12hp brigg & stratton engine gas mixing with oil in engine block..when start engine runs high rpm can not idle & look like gas come out carb air cleaner overflows & flood out & stop engine ...... we changed oil cleaned carb but it is still doing same thing... we thinks it's oil breather

depend on the engine. some briggs have the carb mounted to the gas tank. If your equipment is older, say about 20-25 years old the carb may have screws(idle mix, high speed mixture). Sometimes the main jet is the bowl nut.

Does the engine want to bog down when you move the throttle? Or does it do absolutely nothing?
If it bogs down when you try the throttle, It could be:
water in the gas
bad or distorted diaphragm (if no float)
stuck needle valve
stuck float

I’m happy to help further over the phone at https://www.6ya.com/expert/karl_14766549451f8563

carb needle valve stuck open clean carb possible bad float dont run with gas in oil

Sounds like your carb needs the fuel/air mix adjusted..uses all the gas at high speed but has excess at low speeds...find your adjusting screw and at med throttle slowly screw it clockwise until the engine starts to sputter then back it out until it starts to sputter again then continue to do this until you reach ideal mix where it runs the best it can..then go slowly through the throttle up and down..when set..unjust your idle speed screw to your ideal idle speed..if this dies not fix ..carb will bed cleaned or rebuilt. Good luck.. Note always push or pull your throttle handle slowly to position avoids gas load up causing sputter and backfire.

Check your oil.

If it is overfilled, milky, or smells like gas. Your going to need to change that ASAP. It sounds like the float has stuck in the past or is still sticking, and it has flooded the crankcase with gas, which has mixed with the oil. And because it is overfilled it comes through the breather hose into the carb. This also explains the smoke most probably because the oil is spewing in the carb through the breather and being burned.

That being the case...your miss is fuel related. Your mix is too rich..(too much gas...lack of air)...as the tank empties, your mix will lean out, basically telling you what the problem is. I recommend that you first clean the air filter, as this can cause the engine to **** too much gas. After cleaning, if you can tell no difference, the next step would be to adust the car...(air bleed). If this doesn't work, remove the carb soak in carb cleaner, blow out with compressed air, and install a new carb kit, (seals gaskets, and new seat if equipt with a needle valve and float. Good Luck! I hope you find this to be very helpful!

If it starts lightly smoking black smoke at high RPM it is flooding or getting too much fuel. The float is leaking you may have not set the float height correctly when you put that carb back together. Or the needle and set could be bad.

If you're using ethanol-blended fuel, the ethanol will eat away at the neoprene parts inside the Briggs carb.Going to need a carb rebuild.
Don't run your engine until it's fixed or you could damage it as gas will pour into the crankcase, causing the oil to thin and lose its lubricating ability.

it sounds like the governer needs to be set and the carb needs to be cleaned out> I would get a carb rebuild kit and go to briggs online and look up how to set the gov dont run it till you do or you may blow the motor

Drain oil... it is full of gasoline due to a bad float pin and rubber gromet in your carb. When your float pin and gromet fail, gas just keeps flowing into your carburetor, eventually flooding your motor with gas and oil mix.