Fuel bypassing the carb when not running

[arch252]

I have a couple of riding mowers with what appears to be a similar problem. One is a Troy Built with a B&S 18.5 hp twin cylinder, the other is a JD STX38 with a 13 hp Kohler Command. Gas appears to be bypassing the carbs when the engine is not running. Both carbs are gravity feed, neither has a fuel pump.

When I got the Troy Built the guy told me that there was a problem with the carb, gas was running into the crankcase when the engine was not running. He had taken it to a reputable shop and he told them to fix it the cheapest way possible. They installed an inline fuel cutoff switch that he had to turn off when the mower wasn't running. I did a complete and thorough carb rebuild to try to fix it. I checked to make sure that the float/needle valve were working properly (dropped the bowl after it was reinstalled). That carb has a fuel cutoff actuator on the bottom of the carb bowl and it appears to be working correctly.

With the Kohler engine, I had to rebuild the engine after a screw came loose from the carb choke plate and passed through the intake and the piston stamped it into the cylinder head. Turned out to be not too serious of a problem, but made one heck of a racket when it happened. I didn't really mess with the carb because it was running fine. Once it was back together I found that when I started the engine after it had been sitting a few days, it would blow fuel out of the exhaust. I have not opened the engine again, I checked the oil and it does not appear that any gas is getting into the crankcase, but it would appear that the gas is flowing through the exhaust valve somehow and pooling in the exhaust/muffler.

Has anyone experienced this type of problem. Stupid question, I know, you guys have seen everything. Can someone tell me what might be going on? I know that's pretty limited information but I'm hoping there might be some kind of common, general problem associated with this that someone can point me toward.

 

[SeniorCitizen]

I have a couple of riding mowers with what appears to be a similar problem. One is a Troy Built with a B&S 18.5 hp twin cylinder, the other is a JD STX38 with a 13 hp Kohler Command. Gas appears to be bypassing the carbs when the engine is not running. Both carbs are gravity feed, neither has a fuel pump.

When I got the Troy Built the guy told me that there was a problem with the carb, gas was running into the crankcase when the engine was not running. He had taken it to a reputable shop and he told them to fix it the cheapest way possible. They installed an inline fuel cutoff switch that he had to turn off when the mower wasn't running. I did a complete and thorough carb rebuild to try to fix it. I checked to make sure that the float/needle valve were working properly (dropped the bowl after it was reinstalled). That carb has a fuel cutoff actuator on the bottom of the carb bowl and it appears to be working correctly.

With the Kohler engine, I had to rebuild the engine after a screw came loose from the carb choke plate and passed through the intake and the piston stamped it into the cylinder head. Turned out to be not too serious of a problem, but made one heck of a racket when it happened. I didn't really mess with the carb because it was running fine. Once it was back together I found that when I started the engine after it had been sitting a few days, it would blow fuel out of the exhaust. I have not opened the engine again, I checked the oil and it does not appear that any gas is getting into the crankcase, but it would appear that the gas is flowing through the exhaust valve somehow and pooling in the exhaust/muffler.

Has anyone experienced this type of problem. Stupid question, I know, you guys have seen everything. Can someone tell me what might be going on? I know that's pretty limited information but I'm hoping there might be some kind of common, general problem associated with this that someone can point me toward.

Both carbs need needle valve and or seat attention to solve the problem.

 

[bertsmobile1]

Double on the float servicing.

If the fuel is bypassing the float needle when it is stationary it is also doing it while it is running so you will be running rich.

The Kohler could have either a Nikki or Walbro.
One has a replaceable seat one hasn't .
A full carb service kit is the second most economical way to go.

Ditto for the Briggs.

I fit in line taps to all ride on I work on.
Usually between the pump / tank & fuel filter so you can change the filter without getting fuel every where.
I also try to educate my customers to shut it off and run the engine dry when they have finished mowing for the day as it prevents a lot of potential problems & makes them start better next time.

 

[arch252]

Thanks so much guys, good info. The B&S carb had the metal insert that had to be pressed in. If I recall there was no seat, just the metal insert. That was my first experience with that type of seat. I replaced that and the float when I did the rebuild. Not really sure what I could have done wrong. I guess nothing to do really but tear the carbs down and go through them again. I was really, really hoping you guys would tell me I was just overlooking something else really simple. No such luck!

 

[panabiker]

I have a Troybilt with the same engine. I had the same problem and rebuilt the carb a couple of times, but the fuel leak problem never totally went away. I have heard others having the same problem. My solution was to add a fuel shut-off valve on the fuel line and turn the valve off whenever I am done using the mower. This has been the best $6 I spent on it.

 

[Grassbandit]

Definitely needle seat, yeah adding a fuel cutoff is an easy and cheap fix. Eventually, you should get in there and address the real issue.

 

[SeniorCitizen]

Thanks so much guys, good info. The B&S carb had the metal insert that had to be pressed in. If I recall there was no seat, just the metal insert. That was my first experience with that type of seat. I replaced that and the float when I did the rebuild. Not really sure what I could have done wrong. I guess nothing to do really but tear the carbs down and go through them again. I was really, really hoping you guys would tell me I was just overlooking something else really simple. No such luck!

The needle valve will have some kind of seating surface made for the needle to set on to prevent gas flow. What engineers have determined to be the best is anyone's guess but it's still a valve seat. While you have the carb off hook a hose to the bowl inlet and check the needle/seat function via gravity feed. Water in the hose will work as well as gasoline. You should be able to stop flow by pushing up on the float with about a pound of pressure, possibly less. A feather light touch is all it should take. No need to re-install the carb until the needle valve passes that test.