Excessive XS650 Blow-by
Regarding Ron Folmer’s letter, Downtime Files, April 2007:
Ron, I’ll tell you exactly why your XS650 is blowing oil, and it is very simply cured. It has absolutely nothing to do with blow-by going past your piston rings. In an upright twin where both pistons descend together, the pistons build up tremendous pressure in the crankcase. There has to be a vent to relieve this pressure, or it would blow out the crankcase seals. The crankcase breather is this vent; that is its only purpose. There is no valve in the breather of an XS650; it’s just an opening with a partial baffle plate. If you fill any XS650 to the high mark on the dipstick, it will quickly blow a half a quart of oil out of the breather pipes. Never fill an XS650 higher than halfway between the low and high mark. Also, if you want expert advice on maintaining, troubleshooting and fixing an XS650, go towww.650wiki.org/index.php/Main_Page, where you will find many articles by enthusiasts such as myself. I have owned XS650s for over 20 years, and I currently have three. The condition can be even further aggravated by the following modifications, and you can be sure over that in 30 years there have been changes to your bike.
• Part substitutions may have been made from other model XS650s, which, while they fit, are wrong. There are over 40 versions of the XS650.
• Prior to mid-1975 the dipstick called for too much oil and a bulletin was issued to recalibrate these dipsticks. Your bike may have a dipstick (or an engine) from an earlier bike.
• A well-known enthusiast has a Web site where he recommends opening the breather to reduce crankcase pressure for added performance. Doing this is certain to increase oil blow-out, and your previous owner may have done this.
Note that your bike does not call for 20W-50; it calls for 20W-40, which is available at truck stops and auto stores. Shell Rotella is a good 20W-40 without the additives found in modern automobile motor oils. If you want a list of typical problems to watch out for on an XS650, go to
650wiki.org/index.php/12.09._Foibles_of _the_XS650 Enjoy your ride, you’ve bought a great bike; a reliable classic.