riding mower leaking gasoline

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.

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?

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.

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.

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.

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.

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