Some folks are making the original 360 degree firing engines into a more modern 270 degree firing order.  The stock XS650 engine fires a cylinder every time the engine rotates 360 degrees.....every time a piston comes up one is firing while the other one is between the exhaust/intake strokes.  Modern twins are using a 270 degree crank where one crank throw is rotated 90 degrees....so the firing occurs 270 degrees apart at the shortest interval - and 450 degrees at the longest interval.  This makes a bit of a "lopey" sound to the engine.  It is supposed to result in less vibration  - and racing bikes use this design as it is supposed to provide better traction.....the longer 450 degree phase without engine power is supposed to allow the tire to get a better hook up with the pavement.

In order to get a 270 degree firing order, the crank, camshaft and ignition systems all have to be modified.  A special center

http://hughshandbuilt.blogspot.com/2013/08/270-degree-xs650-engine-for-sale.html  

The engine can be rephased to 277 degrees with the stock center crank pin - and this is a more common approach.

https://www.youtube.com/watch?v=s-j9q6K_828

You can't just spin the head around, can you?

Modern twins are using a 270 degree crank where one crank throw is rotated 90 degrees....so the firing occurs 270 degrees apart at the shortest interval - and 450 degrees at the longest interval.  This makes a bit of a "lopey" sound to the engine.  It is supposed to result in less vibration  - and racing bikes use this design as it is supposed to provide better traction.


I know Harley Davidson was really popular with the dirt track guys,, I really don't understand how setting up one cylinder ninety degrees off Decreases vibration, but I Can kinda get the traction thing.
Good post, Dave.
Any sites that you'd suggest that might help me understand would be fun ..

Dave wrote on 04/05/16 at 13:14:48:

I think you swap heads, left for right and vice versa.