Fuel runs straight through carb and out the exhaust


Old 04-18-05, 10:50 AM
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Exclamation Fuel runs straight through carb and out the exhaust

I have a small, Lawnboy-20 mower that was given to me by my dad. He stated that it starts on first pull, and will run great until turned off, afterwhich it will drain the entire tank of fuel onto the mower deck. Rather than fix it, he just ran it with a low tank.

He surmised that there was no needle valve in it, however after several unsuccesful attempts at starting it I opted to remove the carb assembly and clean it up before ordering a rebuild kit. After pulling the float bowl however, I see the typical little square aluminumn float needle and a freely swiveling float in good condition.

Before I throw a kit in this thing, can anyone think of anything else that would cause the gasoline to run straight throught and out the "muffler".

Old 04-18-05, 04:13 PM
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You're on the right track, just be sure to shake the float and listen for any liquid in there. If so, replace such as well. One thing that concerns me though is that your dad ran it for a time with this problem. Gasoline may well have gotten into the crankcase and diluted the oil and its lubricating ability. I'd be concerned that you may have internal engine damage. The easiest way to check for possible trouble here would be to remove the cylinder head and check the cylinder for any scoring. If so, you should elect to do a tear-down and check the crank, cam, rod, etc. inside the engine for any wear. If not, it's still remotely possible that you have internal wear.
Old 04-19-05, 12:23 AM
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Ailling Lawnboy

Is this Lawnboy a 2 cycle or 4 cycle? My Lawnboy 21 is a 2 cycle and as such fuel leaking into the crankcase won't hurt a thing. Puey61 you're right about the float probably being an issue. A small hole in the float would allow it to slowly fill with fuel after gassed and started. Then after turning off the mower the gas filled float would hold open the needle valve allowing fuel to overflow and leak out. I would suggest removing the float and checking for a crack or pin hole. Heat the float to "cook" out any remaining fuel and plug the leak with epoxy. oldmedic
Old 04-19-05, 06:17 AM
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The same happened to my mower. Each morning I go to start to cut grass, the gas tank was empty. The float on the bottom of the carb sticks down allowing the gas to pass through the carb into the head then into the oil case.
The float & bowl probably just needs cleaned out or the float assembly may need replaced. Each time this happens, the oil will need changed before starting.
The way I resolve the problem, rather than putting more money & time in the mower was to put a cheap inline shutoff valve ($3.00) in line between the tank & the carb. Each time I finish cutting grass, rather than shutting off the key to stop the engine, I shut off the valve to let it starve the engine for gas. The next time I start the engine, I fires right up quickly.

Good Luck,

Old 04-23-05, 07:24 AM
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Needle valve seat

Don't forget the needle valve has a replaceable "rubber" seat that is included in your carb repair kit. If the seat is worn, the needle valve can leak fuel--even if the float mechanism works.

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