Compression results

[Joker2]

Went down today, got it to run with portable tank and my built in tank, same results, I don't think it's fuel. I can run it idling it using the throttle linkage at engine, keeping it at 800-1,000 RPM's when I push it close to 1,500 she kicks and shuts down, she is also running a little rough. After I was done running it I inspected a few plugs, they all looked normal except one, the bottom starboard plug was clean and shiny just how I left it after the last time I was there when I cleaned it. That doesn't seem right. Like I said from previous posts that plug was the one that always had black buildup on it whenever I changed plugs, yet the compression was 120lbs. on that cylinder. Didn't have time to check for spark, maybe that's the problem, waht do you think ?