If your lawn mower or snow blower refuses to have a smooth constant idle speed and tends to repeatedly speed up then slow down – you have what is known as a “ Hunting and Surging” idle. This is a common occurrence with snow blowers and lawnmowers. With snow blowers this is usually because they sit all summer long stored in some hot garden shed with old gas still in their carburetor and fuel tanks. With time, the gasoline fuel will start to break down leaving a residue that can plug the small fuel circuits in a carburetor. With lawn mowers the “ Hunting and Surging” idle is usually caused by dirt getting into the carburetor as these engines operate in a much dustier environment. However, you can have the same issue if your lawn mower has bee sitting all winter with unconditioned gasoline in its tank and carburetor too!
There are 2 main fuel circuits in a small engine carburetor – the Low speed fuel circuit and the High speed fuel circuit. When one of these fuel circuits becomes partially plugged the engine no longer receives a constant fuel flow. The fuel flow becomes variable which results in the engine idle speed ” Hunting and Surging”. The only thing to do is “get the dirt out” – and that means you will need to clean the carburetor!
When cleaning a carburetor I always try to use something like “Mechanic In A Bottle” first. Just add a cap full to a tank of fuel and let the engine run for 10 minutes. If the residue isn’t too bad you will actually hear the idle smooth out and the “Hunting and Surging” will stop.
If this this does not work then you will need to remove the carburetor from the engine, disassemble it and then allow it to fully soak in “Carburetor Cleaner” for a night. The next day you will need to replace the internal carburetor parts with new parts from an appropriate Carburetor Rebuild Kit.
Okay, now that you have done all this work and finally got your carburetor clean and engine running with a smooth idle – you can prevent this issue from happening again next season by using a FUEL STABLIZER additive. Simply add the recommended amount to your gasoline and your small engine should start first pull next season!
Hope this helps !