A couple top end rebuild questions

I did a compression test on my 81 xj550 after numerous running problems. The results were about 75-80 psi for all of the cylinders. It raised a little bit with oil added. 90-100. I performed the test with two different compression testers, fully charged battery, and throttle wide open.

The valves are in spec, engine has about 16,000 miles on it.

I tore the top end apart expecting to find some carnage as the previous owner neglected to adjust the valve spacing, and other maintenance.
I was suprised too see how good of condition it was. The valves had very little carbon on them and the seats were in fairly good condition. They lapped in with little effort.

The pistons look about the same. You can still see the factory crosshatching on the cylinder walls. No scoring or marks. I havent removed the jug from the engine yet. Should I still put a set of rings in it? The factory service manual makes it out to be a daunting job.

 

Run it for a few hundered miles and retest. You may only be facing a set of sticky rings.
Was the engine fully warmed when you did the compression test?

 

The engine was cold because I have a no start condition , before that I have put about 40 miles on the engine since I started working on it hoping it may have worked them in if they were stuck. I didn't want to run it anymore than that as I was worried about running it out of tune and damaging the engine

 

So your compression test isn't necessarily indicative of a problem. Carry on getting her into riding shape and check it again once you can maintain idle long enough to warm the engine fully.

 

Okay I will do that. My only question is; is 75-80 PSI (im sure it will be diffrent once warmed up as you said) enough to get the bike running and dialed in good enough I can actually ride it a bit to see if the rings do break loose if they are stuck. Or will It fight me because of the lower compression?

 

The minimum compression required for an engine to run is roughly 90 psi. Any less and it'll just fart and backfike but not actually catch and run. Regardless I'd still try and get it fired and an run it just to see if the condition improves at all. With only 16k miles its surprising the compression is an issue.