B&S mower sends gas out tank cap

lawnmowernut2001

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Replaced gas tank due to crack in old one. New gaskets under carb. Cap is also new with the "corkboard" piece underneath.

When motor is running, gas comes shooting out about 12-18" out of the 3 holes in the cap. Seems to occur mostly when the motor is running at maximum RPM and not when the motor is "bogged" a little when actually mowing grass.

Something plugged? Governor springs not closing the butterfly valve properly? Appreciate any input.

My tank setup is like one of these:

1613438891604.png
 

bertsmobile1

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these carbs use pressure pulses from the engine to pump fuel up from the bottom of the tank into that little bowl that the main jet sits in
The pressure pulses move the diaphragm up & down
Yours is leaking and allowing the pressure pulses to go directly into the tank
To check run the tank down to about 2/3 full then remove the cap
You should see bubbles rising to the surface if the pump is leaking.

The diaphragm must go directly against the tank then the gasket sits on top
Very common mistake is to fit them wrong way round.

I always hold the carb upside down , slip a couple of screw through it then place the gasket & diaphragm on the carb then the tank.
Do up the screws lightly to hold it all in place, invert then fit the remaining screws
Do them up in a diagonal criss cross mannar like you would do with a head till they are tight
Refit the carb & tank as a unit to the mower.
 

lawnmowernut2001

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these carbs use pressure pulses from the engine to pump fuel up from the bottom of the tank into that little bowl that the main jet sits in
The pressure pulses move the diaphragm up & down
Yours is leaking and allowing the pressure pulses to go directly into the tank
To check run the tank down to about 2/3 full then remove the cap
You should see bubbles rising to the surface if the pump is leaking.

The diaphragm must go directly against the tank then the gasket sits on top
Very common mistake is to fit them wrong way round.

I always hold the carb upside down , slip a couple of screw through it then place the gasket & diaphragm on the carb then the tank.
Do up the screws lightly to hold it all in place, invert then fit the remaining screws
Do them up in a diagonal criss cross mannar like you would do with a head till they are tight
Refit the carb & tank as a unit to the mower.

Thanks for the reply. I do have the diaphragm directly against the tank with the gasket next and then carb.....tank--diaphragm--gasket--carb. I guess I can take it apart and check it and re-assemble again. Is it that sensitive or fussy of an install?
 

StarTech

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Strange I never knew that intake produced anything other than vacuum impulses; unless, an intake valve wasn't sealing. Even the crankcase impulses are not enough to pressurize the intake system as it is vented at the air filter side.
 

ILENGINE

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Or it could be something as simple as the gas tank is overfilled and not allowing room for slosh and expansion of the fuel. if filled up into the neck area of the fuel cap will cause fuel spray out of the cap vent holes.
 

lawnmowernut2001

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The tank was 1/2 full of gas and it was blasting out the "stream/spray" of gas 12-18" out the 3 holes in the cap.

Perhaps need to make sure the spring under the carb is not damaged or deformed during the install?
 

lawnmowernut2001

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So, I took the tank off. Took the carb off and re-did the diaphragm and gasket very carefully making sure the spring on the carb also did not bent/deformed the wrong way during install. Same issue. Stream of gas comes flying out the 3 holes in the cap. The motor does run.

In additon, I took the top off the mower (the pull string starter piece on top) and checked for any blockage, mice nests, etc....nothing. Pretty clean overall.

I'm at a loss. Something must be plugged somewhere?
 

Scrubcadet10

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Wonder if it would be worth trying a new cap?
 
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