To vent or not to vent

1985 XJ700X
I’ve noticed between each of the 4 carbs on the rack, there are Ts. Obviously the center one (between carb 2 & 3) is for fuel intake. I’m assuming the other 2 are breathers. Or are they? I thought I was smart and put hoses on them and ran them behind the tranny, to drip on the ground (if that ever becomes an issue). I looked up the carb online and came across this pic where there is only one T. The other 2 are straight pipes.

This brings me to a strange phenomenon that I’ve been boggled with. While playing with the idle/air mixture… I took the gas tank off, set it on a tall stand, put on a longer see through fuel line as well as a longer vacuum line. I noticed the fuel doesn’t fill the line. It only fills to an inch or 2 above the top of the rack. (Yes my cap vent and petcock have been gone through and aren’t an issue.) As the bike runs and more fuel is needed, it does trickle down and doesn’t appear to cause a starving issue but is just weird. It seems that if the Ts in question were blocked off, it would allow the line to burp and flow better.

 

The t is fuel inlet, other straight joints are fuel transfer tubes. Two outter carbs will be blocked on the outer ends. But open on inner.

 

I understand this. So is your suggestion that they should or shouldn’t be open Ts?

 

No! This is the fuel rail the T is inlet for fuel for carb 1 and 2 , 2 flows to 3 and 3 to four. These are NOT vents the tubes have O rings on them very important so it will not leak fuel.

 

The two center carbs where the tubes go in will be open on both sides. (allowing fuel to transfer to other carbs) The two outer carbs will only be open on the inner sides where strait tubes insert . And closed up on the outer most side. The T will be for a single fuel line from tank, and should have three sides open. Fuel enters the T fills the first carb bowl, once full, the needle closes, and allows fuel to travel thorough the chamber to the next carb and so on.... Both tubes and T should be free and clear with good O rings, for fuel to pass though to all carbs and no leaking. Leaking tube rings can result in not all carbs bowls get filled.

 

This is a rack like mine. Did yours come without the Ts between carb 1&2 and 3&4? Or did you omit them when you tore the rack apart? What was the purpose of having them on the stock racks?

 

But there are two more T's on a higher level than the fuel rail (without o-rings) between carb 1 and 2 and between carb 3 and 4. They should have hoses on them to release the fuel away from the front of the engine when the carbs flow over.

 

Missed your last post but that's what I mean.

 

The ones u circled in red are for fuel delivery...... Not overflow.
The T dont have o rins factory, just a plastic coating on an aluminum T, and ring like molding. Often cracks exposing aluminum T, and leaks. Aftermarket T might be brass or aluminum, but will have actual o rings.

 

Yep

 

My oversight. Thank you for your reply. Do you have any clue as to why my fuel line only fills to an inch or two higher than the rack, even with the tank held higher?

 

Was thinking I'm not sure if the T's for the overflow (sorry for my technical English) should have o-rings or not. They show on the drawing but I thought mine didn't have them.

 

Don't know, maybe it's normal. I'm sure someone with more knowledge of physics can tell you.

 

Open gas cap. See if that helps it fill. plumbing rule 101 is "air behind liquid" for proper flow. Sounds like your tank cap is not venting, causing a vacuum lock(no flow)

 

Carb overflow is usually internal, or on exterior of carb bowls.

OEM fuel feed tube in center T Only is non o ringed, and straight tubes ae quad o ringed. Two on each end of tube.

 

Refer to second paragraph of my opening post I’ve already omitted the cap vent and petcock as the source of the issue. It appears to be trapped air between the petcock and the carbs. As though it needs to burp but can’t. 2 gallons of gas in the tank for extra weight/force. Cap is wide open and petcock is in PRI. If I disconnect the line from the carbs and set it in a gas can it flows big time.

 

Just to complete the 'overflow T's' they are plastic and have o-rings.

 

Sorry did not see that post.....Check float heights, are not causing low fuel level in bowls, which will close needle in the seat too early. the venting your referring to is an internal circuit of the carb base. Or, I'm not to specific informed with your exact model, but does it have a vacuum powered pump diaphragm on the petcock? if it does this can also give you your issue if not working properly. I suspect low float levels tho...with info I'm given

 

Ok, yes those are vent tees for overflow tubes.... Shared by all 4 carbs....3 total. Thanks for last pic it helps alot..

 

Maybe I'm wrong but I thought there are 2, one between carb 1/2 and one between carb 3/4. Not that it matters