How to accurately check cylinder compression

[greg the pole]

Ok. Mechanical question for mc mechanics, or mechanically minded dudes/dudettes!
how the hell do you accurately check compression on a motor that has decompression pins?
I just rebuilt my tenere motor (2nd gen piston, rings, valve seals, cyl. hone), it goes like stink, yet, the compression is lower than when I rebuilt it.

All comp checks were done with WOT.

Prior to rebuild (engine cold). dry/oil added
Cyl 1/Cyl 2 (no oil): 62/72 PSI
Cyl 1/Cyl 2 (oil added): 72/88 PSI
Post ebuild:
Cyl 1 cold/warmed up motor: 60/68 PSI
Cyl 2 cold/warmed up motor: 60/65 PSI
It's possible that prior to rebuild the comp read higher because of the extra carbon build up :. more squish/compression
Another interesting fact is that I just did valves on a friends tenere, with 35km of light-ish use, comp cold read:
Cyl 1 cold: 55 PSI
Cyl 2 cold: 65 PSI
I've attached the Super T procedure for reference. Recommended PSI is (82-108 PSI at sea level)

I'm not worried. Just curious why the numbers above are what they are. You cannot under normal circumstance take out the decompression mechanism. It's part of the exhaust cam. So why did Yamaha not mention nothing about it? Or are the readings including the decompression mechanism working?

https://flic.kr/p/SEwy2U

https://flic.kr/p/SJ8u56

 

[WJBertrand]

Are the rings new and with a resurfaced bore? If so you may need some miles to better seat the rings?

 

[greg the pole]

Are the rings new and with a resurfaced bore? If so you may need some miles to better seat the rings?

Yes. 2nd gen rings pistons, and a fresh hone.
https://thetenerist.wordpress.com/2017/03/04/yamaha-super-tenere-xt-1200-top-end-rebuild/

 

[gv550]

Are you holding the throttle wide open when taking the the compression test?
If your bike and your friends bike both run well and read below spec I'd be suspicious of the gauge accuracy.

 

[Don in Lodi]

Spec numbers seem low, could be they are already working in the compression release. WOT with the injector fuse pulled. See if you can get a couple squirts of oil down the holes to compensate for the cylinder walls getting washed by fuel. Your new numbers sure are nice and even. ::008::

 

[BWC]

First off good to hear it's alive again ::008::
I haven't tried a compression test on the Tenere yet, but like you I wonder about getting any sort of accurate reading with the exhaust valves being held open some amount by the decompressor. I think I would to try a leak down test by locating the piston/valve position just off the decompressor activation. A leak down test will let you know if and where there is a leakage and what components are suspect.

 

[greg the pole]

Are you holding the throttle wide open when taking the the compression test?
If your bike and your friends bike both run well and read below spec I'd be suspicious of the gauge accuracy.

yep.

As per book. first time around I didn't notice it was with a warm motor.

 

[greg the pole]

Spec numbers seem low, could be they are already working in the compression release. WOT with the injector fuse pulled. See if you can get a couple squirts of oil down the holes to compensate for the cylinder walls getting washed by fuel. Your new numbers sure are nice and even. ::008::

I didn't add oil as I know my rings are ok ::015::

 

[greg the pole]

First off good to hear it's alive again ::008::
I haven't tried a compression test on the Tenere yet, but like you I wonder about getting any sort of accurate reading with the exhaust valves being held open some amount by the decompressor. I think I would to try a leak down test by locating the piston/valve position just off the decompressor activation. A leak down test will let you know if and where there is a leakage and what components are suspect.

Agreed. I didn't leak test it before the tear down. Leak down could be done at TDC on each respective cylinder correct? But would it also give compression?
What's the point of the compression check if you cannot capture the pressure in the cylinder for any given time. The decompression pin basically holds open the right exhaust valve of each cylinder, so the thing never seals..

 

[BWC]

Yes, the decompressor pin pushes on the exhaust valve bucket at TDC to relieve some compression to allow our seemingly weak starters to actually turn the engine over :
I think if you turned the piston position to just past where the pin stops holding the exhaust valve open, which is just past TDC the valves will still be closed and you should still get a fairly accurate leak down test. It would at least indicate if there is a problem, and where it might be. Might have to put the bike in gear with the rear brake on to hold against the air pressure from the leak down test not being done right at TDC though.
Next time a do a plug change I'll give it a try to see if it works.

 

[Don in Lodi]

I didn't add oil as I know my rings are ok ::015::

Perfect rings still need something to seal against. A nice shiny clean cylinder wall vs one with a nice velvety sheen of oil, could make the difference.

 

[greg the pole]

Yes, the decompressor pin pushes on the exhaust valve bucket at TDC to relieve some compression to allow our seemingly weak starters to actually turn the engine over :
I think if you turned the piston position to just past where the pin stops holding the exhaust valve open, which is just past TDC the valves will still be closed and you should still get a fairly accurate leak down test. It would at least indicate if there is a problem, and where it might be. Might have to put the bike in gear with the rear brake on to hold against the air pressure from the leak down test not being done right at TDC though.
Next time a do a plug change I'll give it a try to see if it works.

I'm almost positive that at TDC the pin dosen't push on the exhaust valve. At that point, the cylinder should be completely closed.
Another thought occurred... pull the exhaust and blank off the holes ::025:: doesn't mention that anywhere in the book :

 

[greg the pole]

Perfect rings still need something to seal against. A nice shiny clean cylinder wall vs one with a nice velvety sheen of oil, could make the difference.

I'll do that at the next oil change

 

[Checkswrecks]

I'm almost positive that at TDC the pin dosen't push on the exhaust valve. At that point, the cylinder should be completely closed.
Another thought occurred... pull the exhaust and blank off the holes ::025:: doesn't mention that anywhere in the book :

I haven't had anything worthwhile to comment with till this. If you cap the exhausts, you're missing the point to do the compression check. After all, you already know about the rings and it's virtually never that the intakes leak.

 

[greg the pole]

I haven't had anything worthwhile to comment with till this. If you cap the exhausts, you're missing the point to do the compression check. After all, you already know about the rings and it's virtually never that the intakes leak.

I'm being a smart ass.
The whole point of the compression check is a bit useless. If you can't contain the pressure, how can you possibly measure it. Not sure why it's in the book
Both tenere's gave the same reading.

 

[Checkswrecks]

Vroom vroom


Compression check done!
::26::

 

[tuonodave]

Battery condition can greatly affect your readings. Try supplementing your battery with jumper cables to your car/truck. Vehicle running for even higher numbers. A little oil as you know helps the rings to seal and boosts the numbers further. I always use the compression test as a quick, repeatable indicator of a problem then the leak down test to pinpoint the problem, find the actual % of leak and its exact source. Air out the exhaust = exhaust valve sealing issues. Air out the intake= intake valve sealing issues. Air out the Crankcase/Breather = ring or piston issues. Etc....

 

[tuonodave]

As long as the two cylinders are close, seems to run strong/evenly, no smoke or starting issues I wouldn't worry about it. Ride it keeping an eye on oil consumption and you can retest at next major service for piece of mind.

 

[EricV]

No direct S10 knowledge to add. 8) I agree those numbers given for compression are low, low by comparison to other bikes I've had. I think they are accounting for the design, thus the low spec number. Comparative data is good too. It would be interesting to see what, if any, difference a little oil makes in your reading for the new pistons.

That said... we know the Gen 2 rings are thinner/lighter and the Gen 2 pistons are different. The crankshaft and con rods are the same, but the cylinder has been updated in some way, (interchangeable revision though) However, the cylinder heads are different and not showing as a revision that is interchangeable. 23P Vs 2BS.

Just thinking out loud that they may have manipulated the head to keep the compression ratio the same with a different piston design. Altering the combustion chamber in some attempt to reduce carbon build up or ???

That's a long winded way of saying if the bike runs well, forget about what the new numbers are. ::021::

 

[greg the pole]

Vroom vroom


Compression check done!
::26::

well yes ::025::
It's just odd that they even bother putting that page there...