just rebuilt 701- compression low in 1 cylinder...?

badboyr6

badboyr6

Location
Miami
hey guys, i just rebuilt my sj 701, i finally got the ski together, and i ran it on the hose for a few mins, it started up and ran good. A buddy of mine, tells me to check the comp, so i get out the auto zone comp tester and run a test. Front cylinder reads 152, and the rear reads 100...did i do something wrong? please advise!

edit: no hours on motor, ran it for a few mins on the hose.
 
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5810

5810

WHERE-THE-SEAT-AT
Location
Knoxville TN
do you have the right size piston and rings in there , are you 100% positive. what pistons u running.
what exacly u do when u rebuilt it, what caused the rebuild.
 
badboyr6

badboyr6

Location
Miami
the motor was seized when i purchased the sj from a guy on cl. So i took the engine completely down and cleaned it up. Honed the cylinders because there was no scarring, i got a new to me crank from a 650 because mine was a little rusty. i got brand new wsm std pistons and rings. used new seals on the crank, base gasket, and head gasket. I finished installing the water lines on her today, and got it running with the idle, i let it run on idle for a few mins with the hose. i bliped it a couple times and it sounded strong. Tonight i checked the comp and those were the numbers i got.. :(

the ski cranked over just fine with a little squirt from the primer. Im running 32/1 mixture.
 
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ADDict

ADDict

Go run the piss out of it, and then check compression. Those numbers sound pretty far off and I'm no expert, but I've seen some crazy numbers from a fresh no hour rebuild. Go run it, then let us know what you have.
 
badboyr6

badboyr6

Location
Miami
ok, so i went outside to the ski and rechecked compression. i got 120 back and 150 front. i poured a little oil down the front and it went to 155. i poured some oil down the back and it went up to 132...what should i do? run it?
 
tor*p*do

tor*p*do

Squarenose FTW
Site Supporter
Location
NW NC
I don't think compression numbers mean much until the rings "seat"
Like 20 gallons or so of break-in run time,
no WOT, vary throttle a lot
BUT a real engine builder may want to chime in
(or PM Chucky/Supertune. . . .?)
 
SUPERTUNE

SUPERTUNE

Race Gas Rules
Location
Clearwater Fl.
Did the engine turn over by hand easy with no plugs and no intake manifold when you assembled it? If so I would run it, if it was hard and catchy then maybe rings are snagged.
There has been some posting here that rings were package wrong sizes in the boxes, to prevent that from happening always check ring end gaps before assembly, just use the piston upside down and set the ring in the bore at the top then use the piston to push down and square in the bore to about halfway from the top of the block and the exhaust port. Measure with a feeler gauge to see if both cylinders are the same.
.016 up to .024 is the normal range.
 
badboyr6

badboyr6

Location
Miami
I replaced both pistons with the wsm kit. It did turn over very smoothly with the sparkplugs out. I decided to take her out on last Thursday and she started fine. But after a little bit I began loosing compression because it became super hard to restart. My ride only lasted apprx 30 mins. I brought her home and took the head off. Looks like the rear cylinder ate some metal because the piston got chewed up on the side. The cylinder got a nice scratch on the side also, so i ordered pro x pistons 1mm over from jetmaniac. I'm going to have to bore the cylinders over. I did check the comp when I brought her home and I got 130 front, 90 rear. That's why I took the head off. What would be the ring gap for prox 1mm? Also when I take it to get bored what do I ask for in tolerances? I know I have to take my pistons so they match them.
 
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badboyr6

badboyr6

Location
Miami
superjetkev said:
is the cyl with the low compression by any chance the one that seized?? did you measure the cylinders before you assembled it?
Click to expand...
when I first got the boat the motor was completely locked up. Upon further investigation, the reason was because the needle bearings were rusted and wouldn't move causeing the pistons to not move up and down. I didn't measure my bore, I honestly didnt know how or I had to due to lack of experience. But I don't want to screw up this time and I want to get it right.
 
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