Briggs auto choke

[diplexer]

My problem is that my Briggs with the 725 EX engine and the air vane/thermostat auto choke system will not run with the choke spring attached. The engine starts first pull but choke will not open with the spring attached and just chokes out. If I remove that spring it starts right up if I push the air vane so it closes the choke plate and when I let go it runs fine. It just will not open the choke if spring is on. It's the correct spring. Got a new carb and thermostat also. Checked thermostat position and it's within the manufacturers specs. Air vane is not binding and not broken. I'm baffled. Any suggestions? Thanks.

 

[Ocean909]

Its possible the linkages are not in the correct position and keeping the choke from opening.

 

[diplexer]

Not sure what linkages you're referring to since it is the air vane that is connected to the choke.

 

[slomo]

Without touching the mower, choke should be ON. You should see the choke plate in the closed position.

When you pull the rope, engine will/should fire, slowly/quickly increasing revs. These increased revs create more air from the cooling fan on top of the flywheel. More air from the fan gradually CLOSES the choke plate (air vane). At full revs, NO choke should be on. Should be at full song slaying the grass.

Some have an exhaust deal in the muffler pipe that when it gets hot, it OPENS the choke plate slash linkage slash air vane.

 

[Ocean909]

Not sure what linkages you're referring to since it is the air vane that is connected to the choke.

So there is a link that runs from the thermostat on the muffler to a little plastic arm that pushes the choke open and closed when the muffler gets hot or cold. If that is in the wrong position in relation to the plastic arm with the spring on it, it can jam the choke movement.

 

[diplexer]

Not sure what linkages your referring to since it is the air vane that

Without touching the mower, choke should be ON. You should see the choke plate in the closed position.

When you pull the rope, engine will/should fire, slowly/quickly increasing revs. These increased revs create more air from the cooling fan on top of the flywheel. More air from the fan gradually CLOSES the choke plate (air vane). At full revs, NO choke should be on. Should be at full song slaying the grass.

Some have an exhaust deal in the muffler pipe that when it gets hot, it OPENS the choke plate slash linkage slash air vane.

Yes, I pulled shroud and blew off all fins and cleaned all area (although they weren't really that crudded). Problem is engine starts right up but does not rev up quickly enough to let the air vane turn off choke when spring is attached. I'm baffled. The thermostat that moves the air vane was also replaced and is in the recommended position. Almost seems like the spring is preventing choke from opening.

 

[slomo]

Not sure what linkages your referring to since it is the air vane that

Yes, I pulled shroud and blew off all fins and cleaned all area (although they weren't really that crudded). Problem is engine starts right up but does not rev up quickly enough to let the air vane turn off choke when spring is attached. I'm baffled. The thermostat that moves the air vane was also replaced and is in the recommended position. Almost seems like the spring is preventing choke from opening.

I was talking in general terms on how those systems work. Some have an air vane. Some a heated thermostatic exhaust lever.

 

[Ocean909]

Your setup should look similar to the pic below. You should be able to move the link coming from the bi-metal thermostat without to much difficulty. It should move that black arm forward which opens the choke. If it doesn't, I can guess its one of three things.
The arm I mentioned previously is in the wrong position.
The wrong spring is installed and has too much tension for the thermostat to overcome.
The bi-metal thermostat isn't moving or moving enough to open the choke when the engine gets warm