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1975 CB550K
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Discussion Starter · #1 · (Edited)
Hi! New poster and first time bike restorer. Thanks in advance for going easy on me.

Last fall I obtained a 75 CB550. When I began to disassemble, it had two fuel lines going into a petcock with two outlets. I took it at face value and am now reconnecting the fuel lines to a new, identical petcock. Here's the problem:

Before installing the tank/petcock and fuel lines that are upstream from the filters, I thought I'd test it out by putting some fuel in the tank and opening the petcock into a container, to check the flow. When I set the petcock to 'On', fuel flows freely out of one of the lines, but not the other. When I switch to 'Res' or 'Reserve', fuel doesn't come out of either line. Based on the original setup, this does not make sense to me.

Can anyone assist with an explanation of why fuel doesn't come out of both lines, in the 'On' setting? Does a single fuel line supply all four carbs? Does it do so through a single inlet on the carb rack, or two? Do I need a 'T'? Or is the second inlet, near the #4 cylinder actually for vacuum?

Thanks for educating this newbie.
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The fuel petcock has two intake tubes with screens on them, the reserve is the shortest tube and the first to plug. Time to take it apart.
Do you have a service manual for the bike? It's the first thing you should procure for any motorcycle. Failing that refer to a parts diagram somewhere on the net to know what's inside there in advance.

Photos are good both for your future reference putting things back together and for us to assist you (y)
 

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No vacuum lines, only fuel lines
... not sure if I am allowed to link you to the manual or not but there is one online.

I think you are missing part of the tube and a screen filter that goes over it based on parts diagrams.
 

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If the tube is short, reserve will run dry almost at the same time as the normal setting, that's the only real impact. The fuel tap comes apart for cleaning and that bowl bottom part is where solids and water should normally collect.
 
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