Author Topic: Fuel Tank Negative Pressure  (Read 2056 times)

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Offline Kev51

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Fuel Tank Negative Pressure
on: 06 October, 2021, 11:32:07 AM
Since I last removed the fuel tank to change spark plugs, clean air filter etc, I have noticed that when I open the fuel cap to fuel up, it is basically sucked close and you can hear air releasing once I open it.
Had the fuel tank off numerous times over the nearly 4 years / 38,000 kms since new and this has never occurred.
My first thought is that I have the two fuel tank breather pipes reversed, which is difficult because they naturally line up left and right, but possible.
Any thoughts and thank you in advance.
Regards
Kev

#1

Offline Sidewinder

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Re: Fuel Tank Negative Pressure
Reply #1 on: 06 October, 2021, 11:57:40 AM
The filler neck drain is a straight thru pipe, so can only get blocked by muck etc.  The tank vent has a roll over valve which could get gummed up, dirt in the tube or the hose pinched.

#2

Online mustangpilot

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Re: Fuel Tank Negative Pressure
Reply #2 on: 06 October, 2021, 04:52:37 PM
The rollover valve on my Sprint was blocked and caused a vacuum in the tank. The cause of blocking the valve was rust from the corroded steel ball inside the valve.
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Offline Kev51

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Re: Fuel Tank Negative Pressure
Reply #3 on: 07 October, 2021, 10:02:18 AM
Thank you Sidewinder and mustangpilot.
I will remove the fuel tank and check all lines to ensure no blockages exist.
Thanks
Kev

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Offline Cloudy

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Re: Fuel Tank Negative Pressure
Reply #4 on: 07 October, 2021, 09:13:05 PM
Had the roll over valve seize and block on my Speedmaster. Gave all the symptoms of electrical problems. removing's the valve cured it all.
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Offline Kev51

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Re: Fuel Tank Negative Pressure
Reply #5 on: 08 October, 2021, 05:39:27 AM
Cloudy
The strange thing is that my bike is not exhibiting any performance issues at the moment. No hesitation or jerky when accelerating or cruising.
I will check the lines over this weekend especially now our lockdown is ending this Monday and there will be plenty of riding to do.
Regards
Kev

#6

Offline Cloudy

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Re: Fuel Tank Negative Pressure
Reply #6 on: 08 October, 2021, 05:43:31 AM
At a guess I'd say the fuel pump is over coming some of the suction to keep the bike running, My Speedmaster was just gravity feed.
07 Tiger 1050, K&N Air Filter, Snorkel Removed, Stock Exhaust, 20543 Map, NGK Iridium Plugs, Ricks R/R, Ohlins Rear Shock, Ohlins Front Springs, Bar End Mirrors, Leather Seat Cover, Ebay Levers, Ebay Risers, Custom Bars.

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Offline Kev51

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Re: Fuel Tank Negative Pressure
Reply #7 on: 12 October, 2021, 11:02:28 AM
Just to let you know guys, it was a blocked Roll Over Valve that caused the fuel tank vacuum problem. What a rotten position to mount the valve? Had it been mounted in a more readily accessible location, it could form part of our yearly maintenance. Now that I know, every year I will disconnect both ends of the rubber tubing either side of the valve and fill with brake cleaner to ensure the ball bearing (?) mechanism, then blow it out with compressed air, and will remain operating normally.
Thanks again for your assistance.
Kev

#8

Online ZuluTiger

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Re: Fuel Tank Negative Pressure
Reply #8 on: 12 October, 2021, 12:39:16 PM
Great catch and thanks for the feedback.

#9

Offline turtle

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Re: Fuel Tank Negative Pressure
Reply #9 on: 12 October, 2021, 03:02:48 PM
*Originally Posted by Kev51 [+]
Just to let you know guys, it was a blocked Roll Over Valve that caused the fuel tank vacuum problem. What a rotten position to mount the valve? Had it been mounted in a more readily accessible location, it could form part of our yearly maintenance. Now that I know, every year I will disconnect both ends of the rubber tubing either side of the valve and fill with brake cleaner to ensure the ball bearing (?) mechanism, then blow it out with compressed air, and will remain operating normally.
Thanks again for your assistance.
Kev

Kev, fortunately I have not experienced this, but out of curiosity can you describe where this thing is located? Thanks.
Bill
2011 Tiger 1050 SE