Chainsaw problem

I've had a Stihl MS 310 chainsaw for 6 or 7 years and it has always been fairly easy to start. However, I've had a problem starting it on a few occasions over the past year. Normally, I start it in the full choke position and it'll fire on about the 3rd pull. I can then move the control to high throttle position and re-pull and it will run - and, then move it to normal run position. Usually, the cylinder compression is so high, it's difficult to pull. The times that it won't start, the compression is apparently low because the crank cable pulls easily; almost like ther's an air leak somewhere. I've replaced the plug and air filter to no avail. But, the compression loss has me baffled. On the last start failure, I simply let it sit on a shelf for 3 weeks and then tried it again - it was back to it's old self and started up normally. Has anyone experienced this type of chainsaw problem and might you have any insight on what's causing the change in compression ?

The presence of oil or fuel in the combustion chamber will affect the resistance on the rope pull, so it is hard to say, especially if you put the saw up for a few months, as the fuel in the tank will not want to burn all that heartily, as it is old and nasty, so the unburnt fuel lying around really kicks up the compression. Also keep an eye on that saw series, as the fuel line will crack badly,
especially right below the carb, above the fuel tank and give you troubles.