That's Herr Helmholtz messing up things for you, Wob.
https://en.wikipedia.org/wiki/Helmholtz_resonance
The cycle from a positive crankcase pressure differential accelerating the transfer flow to a negative case differential slowing that flow velocity back to zero again takes a fixed amount of time. At low revs this time is less than the total transfer opening time, so the cycle continues, performing more than half a sine wave during the open transfer period.
It's the same phenomenon you can observe with piston-controlled inlet systems, where at low revs blowback can occur although the piston is still on its way up towards TDC, so the crankcase pump is still trying to inhale.
In theory the reed valve inlet system ought to prevent the crankcase pressure from dropping below atmospheric. In practice the reed valve
needs this pressure drop before it will even think about going to work.
Neils gibs-controlled rotary inlet system offers the best of both worlds. Although.... you may remember Roland Holzners 24/7 experiment at Modena a couple of years ago. Roland realized it without any electronics because the CIK would have vetoed that. His system worked, but with the karting world being the karting world, the CIK vetoed it anyway, like it had done with Rolands synthetic inlet inserts some time before. If the CIK doesn't like you, they'll let you know...
Not even a free breakfast, I'm afraid. The problem is not the symmetry of the port timing; it's the duration of the timing. Asymmetry might have worked in the long-gone era of tiny crankcase volumes but these days Helmholtz will hardly be impressed by what the piston happens to be doing.
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