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Discussion Starter #1
Like the title says, my fuel tank keeps pressurising itself. The obvious answer is a kinked vent line, i've blown through the top vent port as well as up from the bottom, both ends are free and clear and I can hear the air blow through. However, when its warm outside and I go to open the fuel cap, it's so pressurised as soon as the lock is released it blows the cap open and the tank literally decompresses?
I've lifted the tank several times to see what the deal is, and i'm stumped.
Any suggestions?
Also would this cause the bike to act like there's a "miss" when it's warmed up? I was initially thinking a bad plug problem, so I changed them out, seems to run more crisp, yet still has a miss, but only when at idle??????:idunno:
 

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Th vents on th tank might b clear but have u checked th drain hoses themselves? Ur rough idle might b th fuel pressure regulator. Mine recently gave out on my 929 an th idle was rough n rich. Im not sure about th pressure causing th rough idle, all I kno is when th tank can't vent, th pressure will cause th flow of gas to stop.
 

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Discussion Starter #4
The issue isn't a negative vacuum, it's a positive pressure issue. It builds pressure whether i'm riding the bike or it's sitting stationary in the back of a trailer.
It's like the same as a fuel can sitting in the sun without spout and vent securely closed.
 

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Remember u have an air vent an an over flow vent. Did u blow air thru both? Maybe if u post a pic of how its connected we might better help u.
 

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Like the title says, my fuel tank keeps pressurising itself. The obvious answer is a kinked vent line, i've blown through the top vent port as well as up from the bottom, both ends are free and clear and I can hear the air blow through. However, when its warm outside and I go to open the fuel cap, it's so pressurised as soon as the lock is released it blows the cap open and the tank literally decompresses?
I've lifted the tank several times to see what the deal is, and i'm stumped.
Any suggestions?
Also would this cause the bike to act like there's a "miss" when it's warmed up? I was initially thinking a bad plug problem, so I changed them out, seems to run more crisp, yet still has a miss, but only when at idle??????:idunno:

It has to be the vent.
Two pipes in the bottom of the tank. The rear is smaller and is the well drain (that's the hole in the bottom of the fuel filler well). The front is bigger and is the vent.
 

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Discussion Starter #9
lifted the fuel tank again over the weekend. It's clear as can be, all the lines are where they're supposed to be yet it still builds pressure??
My tank appears to have a single small line, the size of a typical vacuum line, is that supposed to be connected to anything or just dropped down by the swingarm to vent?
 

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Not sure how these tanks are vented, haven't had to look into it yet. On my FJ1200 my tank used to build pressure when I parked it after a ride as the heat built up under the tank. It turned out that the tank vented through a vent valve in the gas cap. It was sticking from the years of build up. A good cleaning and a new O-ring fixed it right up.
 

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lifted the fuel tank again over the weekend. It's clear as can be, all the lines are where they're supposed to be yet it still builds pressure??
My tank appears to have a single small line, the size of a typical vacuum line, is that supposed to be connected to anything or just dropped down by the swingarm to vent?

That hose is the drain.
There should be a bigger one about the size of the fuel hose just ahead of it.
 

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lifted the fuel tank again over the weekend. It's clear as can be, all the lines are where they're supposed to be yet it still builds pressure??
My tank appears to have a single small line, the size of a typical vacuum line, is that supposed to be connected to anything or just dropped down by the swingarm to vent?
That's th over flow drain hose. It's suppose to run all th way down so ur good there. Maybe there's debris somewhere up th breather tube. When u blow air from th bottom-up it moves th debris out of th way allowing th air to flow up th line but when th pressure builds up, th debris clogs th line when th pressure is tryin to escape.
 

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Take the vent hose off completely and see if it still happens. My bike has no vent line on it all right now, so there's no issue with it.
 

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Take the vent hose off completely and see if it still happens. My bike has no vent line on it all right now, so there's no issue with it.


Yep, I remove the vent hose and drain hose as they only serve to direct the waste further down to the bottom of the bike.
 

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Old thread I know, but I would like to know if it is a bad thing to have positive pressure in the fuel tank on a carburetted motorcycle? Could this cause any problems? Cheers.
 

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Hi matey , think what there taking about here is vacuum in the tank which is a bad thing (stops the flow of fuel) , the normal pressure for the tank should be gravity as the fuel pump creates between 2-3psi for carbed bikes, although gravity feed will work as well, in some bikes pressurised fuel bowls do work especially in the bikes that use DIA as air is sent into the fuel bowl (90s 600 Honda's):wink
 
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