Honda CX 500 Forum banner
1 - 6 of 6 Posts

·
Registered
Joined
·
837 Posts
Discussion Starter · #1 ·
I thought it would be a good idea to drain the tank of gas prior to removing the tank. There's about half a tank of gas in there. When I disconnected the large fuel line leading from the petcock to the "left" carb, not a single drop of gas came out, even though the petcock was turned "on".. Odd? When I moved the petcock to all three positions, with the fuel line haning from the petcock, nothing? No gas flowed in any postion. So, from what I've read I guess I have one of those vacuum petcocks that only allows fuel to flow out once the motor is turning over, developing vacuum - thus allowing fuel to flow?



Now...when the bike has been sitting for some time, I have found it difficult to start the bike, due to suspected fuel starvation. I'm pretty sure this would be a confirmed diagnosis given what I've just witnessed. My bike hasn't been run in one week. If I were wanting to start the bike after long periods of idleness the procedure I've followed was to remove the air filter, spray carb cleaner aimed as best I can at the carbs and wow, the bike starts up instantly. By doing so, I guess there is sufficient working vacuum developed quickly and thus allows the petcock to get fuel to flow.



So, is there anything wrong with this picture? Something doesn't seem right to me. It shouldn't be so difficult to get a bike to start after sitting for long periods. Do I have a properly functioning petcock?
 

·
Registered
Joined
·
2,282 Posts
Yea, your petcock is just fine. That is the way the vacuum petcocks work. If you want instant fuel flow, you can lightly suck on the line going to the carb, or lightly blow on the drain line. It doesn't take much to move the diaphragm inside.



Common theory is that the fuel evaporates from the bowl after several days and it takes a bit to get them refilled.
 

·
Registered
Joined
·
8,423 Posts
Although they are a good idea on paper as a safety feature I personally would hate a Vacuum petcock and would bypass it if I had one,



http://globalcxglvtwins.hostingdelivered.com/viewtopic.php?f=19&t=290



I have taken to switching off my manual Petcock when not in use but have painted the top of my lever Red so I know when it's in the,"Off" position as it's not easy to quickly reach for it if I've left it in the Off position when set off<grin>
 

·
Registered
Joined
·
837 Posts
Discussion Starter · #4 ·
Blue Fox and Shep...thanks for the responses.



Well, I see both sides of the story. I guess my first bike had a simple gravity feed petcock and I was so used to that it just made sense.



I had always wondered about the vacuum petcock threads I have seen pop up on this site. Now I know what all the hub bub is about.
 

·
Registered
Joined
·
3,999 Posts
Some bikes have vacuum petcocks but they also have a "prime" position, for filling the bowls. When you turn it to that position it will allow fuel to flow without a vacuum source. That would be a nice feature but it adds another level of complexity to the petcock,,,
 

·
Registered
Joined
·
837 Posts
Discussion Starter · #6 ·
Allan...I do like that suggestion/solution/idea. It makes sense. Althoughbeit a complex internal working to make it work. Maybe more fuel lines connected? I know....we'd call it an octopetcock...or some such similar monstrosity. Still, I like that idea.
 
1 - 6 of 6 Posts
Top