reasons.. causes.. for low compression

[hwoolfolk]

What are the possible reasons why one cylinder would lose compression?

And.. does it have to mean a new top end?

720 lost compression in the rear cylinder. Went down to 90 lbs. Took the top end apart.. Cylinders actually look good.

What should I look for to KNOW that it needs a new top end?

 

[Bills86e]

Your pistons may now be oval. Same with ring edges. Running lean with continued
no maintenance. Ect. At first sign of something wrong, fix it before you drive it.
99% of Seadoo problems are fuel system related. It would have been nice if Dealers
included a Shop Manual to buyers. Every owner should have one. I give a copy to everyone who buys a ski from me, and a warranty. What year n model you puttin a new top end in? Also buy a pop-off tester, its money well spent. You'll need to have
a machinist check your cylinders, they may be tapered & out of round, maybe save-able.

 

[dsw222]

What are the possible reasons why one cylinder would lose compression?

And.. does it have to mean a new top end?

720 lost compression in the rear cylinder. Went down to 90 lbs. Took the top end apart.. Cylinders actually look good.

What should I look for to KNOW that it needs a new top end?

Same thing just happened to me except mine is reading 75 psi on a 787

ugh...