Compression Test - Bouncing Needle?

[raystone]

Hello, I thought my compression test looked fine, but the engine keeps having trouble, so I re-did the test and filmed it (don't watch it too long on repeat, it'll freak you out hearing the starting motor working it).

Click to see the compression test here (I have the throttle wide open)

Unlike other videos, where the needle bounces upwards and then eventually settles at a compression, mine keeps bouncing back and forth.

I don't think I hear the valves tapping, but from what I've read online, it seems I'll have to take a look at the clearances first. I have no idea on the valve-maintenance history, sadly enough, bought this bike three weeks ago. Black smoke on high revs, sooty plug (air filter + oil changed).

How would you approach this, in case there is an air leak? The spark plug is sooty, but not oily. Although exhaust fumes seemed to smell a bit plastic/oily today for the first time (smoke seemed black to me, but blue/black, hard to say for sure).

I know the gauge holds the pressure until released through the valve, so that doesn't really tell me if/and how quick I may actually be losing pressure.

The engine runs, but far from perfect (story is in another thread, wanted to keep this topic separate in case someone ever has a similar problem).

Manual wants compression between 152 psi and 188 psi. Some readings are perfectly in there, others are at 150 psi like in the video at the end.

(Also: Yamaha Diagnostics tool D03 (Intake Vacuum/Pressure) shows 99, when turning over, it goes down to 64 and then back up. Manual says it's fine when the numbers change. But it also says intake compression pressure should be between 75 and 91N — although I'm not sure if those two numbers even relate).

[RhynoCZ]

Where did you get the gauge, is it certified?

Also, it is advisable to do the valve clearance before you try the compression test.

[Teflon-Mike]

looks pretty normal.
Guage will bounce as pressure pumps up past the one-way valve.
ignore, take the gauge reading, when it's 'done' not during.
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