Compression

Check the valve timing first before you pull the cover to make sure that's the problem. It could be the problem is only in the head. Also, If the chain skipped far enough, it may have bent valves so you may have to take off the head anyway. If the valve timing is ok, adjust the valve clearance and retest the compression. If it's still bad, you'll need to pull the head and do a valve job. The actual compression readings can vary depending on engine wear, cranking speed, compression ratio, etc. The readings should all be over 100 psi and within 20lbs of each other regardless. As for having the throttle open during the test......I've never seen that make any significant difference.