Please excuese the introductory anecdote, I do believe this post is on-topic.
I have a '78 Yamaha XS650 of which an exhaust rocker sized while riding a while ago. Lots of top-end noise, repeated valve collision with the piston, and a bent valve later, I'm in the middle of a rebuild. It appears a small bit of metal made its way through the oil system and welded the rocker to its shaft. I haven't yet found its source, but I will.
For those unfamiliar with the 650, it is a four-cycle upright twin with synchronized pistons (Like the old Brit bikes, Triumph, Norton, etc.) Both pistons reach TDC at the same time, on on compression, the other on exhaust. Similarly, both pistons reach BDC at the same time, one on intake, one on the power stroke. Ignition is therefore spaced evenly
360 deg apart.One of the neat features of this engine is its built-up crankshaft construction. The crankshaft halves are connected by a splined shaft with 13 splines.
Various people involved with these engines (including the similar Brit bikes) have proponed re-phasing the engine to have un-equal ignition spacing. They suggest that improved balance may be achieved through a
270/540 split instead of 360/360. Some people have tried this (to varying degrees) with success. (Googling for XS650 rephase and maybe 277 or 270 will yield lots of hits.)Because the shaft has 13 splines, a 277 re-phase may be achieved by a
3-spline shift. The ignition for these engines is on a per-cylinder basis, so points or electronic systems are easily modified. The trickiest part is the camshaft modification, which may be handled by the do-it-yourselfer, or various pro shops will be happy to sell you a modified camshaft.Since I am in the middle of a rebuild, now would be the perfect time to do a re-phase of the engine. However, I am somewhat unconvinced of the benefits, and would like to do my homework. I took a look at 'Internal Combustion Engine in Theory and Practice' Vol. 2 by Charles Fayette Taylor at the local bookstore, and un-equal ignition spacing garnered all of two or three sentences throughout the text.
So, can anyone clear up whether the supposed benefits of the re-phase are fact or fiction? Can anyone point me towards a reputable technical reference that discusses uneven timing for four-stroke engines?
Thanks in advance,
Rob