Compression Test Question

[TitaniumMan]

Hi All,

I was just searching the forums to see if there are any tricks to doing a compression check on my 99 GTX RFI.

Other than the suggestion to remove both plugs to lessen the strain on the battery and to check each cylinder at least 3 times it appears pretty straightforward.

Will any compression gauge work? The one I was looking at has a 15" hose and 12 and 14 mm adapaters supposedly for "broader applications such as motorcycles, ATVs, snowmobiles, and watercraft". It is only $29 and seems like a worthwhile investment (assuming I don't need a special tool from SeaDoo).

Thanks in advance.

 

[ScubaDoo]

You can use any compression guage that screws in the threads in the cylinder. Be sure to use the ground struds for the spark plug caps on the cylinder heads to keep the power from the ignition coils from building up and causing problems. There is no special parts needed from seadoo to do a compression check... A good psi is 150 per cylinder. If you have any more questions give us a shout.

 

[seadoosnipe]

adapters......!

All compression gages come with adapters because of the short and long type spark plugs.
Using the long adapter in a short plug, will damage the piston. Make sure you choose the right threaded adapter for your application by comparing to the plug you pulled out.:cheers:

 

[ScubaDoo]

Thanks for the extra input seadoosnipe. I assumed that he would use the same thread as the plugs, and you know what happens when you assume something.

 

[TitaniumMan]

Thank you seadoosnipe and ScubaDoo. Very helpful.

Fear not, I know at least enough to match the threads

The machine is still under snow in Quebec so it will be a while before I get up there. I'll report back when I have the results.

Thanks again.

 

[All Things Custom]

If I can add a few things here...

When doing a compression test, make sure both plugs are out and that the throttle is wide open.

Almost all guages read differently. I have 3 and they are all about 3-5 PSI different from the other.

The best rule of thumb is to be + or - 5 psi from each cylinder.

For example I can test with one gauge I have and the PSI will be 140/ 138and with another gauge I have it will read 130/128

Both readings indicate a good compression reading. when you have to worry is when you read 145 / 139 or anything more dramatic than that.

In short, its not so much the max pressure as it is the variance in pressure....

 

[seadoosnipe]

Question?

Mike, I've been doing compression test on several 2 and 4 cycle engines, V-10's down to single cylinders and have never did a WOT type test. I've never heard of that ever explained in my youth till now, which means I may have been doing compression tests wrong for the last 30 years. I'm gonna assume it's so you get enough air and fuel to coat the cylinder walls while testing.
Can you elaborate further on this.
Thanks,
Louis

 

[seadoosnipe]

Research.....

With questions in my head now on the proper procedure on doing a compression test, I've run across some points of interest. I thought I'd pass them along here.

 

[All Things Custom]

It serves a dual purpose

1.) It lubricates the cylinders to get a true operating reading. A dry metal to metal reading will allow air to go past the rings not showing the true operating condition of the motor.

2.) With the throttle wide open you are not fighting a vacum created from trying to pull fuel into the cylinder. Or one carb open more than the other. etc etc etc It just gives a truer reading.

 

[seadoosnipe]

Good point....

Good point Mike, that's why I cut off that post above so abruptly. I decided to do some research on compression testing. I thought that was where you were going with it and for an older model, that's a great idea.
My 1997 model, with the residual on the cylinder walls, engine cold, will draw 150-155 pounds. Depending on how many times I do the test. And from what I know of the boat, it's never been rebuitl. Just well severviced. The original owner was pretty rich and I'm sure, had his own mechanics.
But, because we take the simple things for granted sometimes, I did post my research on compression testing in my thread "snipes Korner" and all are welcome to get some ideas on the way to test their engines.....
I do think and agree with Mike, it's not so much what the pressure is, it's what kind of variance you have. With the 4 strokes, like the 4-TEC's, you'll also learn that the compression test can tell you if you've got a burnt valve or bad compression............
Here's the link to my article.............http://www.seadooforum.com/showthread.php?goto=newpost&t=1838