Your engine is 9 cubic inches (148 cc), ninth design generation, with vertical crankshaft, Pulsa-Jet carburetor, plain main bearings, and rewind starter. It was made on 8 October, 2003. I believe it would be US built - I don't think suction carburetor side valve engines have been made in China, but I stand ready to be corrected if someone has contrary information.
Edit: Joe Carroll has posted later in this thread that these engines were in fact made in China, and it emerges that the one this thread is about, is a Chinese engine. Thanks, Joe.
Here is the illustrated parts list:
http://bsintek.basco.com/BriggsDocumentDisplay/18howHXCnfBhU7y.pdfHere is the operators' manual:
http://bsintek.basco.com/BriggsDocumentDisplay/msvtDR-K_ny7tH217FzoPu.pdfThe parts list shows your engine as having a foam air filter. That type of filter is wetted with engine oil, then wrung out, as shown in the operators' manual. I do not know why you would have needed a new element for it, after someone had apparently stored the mower with the crankshaft horizontal. The foam is very likely to have become saturated with oil, but all you need to do is wring it out then reassemble the air cleaner.
If the mower is stored with the crankshaft horizontal or nearly so, and the cylinder downward, the engine oil will pass through the breather tube into the air cleaner and oil will also run into the intake port. If the intake valve is open it will run into the cylinder, and if the exhaust valve is open as well, it will then run out through the exhaust port into the muffler and possibly onto the floor.
To cure that problem it is necessary to put the four wheels of the mower on a level surface, wring out the foam from the air cleaner, turn the engine over a number of times with the spark plug removed to allow oil in the cylinder to be ejected through the plug hole and exhaust port, and unless you want an unholy mess when you start it, drain the muffler. Refill the sump with clean oil. Because your engine has a Pulsa-Jet carburetor, it has a small lift tank inside its petrol tank. Very likely, if your mower had fuel in its main fuel tank and was stored cylinder-downward, fuel will have run into the small lift tank inside the main petrol tank, and through the second pickup tube that draws from that tank, into the main jet, and thus into the intake pipe and possibly into the cylinder. At the same time some oil may have run backward into the small lift tank from the intake pipe then through the main jet. So, the engine may not run very well when started. There is likely to be blue smoke for a while due to oil in the cylinder, and the mixture may be lean temporarily if there is oil in the main jet or the small lift tank.
Hunting of a governed engine consists of an endless series of speed cycles, with a cyclic period typically of a couple of seconds. It is typically caused by lean mixture, not rich mixture, but it can also be caused by a damaged or partly jammed governor. Your engine has an air vane governor - you should check that it moves smoothly with very little force on the vane, and moves the linkage and the throttle. If your engine is hunting continuously you should look for possible causes of lean mixture. Since your carburetor appears to be an emissions model without a mixture adjustment screw, lean mixture is likely to be due to either not having an effective air cleaner installed at the time, or some obstruction in the fuel system, probably due to dirt. Is the inside of the fuel tank completely clean, or is there some sediment or grass residue? The primary fuel pickup tube in the tank has a fine filter at its bottom (open end), and this is quite prone to becoming blocked and causing fuel starvation under load (usually it is all right at idle, but gets lean at higher speeds and loads).
If your engine is continuously blowing blue smoke, it is possible that it has oil contamination of the fuel, and this could cause symptoms of lean running. Strangely enough lean running can cause the exhaust to smell of hydrocarbons, due to poor combustion efficiency, but the smell is of singed hydrocarbons, and it is quite different to the smell of rich mixture.
After working through these points, if your engine is still hunting and you found debris in the fuel tank, you will need to partly dismantle, and clean, the carburetor. We can talk about how to do this if you find that debris in the tank.
