Super Jet lack of compression

[engineman44]

ok well i got my whole ski back together 701sj has a ada girdled head 35cc domes also put new piston rings in it well i tested it before i ran the motor and was 150lbs well now it is 120 on each cylinder so i re torqued head bolts and girdled studs the studs moved some but not the head bolts so what could be the cause of this ??could the rings have seated and made it lose compression ? if it was a blown o ring wouldnt it not hold pressure im lost

 

[Matt_E]

Rule out the simple things first: loose o-ring on the compression tester, wrong length threaded piece of the tester, not holding the throttle open when doing the test, weak battery, etc.

Once you have ruled all that out, I would pull the head to inspect the bores.

 

[rollin5150]

I had low compression after my last top end rebuild. If both cylinders are even, I would run it to seal the rings. After I ran mine, both came up to where I expected.

 

[engineman44]

ok well was about to go to the beach and my battery died so put the xp battery in there brought it up to 135 still not good imo

fyi have ran 1 tank through it

 

[tor*p*do]

needs more than 1 tank to fully seat rings
as long as it runs good, put 15 - 20 more gallons through it
and retest
or if it runs strong, no worries, may be the comp tester

 

[superjetnj]

seems low to me, recheck with a different gauge, I'm running a pro-tec modified stock head right now and getting 175 on my snap-on gauge, when I was running a ADA head w/35cc domes my compression was 185. Maybe your bore is off, or bad gauge. Did you hone it? Just my thoughts

 

[engineman44]

was supposedly sleved to oem size ran a hone through it to clean up wish i would of checked cylinder now just trusted the guy i bought them from had new wsm sleves put in it

 

[scotts2t]

did you replace the head dome orings? even 150 before the new rings seems low, i think your gauge is toast.

 

[engineman44]

gauge is brand new from autoparts tested it on my compressor and read 120 that was as high as my compressor would go will get my friends compression tester tonight and try that one out

i put all new orings in it bought a girdled ada head kit for my new cylinder

 

[engineman44]

sadly compression tester is good im gonna order a gasket kit but im thinking i may need to order wsm pistons for there sleves i put oem pistons and rings in it

 

[Matt_E]

was supposedly sleved to oem size ran a hone through it to clean up wish i would of checked cylinder now just trusted the guy i bought them from had new wsm sleves put in it

So nobody actually verified piston to wall clearance?
Imo, that's your problem right there.

but im thinking i may need to order wsm pistons for there sleves i put oem pistons and rings in it

No, you need to verify that the clearances are proper. There is absolutely no problem running OEM pistons in aftermarket sleeves as long as the proper clearance is there.
There is no one "OEM Size" that the WSM pistons will fit. You buy new pistons, give them and the cylinder to a machine shop and they will bore the cylinder to the right clearance.
Every piston is minutely different. My bet is that the bores are too big for the pistons. Nobody verified what the size was and then you ran a hone through it making it even bigger.
Those clearances are critical. If it's too big by even just five thousandths, your compression is going to be significantly lower.

 

[engineman44]

thanks well looks like im gonna have to get them bored most likley will it hurt to run it like it is till winter

 

[Matt_E]

Just get the clearance measured properly to verify that thought before you get it bored and spend even more money.

 

[DK-Freestyle]

Compression changes before and after you run it. When we do services at the shop, we run the ski for a bit before we test compression to get an accurate reading, but 30psi seems like a pretty big difference.