08-06-2011, 08:56 PM #1
Compression?? Overheated?? New to watercrafts!! And Stumped!!!
Hello.. this is my first real post to this site and just today I purchased a 1994 Polaris 750 SL With a Nice Leland trailer for 50.00 at an auction. I think I got a great deal for it even though the owner told me it had bad compression and but who could pass up and a deal like this. Well to the question.. When I got home I decided to look into the problems and I noticed that the compression was Excellent on the (P) Cyl. About 60 on the (C) Cyl. and 50 psi on the (M) Cyl. Also Noticed that the heads were discolored Nice and blue on the (P) and grayer on the (C) and almost black on the (M) I decided to remove the heads and there was no indication anything bad the Cylinders P,C,M All look real good with no gouges and the same with the pistons and all original pistons as well. What would cause the over heating? is there an water pump impeller like what is in a outboard motor? and I am stumped about the compression as well.. I did put some oil in the Cyl thinking it may have been washed out but it only made a slight difference. There is no visible signs of cracks and set the heads on a flat stone for my MMC measurement tool and no signs of warpage. This was also a fresh water Ski..
08-06-2011, 09:27 PM #2
The head gaskets could be compromised from the excess heat. That could explain low compression on those two cylinders. Also, if it got that hot it's possible the cylinder sleeves got out of round.
there is no water pump on these. There is a 3/4" ID tube on the starboard side of the pump. This connects to a screen that is in the jet pump nozzle and the water pressure created by the jet impeller feeds water to the engine.
There are several possibilities as to why this ski overheated.
1) The water intake screen in the jet pump is clogged
2) The water supply hose is clogged
3) Water jackets in the cylinders got clogged
4) Thermostat stuck
5) Bypass in the thermostat housing is plugged.
08-07-2011, 05:53 PM #3
Thanks.. John.. I took my dial indicator to each cylinder and the run out on it very min if any.. So I do believe the cylinders are okay. I am going to get a set of head gaskets for it and a thermostat tomorrow and check more on what possibilities you suggested... Where is a good inexpensive place online to get parts?
08-07-2011, 06:34 PM #4
Will the engine run at all?
use the search function to get a lot of information on the 750 Fuji engine. If you want to try, go get a can of copper spray and reseat the heat gaskets with a light spray on both surfaces. Let setup for an hour or so and test compression.
The thermostat is not needed this time of year, so pull it out and test with out the thermostat.
My guess is the overheat was caused by someone testing the engine out of the water or a pump full of crap. There should be a back flush on the water bar. hook your hose to it and run the engine, turn on water. ( no more than a minute or so) turn off water. stop engine.
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