The chain being tight on the bar is probably completely unrelated to the amount of torque you put on the clutch cover nuts.
Check the following:
1. Your chain brake. ie, engage it and then release it. Is it working properly? Just because you checked it earlier doesn't mean it isn't engaged now.
2. Make sure the bar and chain are getting oiled. If they are dry it can cause binding.
3. If the chain is still tight, try removing the bar and chain and then check the nose sprocket on the bar. This should be able to spin easily by hand (sometimes they get jammed up)
4. Check the clutch/sprocket area. This should also spin freely by hand. (if the saw has been running this may be VERY HOT so be careful) Something may have broken or junk can bind it up.
5. When you put it all back together make sure you don't tension the chain too tightly. in the middle of the bar you should be able to pull the chain drivers out of the bar to the point you can barely see the bottom of the drive links and when you release it, it should snap back tight. This will vary a bit depending on how long the bar is. You did clean out the bar groove with a small screwdriver or the edge of a putty knife? That needs to be clean for several reasons.
6. Be aware that the chain tension can vary with the temperature of the chain and bar. A dull chain will get hotter than it should and can heat up the bar and even saw parts causing damage if you press the issue. A chain should always be kept sharp. Don't look at "just a few more cuts" and think it is ok to burn through them with a dull chain. Either sharpen it or come back later when you are equipped properly. If you tension the chain while it is hot, back off the tension when you're done cutting or the contraction upon cooling can damage your crank/clutch.
