Mechanical feul tank pressure = engne breathing

[slugger]

hi, had a problem , i know how it occured and am just wondering if anybody else has heard or suffered from it.

i left my bike (2001 softail ) in the sun all day yesterday, when went to start it in the evening, it had problem starting, then on my journey it blew an excessive amount of oil through the engine breather.

it turns out that my fuel tank had presurised in the heat forcing petrol through the carb into the cylinder bores, past the piston rings and finaly into the crank case, when it was runing, the oil was coming out the breather due to excessive crank case pressure,

a simple oil change or two will cure this no problem, i know there it nothing else wrong with the bike, all i have to do to stop this happening again is to turn the fuel tap off when i am off the bike.

i was just wondering if anybody else has heard of or encountered this problem themselves, we do not have extreme heat in the uk so must be coming in warmer countries.

[spinlondon]

Mines injection. I have however heard this hissing noise, that went away and then came back when I opened and closed the fuel cap. Very disconcerting untill I realised that it was pressure build up due to the heat from the sun.

[Urrell]

What a strange first post.

Is the tank not vented?
How does it draw fuel out?
How much fuel was "blown" into the engine?
I doubt if the sun would heat it up as much as stopping and the engine heating it after a good run!

Oh and welcome to the forum

[Ukkid76]

I have never heard of this before, I'm sure if the problem did exist there would be a cure for it!!

For a start if your bike engine isn't running the furl tap wont let any fuel through as it has a vacuum mechanism installed to prevent this sort of thing happening,

If it were a simple as the fuel warming up expanding and then finding it's way through in the engine then almost everyone would suffer this problem!!

My bike has been parked for hours in temperatures well in excess or 40dg!! the only problem I have is sitting on a bloody hot seat!!!!

My thoughts are telling me you have a problem somewhere on your bike,

When the fuel expands in the tank as it is heated in the sun it is somehow causing you this over fueling and/or pumping it in to your engine!!

It needs to be inspected and rectified asap…….

[banquo]

I had a similar problem a few years back with a 2003 Softail, mentioned on a post on here somewhere... The tank is vented from a beak on the front right, and then via a series of pipes to the left rear. I'd stopped and parked up in the sun, and when I restarted the bike, and was just getting ready to set off, a lady on the pavement stopped and said there was petrol coming out. She wasn't kidding: it was peeing out under pressure out of the air filter, over the exhausts, and had formed a puddle underneath the bike. Of course, seeing the pressure it was coming out, I knew the vent had to be blocked, and sure enough, when I took the cap off, there was a big release of pressure, and the leak stopped. What I assume happened was that the blocked vent allowed a high pressure to build up from vapourised fuel in the sun, but the diaphragm fuel tap held it at bay until I started the bike up. As soon as the fuel tap opened under manifold vacuum, the fuel came in under pressure, so the float needle valve wasn't able to close again.
I had a nightmare dismantling the breather system to find out where the blockage was. Eventually traced it to the metal beak at the front of the tank, as everything else was clear. Poked it through with a bit of wire, and it's been fine since, but I was really surprised that any blockage could be gas-tight enough to actually allow pressure to build up. There was talk on here about tank liners coming adrift and so on, but to be honest, I couldn't see a bit of flaky liner causing such a good seal. It was very rusty inside the beak, so that might have had some effect.

As above, I don't understand how this could have occurred on yours when the bike was stationary and engine not running, unless your fuel tap is shot, or has been replaced with a non-vacuum version. However, I guess there may be a limit of pressure that the vacuum tap will hold back...?

As an aside, back in the seventies, I heard of a guy with a Suzuki T500 who went to start it up. One turn and the kickstart goes solid, so he's jumping up and down on it, trying to get it to move. Turns out the diaphragm fuel tap has leaked, filled the crankcase full of fuel, and on the first throw (being a 2-stroke) the incompressible fuel is pushed up into the cylinder, preventing compression. A very expensive bent con-rod and replacement crankshaft was required, and all he needed to do was take the plugs out...

[slugger]

the fue tank was about 2/3 full so plenty of vapour in the tank, i have noticed before a huge suction when release the cap, there was no leak around the bike until i was riding, the only problem was the failure to start straight away, because of too much fuel in the cylinders, thanks for all help and please keep info coming

[robbiemac]

There was an issue back in 2003 i think with Dyna tanks. It was as you describe Banquo. The fix was as you did, pierce the sealed vent pipe end with a scriber or similar.
Perhaps this is what is causing this?

I had a issue years ago with my old BMW boxer, the fuel would seep past the float needle and fill the left hand cylinder with fuel. Hydraylic lock would ensure the engine would not rotate. The fix for me then was to remove spark plug and crank the motor. A great rooster tail of petrol ensured the cylinder was emptied out. Luckily enough, no great conflagration ensued.

[banquo]

Guess you need to be checking the petrol tap too, as it shouldn't have let the fuel through (didn't on mine).

A 'full' oil change is a good idea too, refilling with clean oil, and then cranking the engine over with plugs out, and before replacing filter, until clean oil comes out of the filter spigot. This empties dirty oil out of the galleries, which would be a good thing if it's contaminated. I'm not sure there's anything especially harmful in petrol residue though, so it seems unlikley to do any damage now. People used to dilute their oil with petrol to improve cold starting in the winter, and although that isn't a recommendation, it does suggest it shouldn't have any terminal effects....

[Ming the Merciless]

It is astonishing how much petrol expands when heated. I have often been caught out by filling up almost to the neck, going to pay for the fuel, then coming back to find that heat expansion has caused a torrent of petrol over my nice shiny bike!

Although the owner's manual always advises leaving a gap beneath the neck, my motivation is usually to get the maximum fuel into the tank, just in case there is not a handy gas station when I approach the 150 mile mark.

Back to Slugger's problem - has the fuel cap been changed on your bike at any time since manufacture? Some gas caps are vented, some are not, and I wonder if this is a possible cause of the trapped pressure.

[slugger]

no fuel cap is standard, changed oil tonight, everything ok, will now ride for 50 miles then change again, thanks for all coments