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Old 23-09-18, 07:35 AM   #1
Davemurphy007
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Default Bike venting fuel through filler cap?

Hey guys, got a little issue with my curvy. It appears to be venting fuel through the filler cap. I canít pinpoint any particular times that it does it (fully fuelled or empty) but the garage always smells really bad of fuel when you open up and you can clearly see fuel vapour trails (and the dirt that sticks to it) down the front and sides of the tank.

Anybody any ideas how easy/difficult this might be to fix? Am I looking at a new filler cap gasket or is there something else in the system I need to be looking at replacing.

Any help most appreciated.
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Old 23-09-18, 07:48 AM   #2
garynortheast
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Default Re: Bike venting fuel through filler cap?

It sounds like the fuel tank breather pipe may be blocked or kinked. Lift the tank and check it.
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Old 23-09-18, 10:35 AM   #3
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Default Re: Bike venting fuel through filler cap?

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Originally Posted by garynortheast View Post
It sounds like the fuel tank breather pipe may be blocked or kinked. Lift the tank and check it.
+1

The rising heat from engine will evaporate fuel in tank and pressurise tank if not able to escape. The other thing is how is the fuel getting up to level of filler, are you overfilling the tank so that when bike leans over on side stand to fuel is able to reach filler ? Overfilling the tank and not leaving an airgap is not a good thing.
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Old 23-09-18, 06:51 PM   #4
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Default Re: Bike venting fuel through filler cap?

filler cap seal?
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Old 23-09-18, 07:14 PM   #5
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Default Re: Bike venting fuel through filler cap?

I wonder if the breather is rusted through in the tank, and somebody blocked it at the bottom to stop it leaking. Then heat from the engine could cause pressure force fuel through the hole in the breather and out the top ??
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Old 24-09-18, 07:50 AM   #6
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Default Re: Bike venting fuel through filler cap?

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I wonder if the breather is rusted through in the tank, and somebody blocked it at the bottom to stop it leaking. Then heat from the engine could cause pressure force fuel through the hole in the breather and out the top ??
That sounds ominously possible especially if it is venting fuel when the tank is not full.However, it would also mean rainwater will be able to go the other way and contaminate the fuel in the tank. Any symptoms of that?
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Old 24-09-18, 11:43 AM   #7
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Default Re: Bike venting fuel through filler cap?

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Originally Posted by Biker Biggles View Post
Quote:
I wonder if the breather is rusted through in the tank, and somebody blocked it at the bottom to stop it leaking. Then heat from the engine could cause pressure force fuel through the hole in the breather and out the top ??
That sounds ominously possible especially if it is venting fuel when the tank is not full.However, it would also mean rainwater will be able to go the other way and contaminate the fuel in the tank. Any symptoms of that?
Potentially. I got to work last night and the bike was idling really badly, which it’s never done before. The bike isn’t stored exposed to the rain though and I’ve only ridden to/from work in light rain so far.

Is the breather pipe the little hole to the left side of the filler cap by any chance? I had the tank up at the start of the year to clean out the carbs (pilot jets) but was pretty careful putting everything back together. I’ll try and get some pictures of my tank and the fuel venting and i’ll get the tank up to investigate further when I finish my shifts toward the end of the week.

Thanks for the replies so far.
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Old 24-09-18, 04:07 PM   #8
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Default Re: Bike venting fuel through filler cap?

That hole to the left of the filler is to drain water that collects in the filler recess. It goes into a pipe that sits inside the tank and comes out under the rear of the tank. A rubber tube then goes from that and drains to the ground behind the engine. Sadly the pipe can rust out in the tank leaving a hole where water drains into the fuel. If this happens it is a major job to fix or a new tank.
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Old 25-09-18, 10:11 AM   #9
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Default Re: Bike venting fuel through filler cap?

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Originally Posted by Biker Biggles View Post
That hole to the left of the filler is to drain water that collects in the filler recess. It goes into a pipe that sits inside the tank and comes out under the rear of the tank. A rubber tube then goes from that and drains to the ground behind the engine. Sadly the pipe can rust out in the tank leaving a hole where water drains into the fuel. If this happens it is a major job to fix or a new tank.
Yes, to test for this, is get a straw and blow down the drain tube, with the tank cap open. If you've got a full tank of fuel, you'll get air bubbles when you blow down the straw
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