Compression test result

Hi,

Done a compression test on my GPZ1100B2.

Results are #1:120 /#2:100 /#3:115 /#4:135

If check the valve clearance and #1 and #4 were too tight on the exhaust cam.

Unfortunatly, I didn't do the test of adding oil in the cylinder to compare the resulat..... and the compression tester is now return to the shop.

Test was done with open throtle.

When I got the bike, it was running good when cold but stop once hot. I suspect it was the temperature sensor related to the FI system. It had black smoke also at high revs. Wasn't sure if it was related to the FI and was running to rich.

I'm going to swap the FI to carbs this winter so not pushing too much on the fuel problem.

My questions are:
- When there is normal wear of cylinder bore or rings, is it usely even on all 4 cylinder?
- I'm gonna do a leak test shortly (As soon as my comrpessor is repair), will I be able to see if the rings are good even if I got two exhaust valves out of spec? Should I adjust them before doing the leak test?
- Could it be possible that the problem is the head gasket leaking?


Thanks in advance for your input!
Joce

Nothing on this?

Will the leak test result be accurate even if the exhaust valve are not closing 100%.

This maybe a basic question but I don't have much experience engine work. Few hints would help.

Thanks again,
Joce

I'm not an expert either but I really think you should adjust those tight exhaust valves before you do anything else. Otherwise any result you get will be confusing or incorrect. The tight and out of spec valves may be the sole cause of your problem of stopping when the engine is hot.

The next thing I would suggest would be to repeat the compression test. I got a cheap tester at Princess Auto for $20. Canadian Tire used to sell cheap ones also. The first time I did compression test I found I had to make sure that the tester was a very good fit in each cylinder and to turn the engine over several times until the reading on the gauge would not rise any further. Also WOT. This makes sure you get the best possible reading. If the readings are still low or the same as in your first post then add some oil to the cylinders and repeat the compression test. Then finally do a leakdown test - I have never done one so cant really offer any advice here.

The information you get from the above sequence should point you in the right direction.

Good Luck

Post edited by: JR, at: 2007/11/02 09:56

Compression test with a hot engine, full throttle, battery on a charger set to "engine start"(ensuring a completely charged battery, and best cranking power), and all spark plugs removed. This will give you the best compression readings. Next, add about a tablespoon of oil in each spark plug hole, and perform the same test as above, this is a wet compression test, and will show how the rings are sealing. Adjust the valves(when cold) and perform the same tests again.

The factory manual will tell you the variance allowed between compression in each cylinder, and the lowest compression allowed before a rebuild is needed.

I have heard of members running some type of fluid thru their engines(in the fuel) that cleans out the carbon in the cylinders. Smokes like hell, and I haven't used it.

Let us know what your compression results are after all rechecking and testing.

Post edited by: CoreyClough, at: 2007/11/02 10:31

Thanks for your advise.

Maybe one additionnal information I should give. The frame is actualy at the powdercoater and engine on the bench.....so won't be able to do a compression test.

This is why I was heading with a leak test.

Well.....while the engine is on the bench it should be a good time to adjust those valve clearances. A leak down test will not be of use if the valves are not closing fully.

Ok then. I'll start with adjusting the valve.

I've check my old note for the bike and it appear that a wet test was done and the compression was increasing of 20 psi when dropping oil in cylinder.

Thanks for your help,
Joce

jcote75 wrote:

Hi,

Done a compression test on my GPZ1100B2.

Results are #1:120 /#2:100 /#3:115 /#4:135

If check the valve clearance and #1 and #4 were too tight on the exhaust cam.

Joce

Are you sure about the exhaust valves being closed tolerance on #1 & 4? Seems that numbers 2 & 3 would be at issue based on the low compression of those two compared to the outer two. If the exhaust valve can't fully seat then your compression will be low in those cylinders. What did the intakes look like on clearances?

If 1 & 4 are tight and those numbers are correct then there is some sort of severe issue in the middle two cylinders. Assuming the compression can only get better in 1 & 4 if you re-shim those to open the tolerance you're going to end up with about 30-40% difference in compression from the lowest to the highest.

First, I'd re-check your findings on the compression normally but since the engine is out of the frame already I'd recommend doing a leakdown test and report back the findings after you set the valve clearances. Re-check your valve clearances (lobe should be pointing away from the intake side), the trick is to use the marks on the cam sprockets not just pointing the lobes away from the shim. I like keeping the clearances within .05mm on all valves not just the exhaust. After that you can do a proper leakdown test. You can do both the clearance check and the leakdown test easily on the bench, just remove the ignition cover and turn the crank with a socket and ratchet. Have all the plugs out to turn the engine to where you need it.

If the engine is still assembled and setting on the bench, you can still do a compression test, but two people make it a lot easier.

Brace the motor with 2x4's or some other wood blocks so it won't move, then apply 12 volts (battery and jumper cables) directly to the starter cable. I have done this with KZ twins and fours, even did it with a hot rodded Chevy 396 (it was setting on the floor though)

If a wet test only bumped the compression 20 psi then the rings are good. I did a 750 four recently, 80 psi dry, 200 psi wet, now that's bad rings

I agree with all of the responses you've gotten. I'd like to add a note or two, though.

KZ_Rage and Steell both make excellent points about the results. And yes, you can re-do the compression test--even cold--with the engine on the bench. You want to see the difference between cylinders as a percentage or as a psi reading. This can be telling you there are other issues to look at. I'm not sure (but will try to find and post later) the allowable difference between cylinders for your engine, but a rule of thumb is that it shouldn't be outside of the 10-15% range.

With respect to the "tight" exhaust valves on 1 & 4, that's not an issue for leakdown testing. The tolerances are from a cold engine, meaning they will be tighter when the engine reaches temperature and everything expands with heat . . . there will still be an acceptable (but smaller) clearance that allows your valves to close. In other words, if you have any clearance right now (cold), your valves will be closed. Leakdown is a static test, where you physically close all valves and put the piston at TDC. There will be leakage, and you're interested, again, in the percentage. There won't be any leakage past your valves unless they're held open by carbon, burnt valves, bent valve stems, damaged seats, or your cam lobe is actually pushing the bucket down (minus, or negative clearance).

Please do recheck the valve clearances, and do the leakdown and post it.

Close tolerance on the valve lash won't show up as a problem on a cold compression test. It would have to be past zero to show up. But close tolerance will make the bike stall out when hot. Usually, the earliest symptom is that it will stall at idle right after a highway run.

You should very likely adjust the valves and then repeat your test. You can test the 4 cylinder with the engine out of the bike. Doesn't need to be braced; just make sure you put it in neutral. Run a wire from a bolt on the engine to the negative battery terminal and touch the POS wire on the starter to the POS starter post. It will spark a little but once you make contact, the starter will spin the engine.

As to the leakdown test... if the valves are so tight that they hang open a tad, it is a waste of time. Air will just blow through the open valve.

Your compression is low and there is too much variance between cylinders. With the 80s engines you have, I would expect to see compression closer to 150 PSI per cylinder (don't have specs handy). With that said, you are going to have to disassemble the engine to bring up the compression so go through both the cylinder head and the pistons/rings and cam chain bits as well while you are in there. While a leakdown test is nice for a shop on a late model vehicle, you can bet that if your rings, pistons or cylinder bores or valves need attention, everything mostly likely needs attention.