Fuel expanding in tank!!

I am experiencing problems with the fuel in my MK2 Lemans escaping from the top of the tank. I always leave an inch or so below the filler when i fill up, and have fitted a new cap ( The one under the cover flap) which is very tight at the mo, but must seal it off surely. And yet whenever i fill up and pop in to pay i arrive back to a puddle of fuel. Ok I realise it leans over when on the sidestand but it shouldn’t be able to escape surely. Then I noticed that when full it also comes out even when upright, apparently because when heated the gasses expand and force the fuel out. Is this right? And can it be stopped or is there a way of venting it somehow?

Nope never happened to me. Sounds like tank has got a hole. When this overflow happens have you taken the cap off to have a look?

Also cap should not seal it will or should have a small aperture to allow air in to replace the volume of fuel used.


The first time it happened was with the old cap and the petrol was shooting up out of a hole in the top of the cap. When I removed the cap it just kept overflowing until it eventually stopped. I will check to see if the new cap has a hole in it as I appreciate the need for a way of venting, but it is a genuine Guzzi part so should be ok. My Spada has never done this and I brim that tank, and the stopper for that tank is part of the flap, not a seperate cap.
I have seen bikes with a tube coming out of the top of the tank, just in front of the filler. Is that to vent fuel or vapour, and where would it go, and why only some bikes???
Any feedback would be appreciated folks.

Does it do this when left in the garage half full?
If yes you have a leak somewhere.
If no then that amount of fuel leaking should leave a trace down the tank or whatever as it comes out via the cap.
Where you have the separate vent pipe is because the aircraft filler cap is sealed for safety in the event of a racing accident. The vent pipe normally feeds forward into the fairing and is looped several times for the same reason toilets have U bends.
If you want to ask some poseur why he has a toilet on his tank I will leave that to you.

I have a vent on my tank because it has an aircraft filler with no vent, it allows air to replace the fuel used, otherwise the engine gets starved of fuel. I don’t have miles of loops in this (but like Ian I’m sure I saw a Ducatti thus rigged at the last Fordingbridge V Twin) the pipe at the top of the tank at the front goes up an inch or so and then curves down below the tank at the headstock. The only time this leaks petrol is if the tank is full and I slam on the brakes very hard.

Are you sure it is not leaking from somewhere else?

I have thought of buying some fuel pipe, feeding it under the tank bag and then finishing it with several loop in the screen.
But then the word buying reared its ugly head so I didn’t.

I have a Le Mans 2 tank on my Spada and have never had this problem. I can have a closer look at mine, but is there some vent that is blocked somewhere, or something missing? If you are stopping an inch below the filler, there should be a decent air space in the tank which should deal with expansion in addition to the cap vent.

Thanks for that guys, so pipe on tank not really required. But still no further forward on why mine spits out fuel. Yes it leaves marks on the tank as it is coming from the cap area defo. No it doesn’t do it when half full, only when full, then stops as the fuel level drops. Runs fine during this by the way. Taps appear well sealed and don’t leak.
It would appear that when the tank is full or nearly full and the engine has been running the fuel or vapour expands and forces it’s way out of the only place it can which is the vent hole in the cap. It pours out at quite a rate. When I lift the flap it is spurting up 2 or 3 inches straight up!!!

CAPA, Corrective Action, Preventative Action.
The CA is to leave at least 2’’ at the top when filling, a pain which leads to extra fuel/fag stops. It may need to be more than that.
PA is more interesting as I have never heard of your symptoms before.
Now the easiest PA is to give me the bike and let me live with the problem but I somehow don’t see you going for that.
Tony Botto, TB Motorcycles and friend of Con, also owns a MKII, he might lend you his cap to see if that gets rid of it.
If it does then you are looking at a new cap.
If it doesn’t then I am stumped as I never had that problem, even in Germany which can get warm.

:laughing: Thanks Ian, but I think I’ll try to sort it myself and/or live with it.
I will drop in on Tony to compare the bikes and discuss, but I have a new cap just fitted, and still happening.
I agree the easy way is only fill to a certain level, but as you say , I may end up with a range of 50 miles or something daft, so not really ideal, even though I like a smoke!!!

It is however a new Moto Guzzi cap, and it will have been made by an aftermarket firm.
Though a faulty cap should stop the bike from running, what you have is unique(something you never want to hear from a doctor).

I have suffered that kind of thing, but only by filling pretty much to the brim and them almost immediately parking the bike up for a while. I think this is due to the fuel being quite cold when you pump it and then having it warm to ambient temperature and expand somewhat also. Which has left me with worrying puddles (I’m at that difficult age), but nothing as dramatic as I think you re describing.

As you say most of us would have seen that but this sounds a lot worse.
I still think Martin should consider the fire risk and let me look after the MKII.

I admire your persistence Ian, but I’m afraid this one’s a keeper, even with a ‘unique’ problem.
Well unless anyone else notices this post and does know what’s going on, I will continue to investigate, and should I find a cause I shall report back.

This is really bizarre I have never had this happen on ANY vehicle. (Except once, BSA Bantam, but only cos the [Indian] petrol pump attendant [remember them?] got some idea from somewhere that it should be able to take 2 gallons. Which of course it couldn’t.Total capacity was one and a half.)

Something strange is going on!

Almost as tho the tank is contracting in contact with cold petrol. (?)


My Spada leaked fuel when I filled it right up when on the centre stand, then ride it a short distance and left it on the sidestand, quite disturbing to see petrol flowing over the tank and onto the hot engine.
Did it once, never again!

Does your tank have the tubular arrangement going down into the tank inside the filler? I assume it’s an anti-splash device.

No, just your standard tank.

I would have a look at the vent, even if a new filler cap the vent could be blocked.Try clearing it with an airline?

The fuel cannot be pressurised in the tank but the air and fuel vapour can so causing pressure in the tank, but only if the vent is blocked. As said above, maybe try not filling so full?

Typo corrected :unamused:

Well, after consulting a couple of Guzzi Guru’s I have discovered the following. The old cap had a hole in it that was non standard, so the previous owner must have drilled it out himself following a problem, I’m assuming he had a problem with fuel delivery due to a lack of venting. The original cap has no vent hole, it just vents via the cap and taps not being air tight fittings!! Very Guzzi I thought. The only way of preventing an overflow is to find a level the bike is happy with, so leave more room for the expasion to happen.
I still think this is wierd, as other similar bikes, including my Spada, has no such problem. There is one final discovery that may have a bearing, and that is the new cap I fitted was very tight so was hard to turn to fully closed. Now that it’s worn in a bit I seem to be able to make one more turn to fully secure , so perhaps the new cap will keep the fuel in despite the expansion when fully sealed. Mind you, if I then find I stall out due to it not venting very well, maybe I’ll end up drilling a hole myself!!! :laughing: