riding mower leaking gasoline

The gasoline line from the gas tank to the carburetor in my riding mower has no shutoff valve. It is a carbureted engine.
This week I filled the tank in the riding mower. Later the same day, I noticed a gasoline smell in the garage, and traced it down to the mower. It was dripping gasoline out the exhaust! The engine hadn't been run in more than a week. The gasoline level in the tank had dropped about a quart!!
The only way I can figure that gasoline could get from the tank to the exhaust is through the float valve in the carb. Do most inexpensive carbureted riding mowers nowadays have a float valve in the carb and no shutoff valve in the gas line??
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yes Ether Jones wrote:

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Ether Jones wrote The only way I can figure that gasoline could get from the tank to the exhaust is through the float valve in the carb. Do most inexpensive carbureted riding mowers nowadays have a float valve in the carb and no shutoff valve in the gas line??
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Yes.
You didn't ask, but a common cause for your problem is a small bit of debris holding the float valve open. The cure is a simple cleaning. The prevention is to be extremely careful to get gasoline and only gasoline into the fuel tank when refueling. It doesn't take much of a bit of grass clipping, dust, dirt, etc. to cause a problem.
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Gideon wrote:

There's a filter in the gas line from the gas tank to the carb. Although I am extremely careful about cleanliness when filling the gas tank, I doubt if any debris in the tank large enough to cause a problem in the carb could ever get past that filter.
I changed the gas line and gas filter two years ago. I spent an hour cleaning the gas line fittings with a toothpick and toothbrush and Q-tips and vacuum before removing the gas line, to make sure no debris got into the fittings. I inspected the new gas line before installing and noticed it had manufacturing debris inside it! I washed it out with fresh gasoline before installing. I'm pretty sure there is no debris in the carb.
I had a friend who took his riding mower to the place of purchase for a "tune up". Among other things, they replaced the gas line and gas filter. His mower hasn't run right since. I told him some high school kid probably replaced the gas line and got debris in there that found its way to the carb.
Back to the gas leak: In my case, I had run the mower out of gas and it sat that way for a week before I filled the tank with fresh gas. A couple of hours later I noticed the leak. I NEVER run the carb dry on this engine; this was a one-time aberration. I'm convinced there is a direct correlation between running the carb dry, and the leak. I have the service manual for the engine but it gives no details about the float valve design. I stopped the leak by tapping lightly on both sides of the carb with a piece of iron rebar.
Has anyone else ever experienced this sort of problem? What's the street wisdom on fuel additives for cleaning the float valve?
The engine is a 4-cycle Briggs 16.5HP L-Head opposed twin. Does anyone know if the float valve seal is brass, or some sort of elastomer?
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Ether Jones wrote:

My single experience with additives : they clog the carburetor by dissolving crud that lines the fuel system, if you use it on an old engine.
I've been using stabil with a Honda generator from the beginning, and borrowed some of that fuel for the ancient lawnmower. The lawnmower ran fine for about an hour and a half and then started missing, and hasn't run right since.
The Honda works fine.
So I'm guessing cleaners aren't a great idea in old fuel systems. Odds are.
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Ron Hardin wrote:

Incidentally, if anybody knows if there's a mixture control on a Briggs and Stratton 4 hp push lawnmower engine, if you start taking things off, let me know. I think it can be fixed by leaning out the mixture, as it runs great when it's just about out of gas. I see no signs of an adjusting screw.
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Ron Hardin wrote:

If you were to bend the lever on the float so as to lower the gasoline level in the bowl, would that make the jets run a bit leaner?
I have a vague recollection of doing that back in the '70s on a motorcycle engine carb.
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Ron Hardin wrote:

I use StaBil in all my small gas engines over the winter.
It was my understanding that StaBil merely keeps the gas from forming varnish. I was not aware that it had any solvents/detergents for dissolving existing varnish deposits.
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Given your described cleanliness, I suspect running the carb dry did cause the float to stick. The little bit of gasoline left in the system after the engine quit evaporated, leaving behind a bit of sticky gum or varnish. The float was all the way down (opening the valve) and there it stuck when you filled the tank.
I highly recommend stabil. It prevents (or nearly so) that sticky residue from evaporating fuel.
In equipment I use frequently I typically do not use stabil. But in my chipper, the generator and every 2-cycle engine I always use stabil. In the lawn mower I start using stabil in the fall as I never know which cutting will be the last. And I try to keep the fuel tanks full (and if there is a shutoff valve, I close it, just in case).
I'm going to try Pri-G this winter... It is a lot cheaper than stabil.
sdb
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