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Discussion Starter #1 (Edited)
Gas tank overflow!

I have a Fat Bob split tank setup.

The trouble I am having is when I need to fill with gas. I fill the right tank and never open the left tank. But for some reason the left tank overflows every time. Gas begins seeping out of left gas cap rim. Not just a little, but a lot.
If I level the bike and look in both tanks, the left tank is always a touch higher and not level. Of course when I lean it back on the stand, gravity takes over and the angle of the bike forces gas to over flow out left cap.

If I siphon gas from left tank while bike is off stand and level, the left tank bubbles and seems to vacuum suck gas into the left tank. I siphoned gas about 2 liters from left tank to a point where I could put it in the stand and it not leak.

It appears both caps are vented. This might be part of the problem.
 

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Don't want to sound like a smart a** but have you tried filling from the left tank first? Either way, it does sound odd that the gas overflows so much coming from the right. bbally's suggestion sounds like a good one
 

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you do understand how gravity works I hope..... the gas flows to the carb because the gas caps are vented

put a piece of wood under the kickstand to raise the bike up a bit. There is also a kickstand wedge that can be installed if your bike leans too much
 

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Killer of Souls
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to me it sounds like a better seal is in order....but I just throwing darts in the dark...
 

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Just Ride
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How's the gasket around cap?
 

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Keep on Ridin’
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to me it sounds like a better seal is in order....but I just throwing darts in the dark...
How's the gasket around cap?
My son's Fat Boy has dual tanks. He always fills the left tank first, caps it, and then fills the right tank. The left cap seal should not leak, so the gasket does appear to be defective. One other thing--far-fetched, maybe, but possible--a bad crossover hose with a bubble on the inside acting like a one-way valve. Check this by holding the bike vertical while up through the left filler neck and looking into the right filler neck to see if both tank levels rise at the same rate. Yeah, I know it sounds weird but I've seen this happen with old brake hoses. But I really think a new gasket on your left cap will get you straight.

--
 

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Discussion Starter #12 (Edited)
Don't want to sound like a smart a** but have you tried filling from the left tank first? Either way, it does sound odd that the gas overflows so much coming from the right. bbally's suggestion sounds like a good one
Looks like the best answer so far, is to fill from left side while on the stand.
The guy I bought it from, told me to NEVER remove left cap and only fill right side 3" from top.
Well, that didn't work.

Yes, I know how gravity works, just not sure why the left side holds more after gas settles.
 

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Asylum Inmate
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Did I mention put a gas gauge on the left side?

Solves the problem on vented tanks of the 80s leaking as they self level when on the side stand. Or you can live with the same problem the guy that sold it to you did! That's why he told you only from the right and never over 3 inches from the fill neck.

Or learn to enjoy the lovely smell of evaporating gas in the garage.
 

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Mississippi Cajun
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My old Fat Boy was a two holer, and the instructions said fill from the left FIRST. Never had a problem.
 

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Did I mention put a gas gauge on the left side?

Solves the problem on vented tanks of the 80s leaking as they self level when on the side stand. Or you can live with the same problem the guy that sold it to you did! That's why he told you only from the right and never over 3 inches from the fill neck.

Or learn to enjoy the lovely smell of evaporating gas in the garage.
Its a Shovelhead. No gas gauges for those....


Left cap is not supposed to be vented.

Previous owner probably put 2 vented caps on there, check and make sure.
If only 1 is vented, make sure that one goes on the right side.


Also, as posted previously, fill left side first, replace left cap, THEN fill right side.


Gas begins seeping out of left gas cap rim. Not just a little, but a lot.
This makes me think that the left side cap that is on there now is a vented cap.
 

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The reason you don't fill the right side first is due to the siphoning effect.
If you fill the right side first and the siphoning effect begins, it will continue till the right side is empty.
The vented cap on the right side will not stop the siphoning effect whereas the sealed cap on the left side will stop the effect from occurring in the opposite direction.
There is no siphoning effect, or any complicated event going on between the two tanks.

There is a crossover hose at the front lower part of the tanks, and a crossover vent hose at the top of the tanks.

This allows the level in the tanks to remain equal, whether the caps are vented or not.

The level in the tanks stays the same as they empty out.

The only reason the left cap is not vented is so that it does not leak when the tanks are full and the bike is on the jiffy stand.
 
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