Compression Test?

[Anonymous]

i hear everyone talking about their compression in their engines and how anything under 110 psi is time for a rebuild. how do i go about getting a compression test done, or doing it myself (somehow to save money) on my '80 gs 400L. I would like to see what state my engine is in. Is it just a contraption like a voltmeter? or how does it work?

[saaz]

You will have to buy or borrow a tester. Preferrably one that screws into the spark plug hole as that is a bit easier to use. It is basically a pressure guage that measures how much pressure is being generated in the cylinder.

Testing is best done when warm. remove all spark plugs, and hook up the tester to each spark plug hole one at a time. Ground out the spark plug wires to avoid damaging the ignition coils. Open the throttle and press the starter button to turn the motor over a few times. In a few rotations of the motor the compression should be at maximum. Any more turns than that to get to maximum indicates a problem. The throttle needs to be open to get a true reading.

If the readings are low (less than 120psi) put a small squirt of oil in that cylinder - this will seal the piston rings. Test again. If the reading goes up to a reasonable level , the problem is likely worn rings/bore. If not, a valve problem.

This is a real simple explanation. If you have a friend who knows how to doi this on a car, the principle is the same.

[Anonymous]

you can buy a tester at canadian tire. I bought one but i got the cheap one($35) so it didn't have the proper end for a motorcycle now I have to make one. So spend the money about 50 dollars and make sure that it will fit on your bike. Take a spark plug with you for size. They will work on any engine. They have instructions in the box as well.

[Anonymous]

None of the ones at Canadian Tire will fit a D (10 mm) spark plug, just the B (12 mm) NGKs. After a bit of thinking it may be D is 12 mm and B is 14 mm.

Steve

[Scud]

Saaz,

You mentioned grounding out the spark plug wires, how do you do that? Is it as simple as putting the spark plugs back in the wires and then resting them alongside the engine?

Thanks,

Scud

[Anonymous]

"B" NGK plugs = 14mm and "D" NGK plugs = 12mm

I use a compression tester that has no threaded end. Its end is rubber and you press it in the plug hole. Works great and you don't need adapters.

[Jethro]

I bought one at Sears for $29 and it comes with 5 adapters, both double ended. It fit's my snowmobile and my boat, both of which have odd size plug bores.

[LW_Icarus]

Id think Id have a bit of a problem holding a rubber compression tester in the hole, and holding the throttle open, and holding the clutch in... whew, thats alot of hands.

a good compression tester will set you back a bit, but is worth every penny if you are going to work on your own stuff.

read the manual's procedure for the test if you can, it ensures the best results. I run the test cold, then hot. and I just unplug the power wires from the coils, rather than grounding the hi-tension leads. easier to get at the middle plugs with the tank off anyway.