Compression Testing

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H

Hunterb

Member
I have a question about compression testing. I inherited a 1997 Seadoo GTX. Fairly typical story. Someone got it, not running, took it apart, messed up the carbs, let it sit for 2 years. Eventually they decide to get rid of it which is where I come in. I like a project.
So, I have now pulled the motor from the ski. It spins easily by hand and develops compression but I have no idea how much. I don't want to go through the process of putting new carbs on it, reinstalling it etc just to find out the compression is low. So here's my question; Can I hot wire the starter and spin the motor and get a useful compression reading? Remember there are no carbs installed at the moment. Will the motor suck air and compress it normally? If so I assume I want a minimum of 130 or so and ideally 150psi.
If you know and can let me know that would be most appreciated.
 
Yep just put jumper cables direct to the starter and read compression. Don't need no carbs on there. :) I'd be concerned at the engine sitting with no carbs. Was the engine sealed up to keep moisture out?
 
Thanks. To answer your question, no the engine was not sealed. There was definitely water in it. A fair bit squirted out when I engaged the starter. I had sprayed fogging oil through the plug holes and through the intake before spinning the motor. When I hooked the compression gauge up it shows 200 on the mag side and 185 on the PTO. This is clearly not what it should be. There is 'milkshake' oil coming out the plug holes and out the fuel pump connection. I have spun the motor a lot and there is noore, or very little, milkshake coming out now but the compression is crazy high. Any suggestions??
 
Water in the engine is bad news. If you want reliable just pull it and rebuild it complete.
 
Thanks for the reply.
It's already out on the bench. I was hoping to avoid having to split the case. Is there no way of clearing the milky oil without a complete tear down? I didn't think it was all that unusual to get water in the motor.
 
Hunterb said:
Thanks for the reply.
It's already out on the bench. I was hoping to avoid having to split the case. Is there no way of clearing the milky oil without a complete tear down? I didn't think it was all that unusual to get water in the motor.
Click to expand...
Water in there for 3 days is a big concern for me as parts begin rusting immediately. I clear the water immediately if any gets in there and I get the ski started up and advise people to ride it for an hour to get all the moisture out. This is for engine longevity. Any other way and the engine is on borrowed time. I've heard of people dumping gas and oil in the engine to displace the water and that helps.

I'd soak everything in oil before putting it back together. You might save it. Good Luck !!
 
Last edited:
Thanks again! I hope it's salvageable. It spins perfectly. I have looked at the pistons through the rave valve ports and they look fine, other than the milky oil on them, but I guess it is the bearings in the bottom end that are the biggest worry. I'll keep spinning it to see if I can get the rest of the water/oil out and get the compression down to normal.
Bruce
 
I don't find the milkly oil is as bad as no oil and just water and rust. Nothing wrong with being positive. Get R Done !! :)
 
Pull the engine down and check for rust in the bearings and cylinders.
You should be replacing the gaskets and seals anyways.
 
I borrowed a borescope camera and poked around as much as I could and can see no rust. Observing the pistons and rings through the rave hole they look fine as well. With no carbs and no exhaust it is currently showing exactly 175 lbs on both cylinders. I have cranked it a lot with the plugs out and there appears to be no more oily mist coming out. Is it possible that no exhaust back pressure is resulting in the high compression. The engine is on the bench.
 
Do you know the compression gauge you are using is accurate? Maybe try another to check. Some auto parts stores lend them out.
 
Only thing that would cause high compression pressure is a bad gauge or water/oil still in the cylinders.
Could be a milled head but highly unlikely, very unlikely.
 
Thanks all for the replies.
The motor definitely had some water in it before I got it. The oil/water that came out looked like a pale chocolate shake. I have not had any trouble with the compression gauge before but it does seem strange that it's reading exactly 175 on both cylinders. I will see if I can borrow a gauge from someone to confirm. The PO said the engine was 'heavily fogged' prior to him removing the carbs and subsequently apparently working on them with a large hammer and an impact rendering them unusable. I will continue to try to clear the crankcase to see if I can get the compression down, if it is in fact 175.
Not only did the PO destroy the carbs but he skillfully lost the only key and no one on Vancouver Island will program them for a 1997 machine. They look at you like you've just brought in a model T for a rebuild.
The struggle continues.
Thanks again for the replies and suggestions. They are much appreciated.
Bruce
 
Update on the compression. I tried another gauge, a push and hold type, and it reads exactly 150 on both cylinders. Hmmmmm. So I borrowed another screw in gauge and it also reads 150 exactly on both cylinders. So my compression gauge that I've trusted for years is not working in this application for some reason.
So on to the next issue. Thanks again for the help. I'm sure I'll have more questions as I try to get this neglected beast back to life.
 
I will add that there are 2 possible opening the motor up tasks. The top end only gasket for the 787 can be found for about $40. Removing the head and jugs and getting a real look at the bores is important. You will also get a much better sense of the crank and how rusty it is inside just looking in past the connecting rods.

There may not be a need to split the bottom end cases. Since it’s already on the bench and it’s winter for the commitment of a gasket set you will answer a lot of questions.

These are very simple motors but running it with rusty guts will loosen the rust and is likely to ruin what maybe good rings and pistons.
 
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