Compression Help

[97GTS]

I have a 1997 Seadoo GTS with a 717 engine:

So i was having some problems with my jetski and I took the top end apart and found that my PTO piston Rings had a little rust on them so I replaced the ring and then put a new gasket kit on the top end the only thing I didn't do was have the walls honed before putting it all back together. I didn't check the compression before replacing anything cause my ski was not running at the end of last year but now when I got it all back together and fixed my original problem I tested the compression today and the flywheel piston is at 135psi and the PTO piston is only reading 120psi... I also cleaned the top of the pistons to get rid of all the carbon build up, I don't think that has anything to do with my compression problems.

Please advise

Thanks
Chris

 

[Bills86e]

Bad compression means useing excess fuel, as it can't completely burn.

 

[97GTS]

Excess Fuel??? Never heard of that... but as I should tell everyone I was wrong on my original posting the piston with the 120psi is the piston that has the original Rings not the replaced Rings...

Sorry for the mix up

Chris

 

[LouDoo]

135 ain't real great either. It's probably time for an upper end rebuild. Either replace the pistons and sleeves, or larger pistons and bore out the sleeves.

Lou

 

[Ninetok]

2nd what seadoolou said

 

[Dr Honda]

When the compression is low... there are a bunch of issues.

1) Going from Bills idea... it can't burn the projected fuel load efficiently.
2) you will get excessive blow-by. This will over heat the piston skirts, and the crank. Also it burns the oil out of the cyl walls, and rapidly causes the engine to eat it self.
3) with the extra blow-by, you introduce exhaust gases into the lower half, and mess up the air-fuel mix.


There are plenty of engines that run at 120 psi... but when the designed compression is 150... the engine will run poorly once it gets too low.

 

[97GTS]

Thanks everyone for your replays...

I think I'm going to replace the Rings on the other piston and have the Sleeves honed and see what happens before I spend the money on a rebuild Kit...

Chris

 

[kevinpaula@comcast.net]

Yep, I think replacing the rings and getting the cylinders honed is your next step. Good luck and keep us posted.

 

[97GTS]

Just wanted to let everyone know that My ski is up and running and I ended up replacing the rings, and I now have 150psi on both Pistons and the ski is running on the trailer... I will do a Lake test sometime next week...

Thanks everyone again for all the help and we will talk to you soon
Chris

 

[kevinpaula@comcast.net]

Great, It's always good when a downed ski gets fixed and back on the water. While doing the rings, did you have the cylinders honed? It's a good idea to have them honed or remove the glazing if the original thatch marks are glazed over and not viable. Also, don't forget to break in those new rings. They need a break in period.

Good luck on the lake.