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Old    SFX (blueplastic)      Join Date: Jan 2004       10-02-2007, 4:13 PM Reply   
Does anybody here have any knowledge or info on engine compression tests?

I am thinking about buying a 2007 Yamaha FX HO jet ski from a guy in another city and cannot demo it before purchasing. So I asked him to take it in to a local Yamaha dealership and asked them to look at it for me and do a compression test. The test came back with one cylinder significantly lower than the rest. Is this a reason for concern since the jet ski only had 70 hours on it?

At 70 hours, compression readings were:
Cyl 1: 190
Cyl 2: 100 (low)
Cyl 3: 180
Cyl 4: 185

After powertuned and cleaning cylinders:
Cyl 1: 200
Cyl 2: 195
Cyl 3: 200
Cyl 4: 195

So, Yamaha dealership is telling me that there was carbon buildup on the exhaust valve for the 2nd cylinder which might cause them to hang open. Does this sound right? Is this normal on a ski with just 70 hours?

They also recommended using Yamaha Ring Free fuel additive in the future to prevent this carbon build up from happening. Does anybody have experience with this?
Old    Art (rallyart)      Join Date: Nov 2006       10-02-2007, 7:02 PM Reply   
One cylinder low like that on a compression test would normally indicate a valve that leaked some. There are things you can do to get a problem fixed temporarily if it's a ring but not if it's a valve unless there was a carbon build up. Assuming that the dealer is not completely dishonest I would say they got the problem.
Sometimes the carbon build up could be from a valve stem seal leaking. If that's the case it will reoccur. The other source of carbon would be bad fuel mix, and you need to know why that would happen on a new engine.
Not much help, I know, but if you get some others with that engine they may tell you if #2 is a problem cylinder or not.
What they tell you could be exactly right but it should not be normal.
Old    Mo Scrilla (mo_scrilla)      Join Date: Jun 2004       10-02-2007, 8:20 PM Reply   
We bought a 2007 FX Cruser HO Tthis summer and we love it.Yamaha is the #1 rental PWC..for a reason, they run great.


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