What's the deal with my double gas tank?

[Anonymous]

Hey boys and girls, i got another inane question for you to tolerate My 84 gs1150es has what seems to be a double walled gas tank. I'm relatively new to this bike and was wondering if you were supposed to be able to fill them both with gas. So far i just fill up the the interior tank because i'm afraid of dumping gas everywhere but i was hoping to squeeze some more gas in by filling up the outer tank. Will that cause any problems? Thanks everybody

[Jethro]

Woha, there mookie. Unless I am mistaken (back me up here people), all our tanks are double walled. I know mine is because the side emblem is screwed directly into the outside wall. Can you see the cavity between outer and inner? How would you go about filling it? No matter, I think it would be a mistake, and am certain it is not intended. I am thinking it is some kind of safety measure, like if the bike goes down and the tank sees some damage it prevents gas leakage. Just my 2 cents.

[Anonymous]

It just looks like one small tank inside a larger tank that are totally separate. I would say there is an approximately 1-1/2 in gap on all sides. When i fill her up normally, i open the tank and then look down in it andf there is a separate hole another inch or 2 down, this is where i normally fill it up. I've accidently gone barely over the top a spilled a little in the outer tank but i couldn't find any external leaks. Also what is there to stop the gas from just spilling over the top of the smaller tank anyways? It seems like it would need another gas cap inside, eh? My old 73 CB750 just had a single walled tank and it never leaked or anything, it just seems like they are wasting gas space and weight if you can't fill it all the way.

[Billy Ricks]

My '85 GS700E has the extra wall on top. It's just so the flush mount cap can be used. The sides are single wall as you can tell if you ever remove the petcock or float assembly. If you overfill it it will make its way into the tank.

[Jethro]

Wow, mine is not like that at all. My tank is most definetly double walled, but there is no opening to it- it's a blind cavity. You make a good point about the spilling over into that spot- in the course of a vigorous ride, it's bound to happen! If you have seen no leaks, I'd say fill 'er up. When you do fill the "inner" tank, do you see any fuel in the other cavity rise? The only concern is if the outer cavity does not drain into the inner cavity. This is all very intersting, anyone else have some input?

[Billy Ricks]

I overfill mine all the time. Since I don't have a centerstand all I do is place the bike upright and the fuel level goes down as the extra fuel displaces air in the tank.

[Anonymous]

Jethro,

You have the same kind of tank I do on my 81 GS750 EX -- in fact you may have the same tank and paint scheme from the looks of the picture you post -- burgandy? Did they use the same frames, but put the 1150 engine in instead? (oh wait, that's a different thread, I think). eh...

...hem...Our tank is double walled at least where the emblem attaches. Whether it is on all of the sides and the bottom, I can't say for sure. When you look inside our tanks you see the hump that rides over the bike frame and the two halves of the tank are just the left and right "lung", (for lack of a better description). I've never quite figured whether all the fuel from the right side can be used since the petcock is on the left side, but my guess is things are pretty equal across the seat end of the tank.

I'd guess for these later model tanks they allow this overflow to spill over into another tank, but they must assume that the fuel cut-off at the pump would dis-allow you from filling up this overflow area. But hey, I only know mine -- and if I overflow (by lifting the nozzle to get that even-penny sale), it just goes on the outside of my tank.

Later...

Roger Moore

[Anonymous]

I have yet to see a double walled steel gas tank on any bike. I believe that cavity around the gas cap is to allow the cap to vent and to let expanding or splashed fuel to drain. You know that rubber hose on the bottom of the tank? It's connected to a pipe that runs up inside the tank to that area. I think if you tried to fill it with gas it would immediately drain out the hose.
I could be wrong, but then I wouldn't be me.
Axel

[SqDancerLynn1]

sounds more like a baffle to me to keep the fuel from sloshing around

[Billy Ricks]

I'm fairly certain Mookie's '84 1150 tank has the same style flush mounting gas cap as my '85 700. It is simply a small tunnel where the cap seals at the bottom of that tunnel. Any overfill drains into the tank itself through a couple of holes in the flange where the cap seals. If you look inside the tank I think you'll find that tunnel, for want of a better term, is only about 3" in diameter and a couple of inches deep. It only holds perhaps 4 ounces of fuel which will easily drain into the tank.