Join Date: Oct 2007
Location: East Central Wisconsin
Most likely the idle circuit is plugged. The engine runs off the idle circuit even at full throttle if it isn't under load. The surging is happening because it is starving for gas off the idle circuit, so the governor opens the throttle plate to get the engine back up to speed, and it pulls off the main circuit and speeds back up. When the plate closes again, it starves for fuel and it starts all over again. On the side of the carburetor, right above the lip of the fuel bowl, there is a jet (assuming its a fixed jet unit from your description) that can be removed and cleaned. It probably has a small plastic cap over it that you can pop off. The jet will unscrew and you can blow carb cleaner and compressed air through it. The metering hole is on the very tip of it, and its pretty damned small (something like .005" iirc). I usually blow carb cleaner back into the hole it screws into in the carb to flush that out as well. When you screw the jet back in, make sure it is tight, but don't twist it off.
The above is assuming you have a newer fixed jet carb. The bowl nut (also the main jet as you said) will be smooth on the bottom with a number stamped there identifying the jet. If it was an adjustable jet carb, there would be a screw sticking out of the jet with a spring on it. The idle jet that I mentioned above would also have a spring on it and would just be a sharp pointed needle. They stopped using those carbs 15-20 years ago due to emissions regulations.
Another possibility is that the carburetor bowl vent is plugged. Some vent back into the air cleaner housing through a hole or through a small tube that sticks into the housing a short ways (usually red). Others vent through a very small hole in the side of the carburetor. This hole is usually on the same side as the idle jet and is right below the flat "boss" that has the carb id numbers stamped on it. Blow carb cleaner through it if you find it.
I work on small engines at my job in town, and I must have cleaned 20-30 carbs already this season on snow blowers with the exact same problems you are describing. When storing a 4 cycle engine, you should fill the fuel tank 100% with fuel and stabilizer mix, let the engine run a few minutes to get the mix to the carb, then shut the fuel off and let it run dry. You'll run into far less issues that way. Our engine distributor has been recommending running premium fuel in all small engines to help offset the problems caused by ethanol in the fuel. In storage, the ethanol separates from the fuel and absorbs water, causing all sorts of problems with carbs and whatnot. That's why you want a full tank of fuel. There is no room for condensation to form in the tank if its full of fuel. If you can find fuel without ethanol, use that. There are a handful of stations around here that sell one or more grades without the ethanol. Also, never run fuel with more than 10% ethanol in any small engine. All the manufacturers we deal with will void your warranty if you do. Another tip is to use a marine grade fuel stabilizer (we use Marine Grade Stabil) to help deal with moisture and ethanol issues.
2000 WJ Grand Cherokee Limited, 4.7l V8, Quadra-drive
Farming is easy when your plow is a pencil and you are a thousand miles from the cornfield. -Dwight D. Eisenhower
Don't curse the farmer with your mouth full.
Ah, cows. A thousand roast beef sandwiches wrapped in a gorgeous leather coat.