I’m looking for opinions from experienced mechanics on the principle of using a compression test to check if the valves are in need of adjustment.
I was watching a video from a youtuber called rod rides and wrenches. He mentioned that if the valve clearances are tight, then the valves would be held slightly open and compression would be down a bit. For instance on his DR650 before compression was 135psi when all four of his valves were below the min clearances. After adjusting the compression was now 150psi, as he’d expected.
The valves on my trophy are coming up for checking and I was contemplating trying as it all seems very logical to me.
If the valves are wider than max, then he says it would result in lost power, so you’d notice it that way, I presume.
If anybody wants to watch the video its here Step by Step Motorcycle Valve Adjustment PLUS WHY Not Just How… - YouTube
Not a mechanic.
The valve shims would have to be touching or more, pressing. The reason for doing a valve clearance is to stop this from happening. Before they are at this stage.
I suspect for this to work, your clearances would have to be disastrously out of spec to start with.
Normally you adjust the GAP, while there is still a GAP.
Also your rings and valve seats contribute to compression, regardless of clearance, shirley.
I think.
The main reason for posting this was to get some feedback and help me clarify my understanding of whether this could work, because I’m thinking like you.
However one line of thought that struck me is that, the valve starts to close as it comes off the cam lobe and the piston is on the up stroke, creating the compression. So if the gap was less than min, it is going to be closing later, as the lobe is closer to the shim, so the engine will loose compression. Eventually, unless the valve gap is non existent the valves are going to close and be pulled up against the valve seat by the springs. That’s my theory at the moment of how compression could vary, before the gap has closed to zero. Trouble is I don’t know if it would be enough of a change to measure. However on the video he did see a 15psi change after adjusting the valves which were below min but not non existent.
Of course this is all on a cold engine, once warm, I believe the gap should close up anyway.
Thanks for replying. It helps me think when people challenge it.
With respect to the worn rings/bore, I guess you would probably see that through poring a small amount of oil down the spark plug hole and retesting. If the compression goes up significantly I assume you have a problem you need to investigate. I believe that is one way to diagnose.
Although the compression builds when the valves are still open, the compression figure is only achieved when the valve is closed. The size of the GAP is irrelevant at that point. The important thing is that there is one.
Since the, minimum and maximum, measurement of an in spec GAP are there to allow closure and maintain smooth opening.
I am thinking, for compression to be lost, the closure timing must have been shortened or irradiated to a point where your valve clearances are severely out of spec.
Normally you would want to check clearances as a maintenance procedure, to always have them within spec.
So to be checking your timing by this method has a, to little, to late, feel to it.
Perhaps as a first check of a new to you vehicle it may have a merit as a shortcut ( never a good idea).
This guy has a garden implement he is using as a spade.
I may be wrong ( not an unusual occurrence).
Unfortunately this is something all too familiar on YouTube. @3speedlowslowgo is correct IMO, a compression test just checks the seal between the valves and their seats which is totally different to checking the clearances to make sure the valves are opening and closing correctly. If you want to check this then it needs to be done properly. Sorry I’m not directing my vent at you, just the **** on YouTube. Personally I’d get them checked by a competent mechanic and go from there. What ever the outcome is you’ll end up knowing your valves are within tolerance at the end of the process. You wouldn’t be happy if you followed his “advice” and down the line your valves burnt out because the clearances were too small. BTW I’ve had a wee dram of malt 
Thanks to both of you for responding. I decided to ask artificial intelligence about this 
No Kidding…I used Bing chat.
Basically it pointed me to a published paper where someone had done the tests on a 100cc engine, changing the valve clearances and then dyno testing after each. Some of it’s conclusions that seemed relevant to my question is as follows.
“c. In accordance with the purpose of this study, namely, if the change in the size of the valve gap is too large or too small, it does not comply with the specifications, it will greatly affect the compression pressure in the combustion chamber so that it will affect the power and torque of an engine.
d. The size of the valve gap that is too small will result in the valve opening being longer than it should be. As a result, a longer valve opening will cause new gas losses (new gas will be wasted into the exhaust channel because the exhaust valve opening takes longer), so the idle rotation becomes unstable.”
This was taken from (PDF) Analysis of Variation of Valve Gap Size on Power and Torque of a Four-stroke Motorcycle (researchgate.net)
So my take is that Rod’s theory is correct, however it comes back to whether this is measurable if the valve is just outside of spec. And of course at what point it becomes measurable.
As has been pointed out I don’t want to burn out a valve or anything such like.
I’ve had fun and read a lot around the subject. So far my conclusion is that I will buy a compression tester, run a compression test before checking the valves. If the valves need any adjustment I’ll run another compression test and see if there is any measured variation.
Unfortunately getting a competent mechanic to check clearances is far from straightforward. I have two experiences of dealers claiming it’s been done when it clearly hasn’t.
A potential symptom of zero valve clearance is poor cold starting, I have practical experience of this on a 955i some years ago, and also heard the same from a Suzuki/Kawasaki mechanic.
I now do my own checks, and would suggest at least checking, if not correction, is within the capabilities of most.
A trustworthy mechanic is a valuable asset and they can move around from dealer to dealer, if you find one make sure you get to know him. A dealer front desk will often try to keep customer and mechanic at arms length. 
Trouble is on a typical across the frame triple or four to access the plugs to fit your compression gauge is a fair task in itself.
So then you’re only a cover removal away from doing a “normal” clearance check. 
Great thread, always like a bit of theoretical.