i have just recently inquired a Yamaha xs400 special and it seams to be firing on one cylinder half the time i have rebuilt the carbs put new spark plugs in it changed oil and im still having the problem any ideas, i was told it may be caused by a bad compression ring

I've rebuilt enhgines with that problem before, I replaced the piston rings, new valves, changed the plugs and cleaned the carbs to within an inch of there lives. Now it runs sweet.

But that may be a bit of a problem, from what I've heard that problem is caused by bad cpompression, usually valves or rings in my experiance, drain the oil to see if it has fuel in it, if it does, you've got bad rings.

Otherwise, I'm not sure, rebuild perhaps, an engine as old as an XS 400 could probably benefit from it anyway. I'm just speaking from MY experiance, I'm not a mechanic, just my two cents.

(That bike has a godly sound by the way, love those bikes)