Riding mower engine question, Briggs and Stratton

[natto fire]

So this thread is kind of a long shot, as I am already going to pull the trigger on a carb rebuild kit, but I wanted to pick a brain or two.

I have an 18.5HP B&S twinII motor that ran perfect when I put it away last fall. I went to start it up today, and it did run for a minute, but died when I tried to throttle up.

I took the air cleaner off and noticed that it was dumping fuel, a crazy amount, into the air box. Like a geyser of gasoline that soaked the filter and puddled in the air box.

I took the carb apart and didn't notice any diaphragm obviously broke, so I am worried about a carb rebuild fixing the problem.

The fuel system is all gravity fed, so it is not a pump issue.

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[stormkroe]

As far as I know, the only way that can happen is an intake valve sticking open (or having a hole in it I guess) unless the airbox is 'down hill' from the carb and the float is stuck, but I've never seen a motor with a low airbox.
I'm guessing valve, something bounced in the way of it closing or the spring broke when you first fired it up.

 

[natto fire]

As far as I know, the only way that can happen is an intake valve sticking open (or having a hole in it I guess) unless the airbox is 'down hill' from the carb and the float is stuck, but I've never seen a motor with a low airbox.
I'm guessing valve, something bounced in the way of it closing or the spring broke when you first fired it up.

I will check that out, but I forgot to mention the fuel was not coming from the jets. There are two holes near the throat of the carb (above the Venturi) where it was spurting from.

This mower was a gift from my grandpa, and he said he rebuilt the carb two years ago, which explains it looking great.

As far as the valves, I am assuming they are ok, as the starter gets bogged down on the compression stroke. I am going to throw my compression checker on it anyways, while I have the carb off of the intake manifold.

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[C1]

Classic stuck float.

Tap the side of the carb (float bowl) lightly with a small ball peen hammer, remove the spark plug & blow it dry, reinsert then give engine another try to start.

Rebuilding the carb again is not a bad idea. Hopefully before it was stored for the winter, the carb was drained.

Oh ya, check the fuel tank for rust, debris/trash as that's what may be getting into the carb & jamming the float valve. When re-rebuilding the carb, you might should flush the fuel tank & gas line too.

 

[stormkroe]

I agree with the stuck bowl float idea, if you can see it coming in pre-venturi (though why isn't it coming out the overflow drain?) that is probably it. How on earth is it getting back uphill into the air box?
Terrible side effect of it leaking there is that it will seep down into the motor, washing out the cylinder walls and filling the crankcase with gas

 

[C1]

Considering what has happened after a carb rebuild, inspect you gas tank. Gas tanks often have a coating to protect from corrosion that can begin to disintegrate. The particles then mess up the carb. Gas containing alcohols are known to be bad this way and leaving alcohol based fuels sit in a gas tank over time damages the liner/inner coating.

If this is what is happening, you may have to (besides cleaning out the carb) replace the gas tank.

 

[mindless1]

It might be good for you to further describe the setup on this mower. Mine has a ~ 17 y/o B&S Intek V-Twin model 405700/406700/407700 19HP engine. It has an all plastic fuel tank and inline fuel filter. I would expect all came stock with an inline fuel filter. Point is that filter should block anything upstream from getting to the carb, except bad gas once it sits there turning to varnish.

Yours might differ some, mine is OHV and has a separate fuel pump, nothing fancy though, looking at the engine manual it looks like it's diaphragm based, not electric.

 

[twinrider1]

Also, if that has been going on for any length of time, gas has been going into the cylinders and leaking into the crankcase.

Check your oil for signs of gasoline. If there's any doubt, change it.
Pull the plugs and crank the engine over for a bit to blow any gas out of the cylinders.