Reversion is what you might see coming from the bellmouth of a carby when you rev the engine up........Generally it is from some type of performance engine where the cam overlap is great enough that combustion gasses being forced out the exhaust valve/port are still enough that when the inlet valve opens, some of these gases escape through the inlet port, just as the piston descends down the bore, and thus the negative pressure draws in the intake charge. This in effect causes the intake charge to be almost a "Double mixture" in strength due to the change in airflow out and then in through the carby.......This explains why in a performance engine like a Manx single, once the rev's drop bellow a certain range, the engine misses badly due to this excessive rich mixture.......once the rev's come up, and the engine clears itself, the reversion settles down and the engine performs as it should.........The cam is partly the problem, but exaust port and header pipe tuning also contribute very much in the way this all happens........this symptom was once called "Megaphoneitis"........It still happens, as it did to my racer........once I lengthened the header pipe quite a bit (near 300 mm) it took away all the above symptoms........In doing so it probably killed a little of the top end power, but the engine has such a large amount of torque the power loss at high revs is not an issue. When I road test my racer around the area where I live, I can feel the cold methanol (from the reversion) hitting the inside of my left leg.......with the small added castor oil in the mix.......better than aftershave I say....... Cheers.
