I would say it's running rich (too much gas). As it starts to lean out when the fuel gets low, it runs better because it's finally getting the right mixture. I don't know what the model number means as to what kind of carburetor/choke it has, but since it has a primer button it should be a float carb. If it has a mixture screw, obviously that needs to be leaned a little, but they haven't been on mowers in many years. I would say your carb float is stuck, leaving the fuel inlet valve partially open. Start using Sta-Bil Marine fuel stabilizer in you gas can along with 2 oz/gal of Marvel Mystery Oil. With the engine off, pick up the side of the mower with the carburetor and drop it several times from about 6" height. If this doesn't get it right, the carb will have to be overhauled.
Also check the oil because if I'm right about the float, it can leak down gas into the engine even when turned off. If the oil level is higher than it should be and the oil is diluted with gas, this confirms the problem.

Excellent info. I'll give it a try and report back. Thank you so much!

A new diaphragm kit should address the rich running. (795083) Gasket on the carb side. The arm where the gov spring attaches gets bent inward on these and may require adjustment.

Thanks Tom. I did notice that the diaphragm looked "bubbled" or separating...probably time for a new one. Thanks for the tip.

Before you tear into the engine or carb, get a can of Sea Foam and mix it with some gas and it through the engine. If you can find it in the spray can, they're hard to find around my area, you can spray it directly into the carb as directed. I read so much about Sea Foam I decided to try it on my old equipment that has never had any of the fuel system cleaned. My rider seems to run smoother and have a little more power, but the biggest imrovement I found was in my push mower. I mixed the appropriate amount of Sea Foam in a tank of gas before I cut the yard a few weeks ago and the mower just runs so much smoother now. It was OK before, but now I can tell it was straining before. Actually the biggest improvement I saw was in efficiency. Whereas before Sea Foam, I could cut about 4000 square feet on a tank of gas, now I can cut almost twice that.

Sea Foam is a great product, and might lengthen the lifespan of the diaphragm, but at this point, it's too late. Replace it and you'll think you bought a new mower. Also, clean the internals of the carb at the same time.

They're right about the diaphragm. If I had known it was a 'Pulse-Jet'-type carb that would have been my rec too. What great carbs they were. I did not know any of them had a primer bulb though - how the heck does that work?

Thanks all. I have ordered the gasket (diaphragm) and I'll let you know if that does the trick. In the meantime, I tried "dropping it" a few times to unstick a potentially stuck float but it didn't help. Thanks again for all the input!

There is no float. This is how your carb looks.........I think.

3.75 Briggs carb rebuild

This is an easy DIY job to replace the diaphragm of the carburetor. A damaged diaphragm will cause engine surging, hard starting, and erratic high speed operation. You may also notice that the primer bulb isn�t working properly.

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This is an overview of the carburetor and these types of carb-over-tank are typical of small Briggs engines. Note the two governor springs. The carb/tank assembly is held in place by a �" bolt to the left of the carb and a 3/8" bolt on the engine head to the right. Remove both bolts and pull the tank straight out with a rocking motion. If possible, the carb/tank assembly should be blown off with compressed air and all loose dirt removed. Use the proper safety equipment, especially safety glasses, before using any compressed air in any of the steps below.

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In the highlighted area you will see the governor linkage still in the throttle plate of the carburetor. You will have to gently separate the carb/tank assembly from the linkage. It is a Z type linkage. It is not necessary to remove the springs or the other end of the linkage, from the governor.

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Note that on the rear of the carburetor is found a white plastic ring and O ring. Sometimes these remain on the tube of the engine housing. If so, place them back into the carburetor body, as shown, before reinstalling the carb/tank to the engine.

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Note the raised area of the diaphragm. This is caused by age, but more often, by ethanol fuels, which damage the diaphragms. A damaged diaphragm will cause engine surging, hard starting, and erratic high speed operation. You may also notice that the primer bulb isn�t working properly.

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Shown here are the dip tube, which brings up gas from the tank, in the bottom left corner. Just behind that and partially obscured, is the orifice that compressed air can be used to clear the dip tube of debris. It also leads to the primer bulb, so any air must be low pressure and the primer bulb must be held in the depressed position to prevent it from blowing out. In the center top, you�ll notice the jet, surrounded by a wire mesh filter. In the bottom right is a spring. Be careful to not lose the spring or forget to install it during reassembly.

Remove the old gasket and diaphragm combo from the carb body. Install a new gasket/diaphragm on the smooth surface of the tank. I always blow out the tank with compressed air to remove dirt and any water. Be aware that there may also be old gas in the tank. I always empty the tank and let it sit in the sun until all the gasoline evaporates before applying compressed air. Align the carb and lower it down on top of the gasket, trying to keep the five screw holes in the gaskets aligned. Insert the screws and partially tighten them and install all five screws. Tighten each screw until it gently bottoms out. Then turn each screw another quarter turn. It is not necessary to use more than a few inch pounds of torque on these screws. Reinstall in reverse order. When installing, be sure to engage the breather tube with the rubber piece on the carburetor.

Thanks again, everyone, for all the help. Baymee's photos were particularly helpful for making sure I had the springs in the correct places.

I replaced the gasket and diaphragm and that did the trick.

The mower is running very strong again, and now I know a whole lot more about the engine.

Thanks again!