Rotax 657x has low compression.

[Zack9r]

Cylinder 1 reads less than 25 psi, cylinder 2 reads about 95psi.

I bought this jetski and it made about 130psi in the rear cylinder, and 90 in 1. It didnt start, i rebuilt the top end, new rings, gaskets all around, etc.

I put it back together, no luck. Lower psi than now. I got oversize rings (.5mm over) and put it back in, now its reading the compression that its making now but its still not starting and quite frustrating. Can someone help?

I suspect timing is off, pulling off the carb next time i see it cleaning it.

 

[mikidymac]

So at first you had at least one bad cylinder and needed to bore and fit oversized pistons.

You cant just install larger rings on standard bore and pistons so you have damaged things even more by forcing in too big of rings.

You need to have a professional machine shop bore the cylinders and fit larger oversized pistons.

You should also rebuild the carbs and fuel system as that is typically what caused your one piston low compression to begin with.

 

[NickCawks]

as usual, I think Miki hit the nail on the head.

When you (and by you I mean a machine shop) look at a situation like this they will measure your cylinders down to the hundredth. they will then decide whether or not you need a bore, hone, and oversize pistons. (most of the time just a hone isn't going to be good enough and especially at that low of compression). once they have measured the cylinder that will determine how much they need to bore out, the pistons you need, and subsequently what rings go on those pistons.

By forcing oversized rings on your pistons and into your cylinder you have ignored the actual problem which is the top end is blown.

I wish we could give you better news than this, but that is the way the cornbread crumbles with these things and the main issue is unless you have access to some pretty serious machinery then you are going to have to send the top end off to a machine shop and spend the money.

Many people on this forum have had a lot of luck in dealing with Full Bore. They do good work and generally do it in a timely manner.

Here in Colorado, we have a local shop called Peak Powersports that does bore and hone at a very low cost. Look around your area and see if there are any local options and get a price quote on it. Fullbore is gonna run you $285 for that motor. My guys over here at Peak do it for $250 which given the circumstances $35 and no shipping costs are worth it. Even if you have to go with sending it off to Fullbore that will cover the bore, hone, pistons, and rings in that single cost.

These machines will piss you off all the time. things will go wrong if you arent well veresed in *specifically* the carbs and tuning them.

The bad news is you need to spend money

The good news is if you do spend that money and then take the time to learn how to properly test and tune your carbs (this forum is a wealth of info on that) you should be able to run your newly rebuilt top end for many many years with no problems.

 

[mikidymac]

Well said NickC.