low compression... another rabbit hole

welp, borrowed a coworkers compression tester. (questionable condition)
1: 120
2-4: 90s
This was after idling for a minute or so.

I'm hesitant to just rip into the valves and start checking things. Because I don't know what I don't know.
What i do know: 23k miles.
questions: my uncle *thinks* someone before him swapped the jugs to make it a 750. I don't know what I'm looking for, to confirm. I have no idea if the valves have ever been checked. Using an amazon borescope, I didnt see anything alarming in the cylinders.
To my knowledge, before i got the bike and started down this path, it hadn't run since 2018.

What would be the recommended next steps? I've seen mention of leakdown testing. also putting oil in the cylinders? I get the oil is to slow down blow-by. what is leakdown testing?

"Leak down testing" is a process of applying compressed air through the spark plug hole to pressurise the cylinder - you can then listen for rushing air from the oil filler - pistons/rings
From the airbox - inlet valve
from the exhaust - exhaust valve
From the outside of the cylinder - head gasket
If you go this route (recommended) there are safety precautions to be aware of. Leakage testers are not expensive on-line and come with the required adaptors
The capacity 700/750 should be marked on the back of the cylinders (unless the stock has been over-bored to 750)

Firstly, you say compression tester is in dubious dondituon. If you have a compressor set your output vavle to say 100 psi. Hook up the compression tester and feed the 100 psi into it. Ie test your test equipment.!!! 
Ensure your testing with the throttle WIDE OPEN. 
How long since it last ran properly? If its a long time, say more than a few years then I would put some releasing fluid ( wd 40 or similar) in each bore and leave a couple of days. I've had bikes which went from 80psi (cold) to 145 (cold) when the rings have declogged.
Once satisfied you've got the best compression you can add 5ml of oil to each bore and test again. A significant increase in compression will show worn rings. No increase, but still low, will tell you the valves are leaking by. 

Without any certainty of your current engine condition or any likely cause of the figures you have it's not the time to tear it apart yet. 
 

If your doing a compression check the engine should be at operating temperature, best time to do it is after a short 10 minute ride, not just idling. Make sure the throttle is opened all the way when cranking the engine over with all the plugs out.

Use a good quality compression tester and if you can have two available to compare numbers.

I would not panic just yet, run the engine for a bit if you can by riding the bike especially if it has not be run for a while. Test the compression after running for a bit.

You also need to check the valve lash, tight lash will cause low compression numbers. Get a factory service manual (FSM) which you should be able to download from this site. Not sure what the FSM says for compression but 90 is way too low, just may be from sitting too long. You don't want more that 15 PSI difference from cylinder to cylinder.

On a 750 the capacity is on the LH side (#1 cylinder)




 

You MUST adjust the valves.  Failing to do so will destroy the engine.  Messing with a compression test is a waste of time unless you verify valve clearance first.

And if the compression is low because the valves are tight, the engine may be damaged already.  You might get lucky though; after adjustment, and a few hundred miles of hard running, the compression may improve.

Start at the basics, and don't ignore critical maintenance.  

How long since it ran properly: I have no yard stick to gauge from. It wasn’t running when I got it. I’ve since gotten it running. Nothing more than some lazy laps around the yard, and a couple laps around the block. No “spirited” riding, by any definition. Just casual “listen for strange things” rides.

i have no clue what service, if any has been done to it. I’ve changed the oil. No sparkly bits!
The compression test was done out of curiosity. Not because of any particular symptom. It starts well/easily and idles pretty smooth I think. 

when looking through the bore scope, the exhaust valves didn’t look burnt (white) to me, and there was no backfiring or strange popping when running. 
But now that I’ve done a compression test, I’m left with was it a bad gauge, or is there something very wrong. If it was actually 90 psi, wouldn’t it be hard to start at the very least?

I will be purchasing a leak down tester. And some valve cover gaskets. I already have feeler gauges to check the valve clearance.
 

In case you didn't know, after your top end work, before you put the valve cover back on, you must release the cam chain tensioner.  Put the engine at 1-4 TDC, and pull the cap off the cam chain tensioner, releasing the spring tension.  This will loosen the tensioner, and allow you to put the valve cover back on.  Once done, put the tensioner cap back on.

Nessim.
Thank you for the insight about the chain tensioner.

I have the FSM printed out and in a binder. I've read over the procedure for checking the valve clearance a couple of times. I understand the process and what I'm looking for. My biggest fear is that I will damage something in the process. Such as breaking a bolt off when removing the valve cover. Or, not getting the timing marks lined up perfect and bending a valve or damaging a piston. As far as what to have on hand when i do this, it should really only be the valve cover gaskets and whichever shims are needed, right? Should the half circle rubber plugs also be replaced every time the valve cover is removed?

As far as the tensioner, should the tensioner be completely removed? I don't know what you are referring to by "the cap" to remove tension.

 

Nessism wrote:

 

650 style tensioner, your picture is 900 style tensioner. 
 

Not sure if this will be of any help or not, but here are some docs I have on the tensioner for the 650/700/750s

 

Ok… confused by the results. 
exhaust on 1, is on low end of .08-.18, as well as intake on 4. 
but all valves are within nominal spec.
A standard .003 feeler slides through all (a whiff of drag on noted valves), a .005 doesn’t slide under any. The .08-.18 converts to roughly .003 -.007 in inches. 

so does that mean rings is the only other option? Or could valves be hanging on carbon deposits?