90 & 93 SN SJ 650 low compression

[CaMo_FroG]

90 & 93 SN SJ 650 low compression

I got these two superjets last year and i dont think the previous owner did much to them.. he did say that the 90' had been rebuilt with wiseco pistons a while ago... and the other didnt have that much use (still stock)

so my question is .....

i just got a new compression tester and tried both of them in every cylinder i am getting right around 70+- PSI ... are my rings shot? do both these engines need rebuilt? isnt it supposed to be around 150 ish ?

 

[suva]

Do they run? I doubt they'd start with that low of compression. Throttle WFO when testing?

 

[CaMo_FroG]

perhaps im not doing something right ? i simple took out one spark plug at a time & .. screwed in the tester and cranked them over with the starter?

yes they both run and are usuable

 

[suva]

Pull both plugs, hold the throttle wide open, crank until the compression doesn't raise any more, make sure your battery is good.

 

[motoxracer1551]

i had a new comp tester show 125 psi consistently in my ski when it was really 175psi. so who knows

 

[CaMo_FroG]

i did not hold the throttle wide open .. perhaps that is why i am not getting accurate numbers. i will try again.

 

[suva]

That will make a difference.

 

[CaMo_FroG]

well i tried again... but only seem to accomplish a max of 75 PSI (tried both engines and did each cylinder twice) ... i'm thinking the rings etc are shot.. im suprised they still work well...

what are my options ? do i have to just replace rings ? or am i going to have to do a full piston replacement with over bore?

 

[tor*p*do]

try a different compression tester before tearing into motors
I think 75 psi would barely run so the problem may be the tester
Or try your gauge on another shi of known, good compression

 

[aramis]

I'd suspect the tester too, especially if it's reading that low consistently on all the cylinders you try it on. I've never had any luck with the "harbor freight" priced testers...

Also, is the thread-in adapter the same length and end diameter as the plug threads?

 

[CaMo_FroG]

ok i checked a few other things mainly tightened all connections on the tester more and found that there was a coupler on the tester that wasnt getting good seal...

so now im getting 125-130 psi is that more normal ? isnt that still low?

also i checked a fellow friends 07 sxr which should be good and am getting about 130 psi on his too.

 

[tor*p*do]

sounds like the gauge is ~20 psi low and the skis are good!