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2005 rxt 215sc compression high, and high end rpms low.

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whosurpopi

whosurpopi

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Just bought the ski, so I don't know much about it. I was told it was rebuilt about 2 years ago. It sound good, and starts right up, with rpms at rest around 1800. But the compression tests at 165 160 165. On the water, its topping out at 6900 rpms, and is markedly slower than my 06 rxt. My mechanic has me scared to run it saying the compression is too high, and could blow the pistons, or the motor may be a non super charged engine, and they added a super charger. Any Ideas? The wear ring looks good, and the impellor looks good too. Would the high psi and the low rpms be correlated, or two different problems? Thank you.
 
Calling [MENTION=43374]Coastiejoe[/MENTION] what should the s/c compression be?

Your mechanic might be barking up the right tree, especially if a diagnostic reveals high detonation knock count this could explain that.
 
The compress should be 140 psi or lower. Your mechanic is right. Non SC engines are 180 psi and lower.
With that said.
The high rpm failure is the spark plug might be failing. If you haven't blown the engine, then the spark plug will fail. They are not supposed to operate under that much pressure. To keep running like this you will have to keep replacing them quite often.
 
Compression is a bit high. If it was a rebuild then the original 215 motor would have been rebuilt. But if they just threw the supercharger onto an NA motor then there is your problem. Was there paperwork on the rebuild?


One more thing to check is if oil is getting past piston rings into combustion chamber. This will also give you high compression readings.
 
Thanks guys, Im taking it back in about and hour and the mechanic is going to check timing, and if that was right, maybe dig deeper. I don't have anything as far as the rebuild, and the east coast mechanic that supposedly did it is not returning calls.
 
Good news, the timing was off two ticks, and it now has compression in the 130's. The rpms are in the mid 7000's so a little more work to do, but its running better.
 
Ignition timing has no effect on static compression.

If the camshaft was advanced by a full tooth (installed incorrectly) the compression would likely measure high (and the engine would tend to run out of steam early). I find this second possibility low, anyone installing a timing chain would not likely make this mistake.
 
Sorry,, I just saw this message and didn't get the "ping" from my name above.

I am with the others, SC engines are lower PSI. I am curious as to if the same PSI gauge was used in the first test as well as the second test? Also agree that changing the digital timing will have no effect on the physical compression in regards to doing a compression test. Also agree that if the cam was off then yes, it could effect the physical compression.
 
whosurpopi said:
The cam was off, the timing chain was installed incorrectly.
Click to expand...

Wow, someone wasn't paying attention. You said 2 ticks, I guess that means 2 teeth. Say for instance there are 22 teeth on the crank gear and 44 (2:1 ratio) teeth on the cam gear, 360/44 is 8.2 degrees of advance so just 1 tooth is quite a lot!

I've added up to 3 degrees on the cam before using a crank gear with several keyways intended for the purpose b/c I wanted a stronger low end grunt but top end was slightly weaker IMO.

Conversely, if you want a little more breathing for the top end retarding the cam 2~3 degrees can sometimes help, it's worth trying if you're racing and nobody has proved it doesn't work yet. Gotta be careful though, too much retard and the exhaust valve might still be open as the piston comes back to top dead center.

Good going.
 
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Great Learning. Good stuff. Valve Timing and Chain replacement with engine proformace issues. What can go wrong or what not to do and watch for.
 
There are two teeth on the cam gear that either will work because it has slots for final adjustment with 3 bolts that tighten it into final position. Anybody who missed it by two teeth and can't align the crank and cam on a 4tec has no business working on the motor and I would be highly suspect of the rest of it.

Basically, you lock the crank in place using the tool in the detent position and you lock the cam in place with the tool in the detent position and then assemble with the cam gear aligning marks with the chain tight in the driven direction and tighten cam gear in place, very simple and clearly explained in the manual.

Regarding mid 7s rpms, how full is your oil? Do you have the correct impeller and everything in pump in excellent condition?
 
Has anyone inspected the inside of the exhaust hose from the engine manifold to the waterbox? Could be one of the exhaust hoses has delaminated internally and is blocking exhaust causing high exhaust back pressure.
 
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