Motorcycle Repair: Yamaha XS650 carbs, vacuum leak, vacuum lines

Question
My '79 xs650SF restoration runs great except for one area.  If you give it full throttle, it bucks and jumps but doesn't go.  I have had (2) mechanics (benefit of the doubt) "CLEAN" and adjust the carbs but it hasn't helped.  Could it be a vacumn leak through the intake/s?  The float level set too low?  The o-rings around the primary jet assembly?  I give up?  The bike idles pretty well and runs good except as described.  HELP!

Answer
Hi Randy.
 There is a test for a potential vacuum leak at the manifolds.

Make sure that the vacuum lines are new and not dry-rotted or cracked anywhere.  Then, using a can of carb cleaner (spray type) and with the motor running at idle, spray it over the manifolds.  Do this from both sides and get all of the manifolds, one at a time.  Every time you spray it. the idle speed should stay the same.  If it changes dramatically when you spray the manifold, then you have a leaking manifold.  Make sure that it is tight and sealed correctly to the head and to the carb and test it again.  If it happens again, then the problem is the manifold itself and not the connection.

There is no fix for leaking manifolds, they must be replaced, if this turns out to be the problem.

Let me know what you find and we can proceed from there.
Good luck.
FALCON