Manky Monkey Motors

Those who know the XS650 Yamahas will know that one the tappet inspection covers is square, whilst the others are triangular, here’s why.

The predecessor to the Yamaha XS650 was the XS1 and XS2, visually about the same.

The XS2 had an electric start, along with a de-compression system fitted LHS exhaust inspection cap. This bike was one, if not the, first Japanese ‘big’ twins to have an electric foot, and it’s said it had to work hard, hence the fitment of a de-compressor.

To start an XS2, ignition and fuel on, neutral selected (ahem!), pull the handlebar lever, this lever both lifted the valve and engaged the started motor (integral starter button, no separate starter button), then hopefully the motor was up 'n' running.

The XS2 LHS exhaust rocker arm had a "tab" it to engage with the pivot arm, which suggests an operating de-compressor system can't easily be retro fitted to a more modern bike without having the tabbed rocker arm sourced (rocking horse poo?), or an existing one engineered to suit.

Later models had the de-compression system removed, and the square-shaped four-bolt cover was profiled to match the bike’s other three triangular-shaped three-bolt inspection covers.

Some photos showing bikes (XS2) and how it was set up, along with an interesting link.

What some of the bits looked like

More of the handlebar lever (only pic I could find was of a broken one)

Schematic

De-comp inspection cover (damaged)


Link to a review of the XS1, interesting reading from 1972

http://www.cyclechaos.com/wiki/Yamaha_XS1/reviews

The present model, the (1972) XS 650, is not Yamaha's first venture into producing a four-stroke; two years ago they offered, for the first time, the XS1.
This was basically the same as the present model except that it lacked a disc front brake, electric starter.
The XS1 engine was, in fact, a scaled-down version of the Toyota 2000GT car, which was also built by Yamaha engineers. So you can see that although the Hammatsu company are famous for making very quick two-strokes they are not complete novices when it comes to producing a four-stroke.

Here's a helpful site re tech queries for the XS650, and predecessors

http://www.xs650.com/

The cover in that photo appears to be broken -the end of the outer cable stop is missing. The cover on the XS engine I bought yesterday's broken in exactly the same spot.
I'd quite fancied fitting the decompressor to my later engine, simply because I've never seen one in the flesh before & it's an interesting detail. Sounds like it's more hassle than it's worth though. Thanks for the info Tony. 

No real need for a valve lifter on a twin, but try kickstarting something like a 500cc Velocette single without one :D :D :D :D :D

Or a Panther  ;D

 ;D

 ;D ;D ;D ;D ;D ::)

I still bear the scar of "Goldie kick back". Tore a lump out of instep and ankle, not to mention my best boots  ;D

Had an XS 650 that was upped to 840 ....a right pig to kick ...for a while anyway  :)