You may need to read the manual again. To relieve the fuel pressure you first disconnect the FUEL PUMP UNIT 4P CONNECTOR. It's the big square plugin electrical connection. It sits on top of the plastic covering over the fuel tank. Then, crank and let it run until it stalls. Then, DISCONNECT the negative terminal on the battery, THEN, you can disconnect the fuel pump hose.Is there a trick to remove the line off of the fuel pump with the green clip? It says to remove it first to relieve fuel pressure. I got a buddy to push in one side of the green clip with a small screwdriver while I did on the other side while pulling up. It isn’t budging. I’m worried I’m going to break it.Hi: The dirty fuel filter can cause the bike to over heat and slow down -- and back firer -- jerk climbing hills. Burning oil is something one has to find by checking things.
I'm more concerned about the black/gray smoke you saw and the engine compression.
@retro has a great post about checking compression to be sure it's done correctly. He discusses the need for the right gauge parts to get the proper seal while checking. Sorry, I don't have it bookmarked, but a search will find it.
Also, just want to be sure we both have the same definition of "throttle open". By throttle open the manual is telling you to hold the throttle DOWN/IN, as if you're giving it all the gas you can and hold it there while checking the compression. Some people misread this to mean you don't touch the throttle while checking compression. If you DID hold the throttle down it does not necessarily mean your compression reading is correct. Again, read the post by @retro.
Based upon everything you've said I agree with @misterclean59 that your overheating problem may be solved with the radiator cleaning/coolant change.
So, if it were me, I would:
1. check the compression correctly
2. do leak down test
3. pull the top end and check the valves
Lastly, I bet you may have sunk that Rancher at some point. If not, I bet you've enjoyed some good mud more than once. You don't get a radiator that clogged if you're just riding on dirt trails.