2 petcocks - 3 cylinders

[bubbagump]

Help!!! I just bought an 80 xs850. The tank has two petcocks which feed cylinders/carbs 1 and 3. Carb 2 in the middle has an identical inlet as 1 and 3, but has a rubber cap on it. Now one of two things are happening. Either 1 or 3 are "sharing" the fuel with 2, or more likely 2 is not even firing.

Power is lower then I recall my CB750k and 650cs were, and idle is a little wacky, but I would think that if 1 of 3 cylinders are no firing then it would be running really bad. It does run faiirly smooth. I was told the motor was recently rebuilt, which it looks like, clean, newer gaskets, ect. I guess the only way to tell is to check the compresson.

Any ideas on the carb inlet thing?

[threebanger]

Those lines are vacuum lines that operate the petcocks and allow them to feed gas to the carbs. If you look closely, there should be another slightly larger line going from the petcocks to between the outer carbs and the middle one.
The plug on the center cylinder is needed so that cylinder doesn't suck air and make it too lean.

That's a simplistic explanation. Can someone elaborate for him?

[prometheus578]

I have a coupla 81 850's in the garage. From what I can remember.. the carbs are all linked together. what I mean is, one fuel line can supply them all. The reason for two petcocks is due to the design of the frame and the tank.
The backbone of the frame essentially splits the tank into two halves. There must be a petcock on both sides or, as the fuel level drops, fuel would be trapped in the side without a petcock and unavailable for use.
Hhmm just reread the original question, and Threebanger's answer. He is correct concerning the vacuum lines from the carbs that activate the petcocks. Skinny lines vacuum... larger lines, fuel.