Fuel Petcock issues.

buzzstpoint

XS400 Enthusiast
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Location
Stevens Point, WI
81 XS400 Special.
Picked up this bike and I'm working out some issues.
Cleaned up the tank and kept taking it for short rides. Seems to run good, Took it for a 10 mile ride and it gets a bit worse.
I think I narrowed it down to the petcock. Seems If I turn it to the Res it will run fine. Shortly after starting I can take off with the petcock in the On or "mid" selection the bike will stall out. But switching to pri or Res the bike will fire up and run.

After looking a bit closer, I see a vacuum port plugged on the back of the petcock, and on the intake boots a vacuum line connecting the right and left.

Thinking a rebuild of the petcock is in order.
 
If the petcock has a vacume port then there should be a vacume line connecting it to the right carb and left carb should be plugged off.Also the mid point on my petcock is off same bike as you have.
 
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