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I have a Brute 22" mower with a Briggs 6.75hp 190cc 675 series AUTOCHOKE engine.
at one point the bolts on the muffler fell off and after replacing the bolts, and adding new exhaust gaskets the mower developed this issue.

IT will not cold start unless I force open the Lever that's connected to the autochoke bi-metal thermostat spring between the motor and the exhaust.
Until the engine is hot it will stutter as soon as I let go of the choke lever, after the engine is hot and the thermostat is fully engaged it runs and starts perfectly fine, but when it has cooled down and the thermostat has retracted to less than 80% or so of it's full yoke it refuses to start.

I've replaced the bi-metal thermostat spring with a new one, replaced the air filter,carb work(replaced carb seals/gaskets, float needle, Forced carb cleaner & compressed air through ports), and ran damn near a quarter of a can of sea-foam through the gas and carb cleaner through the intake with no success.

Does anyone have any other ideas as to what may cause this issue?

You said you replaced the thermostat that mounts between the engine block and muffler. Did you make sure you adjusted it according to the instructions? Also, did you replace the spring on top of the carb that pulls the choke open after start up?

Rivets said:

You said you replaced the thermostat that mounts between the engine block and muffler. Did you make sure you adjusted it according to the instructions? Also, did you replace the spring on top of the carb that pulls the choke open after start up?

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I tried to find guides in reguards to the autochoke repair but failed , I thought about how to adjust it but was unsure how to a it appears to be a self contained unit.
Also there's a spring that pulls the choke closed (against the thermostat lever)
And a spring that resists the the throttle. I don't recall seeing a spring that assists pulling the choke open.

Assuming you replaced old with new on the thermostat there should be instructions in the bag on a small bit of paper.
You have to have the top of the thermostat adjusted between the 4 lugs on the choke lever.
Sounds as though you have the right springs fitted.

The thermostat's top just hooks around a flimsy plastic lever and has a 2" space it slides back and forth onthat is hooked to prevent it from sliding off. Also packaging came with nothing other than invoice.

kiyolaka said:

The thermostat's top just hooks around a flimsy plastic lever and has a 2" space it slides back and forth onthat is hooked to prevent it from sliding off. Also packaging came with nothing other than invoice.

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The thermostat should not be hooked around anything.
The thermostat top or spring should fit in between the "U" shape of the choke lever.
If you look on the choke lever close to the "U" there are 4 notches.
Adjust the thermostat from side to side in between 2 notches and bend the spring for forward to backward between the other 2 notches.

pugaltitude said:

The thermostat should not be hooked around anything.
The thermostat top or spring should fit in between the "U" shape of the choke lever.
If you look on the choke lever close to the "U" there are 4 notches.
Adjust the thermostat from side to side in between 2 notches and bend the spring for forward to backward between the other 2 notches.

Click to expand...

http://ww2.justanswer.com/uploads/mr2cycle/2012-04-21_225345_thermo.jpg
Did a google for the thermostat and found this pic, is this the correct positioning for it?
The original fell off when the muffler did so when I placed it back on I thought it hooked around bur if this is correct it may explain!

Voila! Starts and stays on firs pull. That clip threw me pff as it looks like it was supposed to go aroind the other lever, but it wasn't and I was holding the choke closed
Thanks to all who replied