Outdoor Power Equipment and Small Engines - Pressure in fuel line making gas in tank bubble?

I've got an older (10-15 yr old?) Craftsman rider mower with 12.5 hp Briggs engine. After mowing the grass for maybe 1/2 hour or so, I can shut off the engine, and hear the gas cap venting. When I open the tank, the gas will be bubbling in the tank. What could be causing this? It's like there's too much pressure in the line, and it's forcing it back into the tank.

Thanks!

Hello NotMrWizard!

Very strange. The only thing I can think of offhand is that the fuel line may be too close to the exhaust, or the muffler mlown out, heating the gas in the line to the boiling point.

This certainly is strange and I think it would be near impossible for the gas to boil since the boiling point for gasoline is around 440 degrees F. I'd be more inclined to say you have a fuel line leak although, again, this is a strange circumstance that I have never heard of.

Thanks for the replies. One thing I noticed now, is that in my lawn, I have several (10-12) brown spots of grass, almost like a little bit of gas dumped out in those areas. They're not very large, only a few inches in diameter, but I'm curious why in those areas only, and not trailed everywhere. I know there is no fuel leak while it is just sitting and not running. I'll do some looking into it, see if I can find one while it is running.

Thanks!

Are those brown spots in places where you would have stopped the mower momentarily, like to back up or something similar? Brown spots are also an indication of a blown exhaust. It burns the grass if the mower is stopped in that spot long enough. It could also be a fuel leak as mentioned.

I just ran into the same problem when, after the neighbor topped off the gas tank, I started mowing and mower started to cut out and die when not horizontal. It restarts but acts as if it is starving for fuel when under load.

I checked tank for fuel level and noticed that although it was full it was bubbling as if it were boiling. Dangest thing I ever saw. I suspected water in the fuel but when I saw the bubbles it confusede the issue. I will let it cool and plan to change the fuel filter. I'll update my findings.:confused: