I have a 9hp Briggs long tail. Wanna say its a late 90's model. Ran perfect last winter, let it sit all summer and got rust in gas tank. I replaced gas tank and fuel filter and fuel lines. Also changed oil. Motor runs great while sitting level. But when I lower the prop below level the motor runs for about 30 seconds and shuts off. If I keep motor close to level it runs great in water or air but in water it's a struggle to keep it high enough. I checked to make sure there is enough oil. I took the carb bowl off and sprayed SEAFOAM all over inside of carb and around float. Gas runs through carb when tilted back. I have no idea what the deal is. Like I said I can run motor at full throttle in water I just have to keep motor as level as possible or it will shut off. And will start on first try.

Is your carb float sticking? Just an idea.

Is it an electric start model? Those were equipped with a mercury tilt switch; a misadjusted one could be the source of your trouble.

Yes it is. But I took the tilt switch off of the motor, it looked burnt out. I thought that only affected the ability to start motor. Who that shut my motor off during use

You're right, tilt switch should only impact the start circuit. One other thought is the low oil level cutoff. I think it's normally open, so you should be able to unplug it and test the impact.

I also think it's the low oil cut off, just unplug it and see it it helps. if that's the problem they are easy to change and don't cost much.

What's the low oil lvl cut off look like? My motor is a 97