ZG222 Compression specs

[leeton31]

G'day all,
Can anyone explain to me the huge differences in the compression specifications as printed in the Kubota Service Manual for the ZG222.
See image below.
Number 1 cylinder 173 psi
Number 2 cylinder 52 psi.
Thanks

 

[ILENGINE]

Compression release on cylinder 2 only possibly

 

[StarTech]

What you got is an automatic decompression system that is activating the #2 Exhaust valve.

Read PDF page 65 (1-M3 manual page)

 

[leeton31]

Thanks for that, either my automatic decompression is not working or is not on this motor, as I did a comp test and it showed 150psi on cylinder 2, I will do some more research.
Appreciate your help!

 

[lugbolt]

#2 has an automatic compression release that "engages" at "cranking speed". That said, "Cranking speed" can be altered by using a jump starter or whatever. IIRC the ACR disengages at 220-250 RPM and the starter will crank it faster than that with a good hot battery. So when doing a compression test, leave the other (#1 CYL) plug in to help slow it down a little.

all the acr does is to relieve compression pressure at very low RPM in order to ease the load on the starter. Once RPM rises, ACR disengages and compression pressure rises back to normal

been through this and the books will throw you for a loop if you don't know.

Another issue us that when the valve clearance opens up (and it rarely gets checked and/or adjusted....), the acr won't open the valve quite as far as it needs to to dump excess cranking pressure and your compression gauge will show real high.

 

[Tiger Small Engine]

#2 has an automatic compression release that "engages" at "cranking speed". That said, "Cranking speed" can be altered by using a jump starter or whatever. IIRC the ACR disengages at 220-250 RPM and the starter will crank it faster than that with a good hot battery. So when doing a compression test, leave the other (#1 CYL) plug in to help slow it down a little.

all the acr does is to relieve compression pressure at very low RPM in order to ease the load on the starter. Once RPM rises, ACR disengages and compression pressure rises back to normal

been through this and the books will throw you for a loop if you don't know.

Another issue us that when the valve clearance opens up (and it rarely gets checked and/or adjusted....), the acr won't open the valve quite as far as it needs to to dump excess cranking pressure and your compression gauge will show real high.

Anything around 120 psi is really good compression on a 4-stroke twin cylinder engine.

The compression release mechanism is on the cam and is on both cylinders, not just one. Also, there would not be a 120 psi differential with and without release. Also, 170 psi compression reading on a healthy 4-stroke is really high. That is more a reading you are looking for in a 2-stroke chainsaw.