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Compression testing

catch2otwo

catch2otwo

Active member
Joined
Feb 1, 2009
Location
Vacaville CA
Moto(s)
Multi
Name
Cedric
Bike in question 2001 gsxr600

Was told bike has a bad top end.

Checked valve clearances - all in spec

Didnt have a good battery. Cranked it over by hand with a compression tester and got 75, 105,120, 90psi. I know you are supposed to do it warmed up with a good battery. Just wondering if hand cranking it, how close would you get to spec if everything was good.

Just seeing if i should invest more time into this or if those numbers are a tell tale sign of internal issues
 
Also just did a leak down, every cylinder was 0. Is that possible? Cylinder at tdc both valves closed. Heard zero hissing or leakage. Triple checked i had the regulator set up per motion pros instructions
 
First off, the speed you crank the engine over at can be important. Hand cranking is way too slow for accurate readings.

Remove all four spark plugs, hook up a car battery to the bikes battery and do it again. Positive to positive, negative to negative.
 
ST Guy said:
First off, the speed you crank the engine over at can be important. Hand cranking is way too slow for accurate readings.

Remove all four spark plugs, hook up a car battery to the bikes battery and do it again. Positive to positive, negative to negative.
Click to expand...
copy that, ill just pick up a battery tomorrow
 
Was that 0 percent leakage? If so, that sounds better than a new bike. Find out what your bike should measure at and make sure your meter reads in the same way.

What, exactly, is your zero numbers indicating? Percent leak down or percentage of pressure held?
 
Unless you need a new battery, I’d use a car battery to do the tests. Cranking an engine over, even with all the plugs removed, is tough on a motorcycle battery. A car battery has much more capacity due to its size.
 
And if you need a new battery, take a volt meter with you and I’d reject any battery that is below 12.5 volts. And charge it up before using it.

And if using a car battery, leave it in the car and use jumper cables to hook it up. Much easier that way.
 
ya, the zero on the gauge shows 0% leak down which is why im questioning.

Whatever literature I can find, shows 0-10% as awesome, 10-15 as acceptable etc. The motion pro setup im using has a regulator. My shop air goes into the regulator, I turn the regulator until the needle goes to 0, which is actually 60psi. I plugged it into the hose connected to the spark plug hole, and the gauge did nothing. To verify the gauge I plugged my blow gun into it instead of the hose, and opened it, showed 100% leakage which makes sense.

I do need a battery, the one that came with the bike metered out to 0.5v. doubt it will take a charge.
 
Yes, 0.5 volts is beyond dead. But if you have a decent car battery to use, do so. If it turns out the engine is junk, you will have saved the cost and hassle of a new bike battery.

And since the bike battery is so low, I’d remove it from the bike and connect to the car battery by clamping onto the ends of the cables that would normally go to your bike battery.
 
If you need to buy jumper cables, don’t go cheap. Many are incapable of passing enough current to start a vehicle. I remember a neighbor who was trying to jump tart their car with crappy hardware store cables. It wouldn’t even turn over. I said “wait a minute” and went to get my cables. I hooked them up and it started instantly. And
 
alright, with a fresh car battery
cyl 1 180psi
cyl 2 190psi
cyl 3 210psi
cyl 4 190psi

The spec from the manual shows 156-213psi, minimum 128psi and no difference greater than 28 between any two cylinders. Looks like im good here. still not really sure why the leak down shows 0%
 
Good numbers even if number 3 is more off from the others. Does the engine run?
 
I havent put fuel to it yet. Just wanted to make sure everything was on the up n up before i fired it. So far so good.
 
I would imagine that it should fire up as long as old/bad gas hasn’t mucked things up.
 
Who knows? Could be an uninformed statement by lousy mechanics. Or??
 
Did you have the throttle wide open when you were cranking? On my dirtbikes they tell you to open the throttle when you crank or else the numbers will be low.
 
yep, i forgot about it on the first cylinder and got a lower reading. All the readings were throttle wide open and some oil in the cylinder. cranked till the gauge stopped climbing.
 
Generally you take a dry compression test with the throttle held wide open first. If low, you can add a little oil to the cylinder to help diagnose the issue. If it rises quite a bit it's the rings, not much at all, it's the valves. Adding it first can give you an inaccurate reading.

I don't know why you are getting zero percentage on your leakdown. That is not normal.
 
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