Vacuum in gas tank?

Oldschool

.040 Over
Joined
Aug 4, 2015
Messages
70
Location
Milwuakee wi
Ride
R3 roadster
The rocket has been down for a couple weeks due to needing new tires. Not a big deal since I have to work overtime at work and try and stain 23 wood windows on my place . I should have it up and running in a week or two. Which leads me to my question. My last ride a few weeks ago was about 3 hours long, with my g/f on the back. I wasn't riding overly agressive for the most part. The gas light came on, and I stopped to get gas at a random station. I tried to open the flip up stock gas cap and it didn't want to budge. I thought the key cylinder broke so I muscled it after a couple times fiddling with trying to get it open. The cap popped off the tank and made a woosh noise as air rushed into the tank and it grew visably bigger (the tank that is). Clearly there was a hell of a vacuum in the tank. I never had that happen before and wonder if there is a breather that is plugged up or some other reason it happened? It was parked in a area that was around 80 degrees and I road it to a area that was pushing 93. I would think that would have increased pressure if anything.

any ideas would be helpful
 
There is a breather vent line that runs the down the left side of you're tank. One for overflow and one for vent. Possible it plugged up? I've never seen that problem and in much higher temps. Any significant altitude changes on your ride?
 
My initial thought was "why would you vacuum inside you gas tank"?
Really I have never noticed any vacuum in my tank when opening it.
 
cardinal93 said:
There is a breather vent line that runs the down the left side of you're tank. One for overflow and one for vent. Possible it plugged up? I've never seen that problem and in much higher temps. Any significant altitude changes on your ride?
Click to expand...

Same altitude overall. I will lift the tank and check the hoses the next day I have off. I would think the hose could have been pinched? I did replace the valve cover gasket on the top of the engine a year and a half ago, but I haven't had any problems on the numerous fillups since.
 
Can't imagine the hose got pinched without lifting/lowering the tank, but stranger things have happened. Heck, a bug could've crawled up in there?
 
Does the bike have the evaporative loss system?
 
Hornets nest in the end of the breather tube? Seen them build in some strange places. Chainsaw exhausts, .22 rifle barrels and air inlet on air tools. Breather hose wouldn't be out of the question.
 
Funny you mention that, I had a wicked wasp nest under a stump I split wood on, not even 15 feet from where my bike was parked outside for a couple weeks. In two days I will find out
 
Don't forget about the yellow sac spider. They build nests and block the vents on cars all the time. My son couldn't put fuel in the tank on his Ford Explorer. The spiders had built a nest around his tank vent. $100 to remove the tank and the nest. Read about it on the internet.....it's a real problem.
 
Mine will do that occasionally (the vacuum). Particularly on long rides where I drone on at 70-80 mph for 140 miles or so. I pull up to the pump and the cap is hard as hell to open. Not an issue if I've been trolling around town.

Anyone have any insight into which tube is which under the tank? Is the front-most tube the vent or the overflow?
 
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