60psi compression-pics of top end...

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bambooi

X
Did a compression test. 60 psi in the rear cylinder... After riding I saw a line that was disconnected that went into the carb. I went online and found out it was a pulse line which is used fuel system. I pulled the plug and that side was lean. The other side was rich. I guess this is why I lost to the blaster, b/c I called up the dude I bought it from and he said something was wrong and I should do a compression test. He said the SJ when he had it outran his 55mph couch. The front cylinder has around 150psi of compression. I don't know how long the pulse line was off either...

I pulled the head and there are burn spots on the top of the piston. On the side of the piston/ piston skirt I see some scoring. With the pulse line disconnected, it did not get the fuel required. :cry: Also it looks like the crank is burnt on the rear cylinder because the front cylinder does not have that type of darkness.

So, I guess I'm going to get a machine shop to check out the cylinders to see what size pistons I should get if I have to go to the next size or not. It cranks up fine and everything, but low compression on the rear cylinder. Pistons rings don't look bad either, but I wouldn't know...
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Does anyone know what those zig zag looking lines on the side where the pistons go into are? I bought this SJ modified, as most of you already know. I guess I will need a topend kit now... Does anyone know if the pistons that are in there are 760 pistons? This is my first jet ski too. :-( Is the crank burnt/messed up?
 
Matt_E

Matt_E

steals hub caps from cars
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at peace
First of all, that crank isn't burnt - the piston rods are supposed to look that way.
Assuming you have a 701 engine with a 61x topend, it looks like you have oversized pistons in there (0.5mm over), so you're at a 81.5 mm bore.
Take the cylinder and pistons to a machine shop - I think you'll be fine with an 81,75mm bore or even 82.00 mm bore.


EDIT: Those zigzag lines are oil delivery channels for the crank bearings (I believe)
 
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Freestyleriverrat

Guest
You have some scoring but other than that everything looks good to me. Crank looks really good, looks new to me.
 
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SkiDaddy

SkiDaddy

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Orange City, FL
You're pretty lucky, it could have been much worse.

If you have some aluminum transfered to the cylinder you can (carefully, w/eye protection) use Muriatic acid to get it off. Then either lightly wet sand the piston w/600 grit in a figure eight pattern to clean up the scoring, or replace that piston w/another. (recommended).
 
waterfreak

waterfreak

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look's like you might be able to get away with a new set of pistons and a good hone job
those look like wiseco's 81.5mm
Also, make sure you replace the head gasket or o-rings if it is aftermarket. looking at the pistons and cylinders, it does not look like it should be down to only 60 psi!!
 
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bambooi

X
that is what my friend said also, but the front cylinder has 150 psi on it. It supposively has a cometic HeadGasket on it. Is it okay if I use a top end gaskit that comes w/ a top end rebuild kit? Also I looked up the part number on the piston and its a yamaha 760 piston...
 
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