Compression test.

**liljegren** · 19th July 2010, 20:39

What sort of comp. result would you term 'healthy' for a GS 550? Matt.

**Paul in NZ** · 19th July 2010, 20:49

It depends.... Have the valve clearances been checked, does it actually run, is it warm / cold... Is the battery cranking it fast enough so many variables... Ideally they would be quite even...

**j_redley** · 19th July 2010, 21:05

Most engines should have minimal difference. Don't know about that modal personally as all vary, but the specs on the 250GsXr says each piston should be over 1000, with a difference of less than 200

**liljegren** · 19th July 2010, 21:18

Thanks , have done clearances/shims, starts really easily, and runs well at higher revs, but bogs at lower end, checked comp today, between 95 and 130 PSI. Just bought the comp tester second hand and wondering whether this is a bit low, maybe the tester is faulty. Will re-test with another to check. Note the motor was cold, would it increase substancially when hot? I guess it should do...Matt.

**SS90** · 20th July 2010, 01:58

Originally Posted by liljegren

Thanks , have done clearances/shims, starts really easily, and runs well at higher revs, but bogs at lower end, checked comp today, between 95 and 130 PSI. Just bought the comp tester second hand and wondering whether this is a bit low, maybe the tester is faulty. Will re-test with another to check. Note the motor was cold, would it increase substancially when hot? I guess it should do...Matt.

A compression tester is just a tool, and if you don't use it correctly, then you will not get the accurate results.

I am sure Youtube or some such thing will have some "expert" who has had enough nounce to be able to record a video of "how to use a compression tester" that thousands of people will think is gospel, but here are the basics.

1) The engine must be at N.O.T (normal operating temperature), this is done by riding the bike until it is the same temperature it is at the normal running temperature.

2) remove the plugs, as well as the low tension wires from the ignition coils. If you can get to all the plugs with-out removing the tank, you can disable the ignition coils by unplugging the CDI on most models (not EFI, as generally this will also disable the starter button.

3) Fit the compression tester to the first of the plug holes (hopefully you have the screw in tester, as the ones that you simply push and hold with your hand a generally less reliable, but, you have what you have.

4) With the throttle wide open (this is important), crank the engine until you see the guage stop rising for at least 4 turns of the engine. Record this number

5) proceed with the other cylinders in the same fashion.

Note, if you do not have the throttle wide open, then the engine will not be able to swallow enough air, and the compression will read low, as will not having the engine up to N.O.T (cold valves and rings do not seal as well as hot ones)

If you have one cylinder drastically lower than another, the problem will be either leaking valves or rings (generally speaking) to test if it is rings, pour a Teaspoon of oil into the suspect cylinder, turn the engine over for a few seconds with no compression tester, then refit and test again. If the compression increases a reasonable amount (say 20%), then there is every chance that your rings are worn, although this is normally a given, due to the fact that if rings are so worn that compression is substantually low, the cylinder will be burning oil at a phenomenal rate......

If adding oil to a cylinder does not increase compression, then then it is likely a leaking valve, although again, there are other symptoms of a leaking valve, namely an excess of oil in the airbox (some is acceptable, and a normal part of operation, but, an air filter dripping in oil is a good sign that your compression rings are worn, as the crankcase breather connects to the air box on all 4 stroke bikes, and if there is too much compression getting past the rings, it will generally carry oil with the leaking compression through the crankcase breather, and into the airbox.

If you suspect this is that case, then a leakdown tester is the only sure way to diagnose where compression is being lost.

That said, there may be a good chance that using the correct testing procedure to test your compression will result in more consistent measurements.

**Paul in NZ** · 20th July 2010, 12:17

And SS90's explanation is about as good as it gets.....

**neels** · 20th July 2010, 13:17

Originally Posted by Paul in NZ

And SS90's explanation is about as good as it gets.....

Pretty much.

If you're after a number, atmospheric pressure is about 14.7psi, so the answer should be 14.7 x your compression ratio give or take the accuracy of your tester.

**Crasherfromwayback** · 20th July 2010, 13:42

And hopefully all cyls will be within 10% of each other.

**liljegren** · 20th July 2010, 19:58

Hey, many thanks folks. SS90, I wish you could live in my garage for a while, I could learn so much, thanks for the info mate. I guess I now have acold reading, will do it properly now, and report back with the correct readings!! Cheers, guys.

**ajturbo** · 20th July 2010, 20:02

Originally Posted by liljegren

What sort of comp. result would you term 'healthy' for a GS 550? Matt.

well.... lets hope that it has more than my engine.....

**pete376403** · 20th July 2010, 21:31

go here http://thegsresources.com/ register and ask in the forums. Someone will have the exact numbers.
The technique described by SS90 is as exact as anything I was ever taught