My cleaning of the carburetor is usually not very involved, typically including making sure the choke plate and throttle are free to rotate (and cleaning the shaft and springs if they doesn’t freely rotate), and inspecting the venturi and jet and anything I can see without disassembling it, cleaning wherever I find dirt. I’ll typically use compressed air and/or gasoline to clean the carburetor, wiping it clean with a shop rag then stuffing the venturi with a piece of rag or paper towel to prevent contamination while I work on the rest of the saw.
By the way – I like to plug up every orifice that leads to the combustion chamber whenever one is exposed; accidentally drop a screw or a small clod of dirt through the exhaust port, air intake or spark plug hole and you’ll always remember the time that plugging those holes can save.
Next I’ll look over and clean the engine exterior, more specifically the cooling fins. I take a toothbrush and compressed air to the fins and even remove the muffler to get at the brick dust deposits around the exhaust port. Pistons and cylinders tend to go bad near the exhaust. The heat from the exhaust itself will tend to build up and harm the piston rings and cylinder, but when brick dust accumulates (and remember that clay brick is an excellent insulator) on those fins near the exhaust port, it keeps the area that much hotter, increasing the chance the engine will overheat and the rings will melt to the piston. At that point you either have a $900 paperweight, or a $300-$500 repair.
Another area where there tend to accumulate a lot of carbon, oil and gunk is the decompression valve. The function of it is pretty straightforward – when you depress it, it opens up a small pathway for air to escape during the compression stroke of the engine, greatly reducing compression, making it easier to start. Once the engine starts, the frequency of compression strokes blasting air at the valve head are enough to push it back out, closing that escape pathway, giving the saw full compression to start turning the blade. In time the buildup of carbon can affect the seal, allowing air to leak during compression, reducing saw power. To clean it I’ll just remove it from the engine and soak it in gasoline. But before you remove it, clean as much dust and deposits as you can from the area – otherwise they may end up falling into the cylinder. Once you have the valve out, stuff a rag into the hole where it was, then clean the valve. Pictured is before and after cleaning of the valve on this saw.
For the cylinder and piston themselves, I just look through the exhaust port to check the condition of the piston, rings and the cylinder walls. You can see a little bit of brown on the piston pictured below, but that’s our Stihl TS 400; it’s 9 years old, so I’d expect to see a little wear and tear on that brick saw. Buildup of brick dust around the exhaust port is part of the reason for the burning of the piston.
I’ll then check the spark plug (cleaning the area in advance like I did for the decompression valve), and replace it if it appears that the electrode or side terminal (where the current arcs, causing combustion) is getting misshapen, or is fouled for one reason or another.