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Discussion Starter #1
The last few rides, I have noticed the top of my tank is wet. It seems gas is leaking out the top. At the fullest, it was a few ticks above half full. Currently it's at 1/4 and still getting wet.

Any ideas?
 

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Still leaking at 1/4 tank? Time for another cap or try to replace the seal, or rebuild the cap. Don't think I've ever seen parts, other than complete caps.

Is the vent at the rear of the tank open?
 

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Inside the cap is a bunch of stuff that can go wrong that is designed to vent the tank "properly". Mine lets the tank get pressurized and then blows gas out the vent pipe as the tank absorbs heat from the engine.

There are o rings on the screws that retain the gas cap assembly that degrade over time, they also set the base height of the cap sealing surface. Replaced mine and the gas cap got way easier to open & close.

The cap main seal rides on a moving surface supported by little springs that can get stuck.

For a function that could be performed by a cork with a hole in it the Vulcan Gas Cap is inordinately complex.
 

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Discussion Starter #4
Here is a video of the leak.

https://www.dropbox.com/s/lmtmo7ktejhd2d5/MOV_20180617_1529161.mp4?dl=0

I just filled up after a ride. As soon as I lock the cap, it starts flowing out! It's not the cap-to-tank seal that's leaking, but I suspect the vent isn't checking closed. The cap seal is dry, but not dried out. I might have overfilled the tank this time, but that doesn't explain the 1/4 leaking.

Because of the rate of flow, I drove home from the gas station (0.5 mile) with the cap unlocked. It leaked less that way.

I suppose I'll have to replace some parts on it. But why would it flow so much more, once I lock the cap? And it was instantly too, like a pump turned on! I get thermal expansion of the gas from engine heat, but I'd expect the pressure to build for a moment before starting to flow.

I ended up parking it in my garage with the cap unlocked. I'll check it after the bike cools to see if it acts different.
 

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Discussion Starter #5
I checked the bike this morning, and there was no leaking with the bike parked and the cap closed and locked.
 

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Fuel should never leak out of the gas cap if the gas tank vent system is clear and all the seals and gaskets are good on a '88 or high model year. The gas should go through the gas cap vent system, down a tube that runs through the gas tank where it exits at the base of the tank (under the seat) and down the rubber tube connected to it. '85 - '87 model years did vent directly through the gas cap, so those model would leak like the one you have.

  1. The gas cap is from a '85 - '87 model year (or a tank)
  2. The gas cap seals and/or gaskets are bad
  3. The O-Ring at the gas cap vent exit is missing (little nipple near the cap hinge)
  4. The springs that seal the cap when closed are weak (I believe there are 5 or 6 of them)
I cannot explain why it happens at 1/4 tank, unless the vent tube inside the tank is compromised and sending expanding gas up the line to the gas cap (long-shot I know).
 

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Discussion Starter #7
Fuel should never leak out of the gas cap if the gas tank vent system is clear and all the seals and gaskets are good on a '88 or high model year. The gas should go through the gas cap vent system, down a tube that runs through the gas tank where it exits at the base of the tank (under the seat) and down the rubber tube connected to it. '85 - '87 model years did vent directly through the gas cap, so those model would leak like the one you have.

  1. The gas cap is from a '85 - '87 model year (or a tank)
  2. The gas cap seals and/or gaskets are bad
  3. The O-Ring at the gas cap vent exit is missing (little nipple near the cap hinge)
  4. The springs that seal the cap when closed are weak (I believe there are 5 or 6 of them)
I cannot explain why it happens at 1/4 tank, unless the vent tube inside the tank is compromised and sending expanding gas up the line to the gas cap (long-shot I know).
I believe my issue is #3 on the list.
There's no indication the tank is not original '89.
The gas cap itself appears to be sealing.

I think at 1/4 tank, it's fuel splashing right out the tank. It doesn't flow like the video at 1/4 tank.
It only gets wet while riding.
I am ordering 92093 and 92055 (see image).
It looks like 92093 is for California only, but I figure it couldn't hurt.

It could be another issue. I might have a blocked vent line causing the issue.
I saw at http://www.vn50.com/forum/19-electrical/102769-fairly-quiet-electrical-i-think-sound-everything-shut-off.html where roadpouring recommended blowing out the vent lines with compressed air.
Thorn disagreed with blowing air up the vent lines.

Thoughts?
 

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In that other thread, I disagreed with blowing air through the lines when there wasn't a problem. :) If you think you have a blockage, I'm not against it. I'd say blow out the vent line that goes through the tank any time with no worries. If possible, I'd still disassemble the gas cap to clear a blockage instead of blowing air through.

I just now watched your video, and that's crazy. The gas level is obviously below the neck of the tank, but when the cap seals, pressure in the tank is squirting it up out the cap. The system SHOULD be venting VAPORS out the cap and down the vent tube and hose. Liquid should not be there, especially if the tank is 1/4 full (instead of, say, overfilled at the gas station). Is there any corrosion in the tank? Is it possible you have a pinhole leak in the vent tube inside and near the bottom/rear of the tank, and a blocked vent hose? That could force expanding gas backwards/upwards through the vent line and out the little hole at the gas cap. Seems unlikely, but I don't know how else that could be happening.
 

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I believe my issue is #3 on the list.
There's no indication the tank is not original '89.
The gas cap itself appears to be sealing.

I think at 1/4 tank, it's fuel splashing right out the tank. It doesn't flow like the video at 1/4 tank.
It only gets wet while riding.
I am ordering 92093 and 92055 (see image).
It looks like 92093 is for California only, but I figure it couldn't hurt.

It could be another issue. I might have a blocked vent line causing the issue.
I saw at http://www.vn50.com/forum/19-electrical/102769-fairly-quiet-electrical-i-think-sound-everything-shut-off.html where roadpouring recommended blowing out the vent lines with compressed air.
Thorn disagreed with blowing air up the vent lines.

Thoughts?
I don't think that pressurized air is necessary, nor does everyone have the tools to do so (I don't). Things that block the gas cap vent can usually be dealt with using carb or brake cleaner (non-chlorinated only), pipe cleaners, etc. I would first try the POOGS tried and true remedy procedure: disassemble the gas cap to inspect and clean the vent (only takes about 10 minutes), spray some carb or brake cleaner down the vent hole (between the gas cap hinge) until it runs clear at the other end (default location is in front of the rear tire), and make sure that the seals and gaskets are there and not dry rotted or shrunken. If you think a the O-ring is missing them replace it. Go to the POOGS "sticky" for the details. I think that this will help solve your problem or at least identify the root cause.
 

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Discussion Starter #11
Short answer: The O-rings for the two gas cap screws were shot.
New O-rings and no leaks any more!

Long answer: The pressure from the cold gas over a hot engine was enough to force it out the screw holes, but not enough to open the pressure relief valve. The bottom of the screw holes are below the full tank liquid level, so it just pushed gas right on up and out. There was no blockage in the vent system. With the shot O-rings, any splashes worked their way out the holes.

Make sure to use metric sized O-rings. I bought mine locally, first trying a small engine repair shop (3 blocks from my house). My first repair was standard sized (eyeballing fit off old worn out rings) and they leaked as well. Next and final attempt was the local hardware store that also carried metric O-rings. I wish I wrote down the size, but they can be found online as well. Or just take the cap assembly into the store and find one that fits. Thickness was 1.5mm.
 
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