I agree that it's likely a carb issue, if in fact the rest of the fuel system (tank/line/pick up) are all clean. Did you empty and clean the fuel tank? If so, you probably have a defective diaphragm in the carb. You should be able to get a kit for the carb. I bought one a while back for a Tillotson carb on one of my Johnsereds saws.
I didn't need it however, because the problem was actually a defective spark plug. Have you tried changing the spark plug? Even if the plug you're using sparks outside the cylinder, it may not do so under pressure. That's what my defective plug did.
