I remember right the last generation of can ams used a Rotax engine with a rotary valve.

To make the engine narrower they moved the carb up behind the cylinder where it is normally on the bike.

The result was a long intake boot going down to feed the rv. This resulted in a delayed throttle response, which took some getting used to. I remember somebody comparing it to "turbo lag".

If you took the time to adjust your riding style it was an awesome motor.

John Martin raced the can ams and he could sure make them fly.
ns503
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ns503 1/2/2021 6:52am
1/2/2021 6:52am
JustMX wrote:
If I remember right the last generation of can ams used a Rotax engine with a rotary valve. To make the engine narrower they moved the...
If I remember right the last generation of can ams used a Rotax engine with a rotary valve.

To make the engine narrower they moved the carb up behind the cylinder where it is normally on the bike.

The result was a long intake boot going down to feed the rv. This resulted in a delayed throttle response, which took some getting used to. I remember somebody comparing it to "turbo lag".

If you took the time to adjust your riding style it was an awesome motor.

John Martin raced the can ams and he could sure make them fly.
Can-Ams had rotary valve motors from the beginning to the end - not just last. Along with some reed engines. And a few 4 bangers. All very potent performers. I've got a bunch of them in various stages of basket-ness in my basement.
Dirty Points 1/2/2021 7:19am
1/2/2021 7:19am Edited Date/Time 1/2/2021 8:07am
Kevin Cameron discusses it in decent depth in this retrospective on the 1964 Yamaha YA6.

In a nutshell, while really excelling at full throttle, rotary valve engines have poor "carburetor signal" leading to difficult fuel metering at partial throttle and often poor throttle response (the aforementioned 'turbo-lag' feeling). Applications where engines that are frequently at full throttle as PWC, outboard, roadrace may have maintained rotary valves, while dirt bikes generally chose reeds.

It is interesting to postulate that, with digital FI, that issue could be moot, so maybe another iteration would be worth investigating.

The other obvious disadvantage is the engine/intake needs to be quite a bit wider to accommodate the rotary valve, and nobody likes a wide dirt bike.

Pit Row

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