'98 GSX Limited.......correct compression please.

[flattracker60]

Going to look at a 50hr (never been rebuilt) 1998 GSX LTD. This is the 951 carb motor (just so were clear).

With a strong battery, what should ideal compression be please?

Wondering what it would be if it were perfect, and no lower then.......?

Thanks in advance,

 

[donkey]

make sure u check it with the throttle open,
near perfect ,150
near dead 130
hope it works out for you,,ive been looking for a gsxl winter project myself,hope to see piks!

 

[LouDoo]

A 951 carb. with 50 hours should be 130-135 psi.

Lou

 

[Dr Honda]

What Lou said. the 951 engien is 135 psi.

To give Donkey a little credit... all the other seadoo 2-strokes should be 150 psi.

 

[donkey]

oh crap,,sorry,,hmm,

 

[LouDoo]

It's O.K., no harm done, it was late when you made the post.

Lou

 

[SabrToothSqrl]

Compression is only one measurement. While 130 is considered perfect, my gauge shows 110 and 120, but the engine runs 100% perfect, sadly it's my only gauge, so maybe it is closer to 130.

Compression will only tell you if the engine is dead... so if your getting 120 and 60. Rebuild time. 50 and 50, rebuild time. 50 and 0... etc.

If you get 110 and 110, the only real test is to drive it. in perfect shape the 951 should spin 7k WOT IN THE WATER. This means new / perfect engine, pump, impeller.

As they age, the max RPM under load seems to drop... 6900, 6800, 6500.

the problem is jet pump thrust is exponential. losing 500 RPM on the high side means losing 10 MPH. the difference between 60 and 55 or 50...

so, ride it for best results.

 

[68ragtop]

And to add to the dilemma, a deceptive seller could squirt some oil in the cylinders & the pressures can come up some. Gauges can definitely be an issue, if you are going there with a cheap harbor freight gauge, I wouldn't trust the readings you get at all. If you don't have a good one, see if you can borrow one. A borescope is also a great tool if you have access to one. Mine has a 45 degree mirror that I can put on the tip & allows me to look at the ports & cylinders walls to look for possible damage. However, all the being said, if you can't get your hands on some good test equipment, make sure both MAG & PTO sides have “good”readings that are near the same & within a few lbs of each other 2-4 lbs difference could be normal, 6-10 lbs, there is very possibly something wrong. even a cheap gauge can help you check that.

The most accurate test in my opinion is on a completely warmed up engine, both plugs out & WOT like mentioned above. Spin it over until the gauge stops going up. The initial lower readings are not the cylinder building up pressure, it’s the pressure in the gauge hoses getting up to the same pressure the cylinder is capable of. In the first few compression strokes, the hoses are essentially part of the combustion chamber & will always start out low. A good compression tester will have different hoses that will have the exact same threaded reach into the spark plug hole as the spark plug does. It will have a check valve at the very tip of that hose. My $.002

 

[SabrToothSqrl]

There is massive conflicting info on hot vs. cold test.

 

[68ragtop]

There is massive conflicting info on hot vs. cold test.

I completely agree.
It seems there are more recomendations for hot testing with 4-stroke engines, but my personal thought is, the engines only operate warm/hot so why not check them in their operating enviroment? But, I like the idea of checking cold too. If everything is equil temperature wise, the results between cylinders should be as well.

I guess I should have left out the warmed up comment, didn't mean to start another debait. If I was trying to check the health of an engine, I would probably do both, cold & warm & compare my results.

 

[SabrToothSqrl]

Actually, if you read the racing manual, the pistons are made OUT of round due to the wrist pin.. as they heat up they become round! So yea, testing cold means they are out of round... but yea... My guess is when warm they will be oiled up it will up the reading, making it more likely to be within spec. I only use compression tests as I listed.. to see if the engine is either 'likely working' or 'shot'. It won't tell you if it's going to run 5K WOT or 7K WOT. only thing that will is a real world water test.

I also haven't found any difference between WOT test and idle test for a compression check. at 500 RPMs, I'm not sure it has a difference... the racing manual even says not to use compression tests as a performance test. If I find the page, I'll post it.