Topic: Compression Test Propcedures

[TwoTired]

Bring one of your spark plugs with you when you get the comp tester.  So, you'll get the right adapter to use.

Cheers,

[OldSchool_IsCool]

Thanks TT, bringing a spark plug is a GREAT tip!... would have saved me a 2nd trip to the parts store! GRRRRR!

Anyway, once I got the kit with the 12mm adapter, I managed to run two tests.  First test had the following results:

100, 90, 115, 130

The second test had these numbers

90, 90, 100, 125

For the first test, I kicked the motor, did NOT adjust the valves and tested the cylinders from #4 down to #1.  More or less, I pulled the plugs out and started kicking!  I wanted to see the compresson number baselines that I have been running without any adjustments.

Thinking that maybe the lower numbers on #1&2 may be due to Wheaties depletion, I dropped the battery in and ran the second test from #1 up to #4 using the starter. (The battery has been sitting in a warm house for a week and on a charger for about 10 hours)  Each cylinder pumped up to max within 3 or 4 compression strokes.

I had planned to run test series #3 with a squirt of oil, but my oil can pump broke, so I put it all up for the night.

Anyway, now for the first salvo in the wildly contradictory analysis contest.  I'm thinking a possible reason for the low compression on 1&2 may be due to my original complaint with the bike, that being a bad head gasket (see my other thread on left side oil leak if you want the details).

You may fire when ready!

[750goes]

Adjust your camchain, adjust all your valves to specifications and do the test again and post your results - sorry for your leg...I feel your pain

oh.....put the battery on the charger again.........

overall - your compression is down  - but do the valve adjustment...........it could well make a difference if your clearances are out..

[OldSchool_IsCool]

Nope! not the answer I was looking for. 

OK, I already pulled the battery back into the house and will charge it overnight.  Camchain and tappet adjustments by the book before the next round of tests.

I did mess up one of my tappet covers though GRRR! guess that will go on the list of unexpected parts needed when I reassemble following the top-end regasketing.

[OldSchool_IsCool]

OK, NOW the wildly contradicting analysis can begin.

After struggling with a stuck tappet cover, I finally got the tappets adjusted.  Several of them were tighter then spec.  To the point that they were tight to the valve when they should have been slack.  Oh BOY!

Anyway, here are the results:

115, 100, 105, 115 (dry)
150, 120, 120, 135 (wet)

By wet, I mean that I put about a tablespoon of 10w30 into the cylinder immediately before testing.

[TwoTired]

At first blush, it would appear that your rings are leaking. 

But, then I wondered about all that oil lurking in the cylinder that is not compressible.  I felt that a tablespoon is a lot of oil for this little engine.
So, I made a couple of quick (and probably inaccurate) calculations.
One tablespoon of oil is 14.7 CC.
One cylinder displacement 137.5 cc
Compression ratio is 9 to 1.  (don't know if this includes the late intake valve closure or not.)
Divide 137.5 by 9 and you get a dry combustion chamber volume of 15.3cc. (??ballpark??)
15.3cc-14.7cc is the initial volume lost due to oil insertion and is going to make a big change in compression ratio and compression test numbers even if the rings are sealing perfectly between the dry and wet tests.

Certainly some of the oil should get spit out or vaporized over repeated cycles.  But, how much?  (Certainly all if they are firing cycles.)

I've never put that much oil in a cylinder for a compression test, as the idea is to see if a viscous fluid can help temporarily seal the rings, not change the comp ratio of the engine.  Just one pump of 40 wt from my oil squirter is all I ever used.  And, even with that I usually see about a 5 lb increase across the bank. (Air with oil in suspension is less compressible than ordinary air, too.)

Another variable is the volume added to the chamber from the compression tester.  Do you think you might get a tablespoon of oil into the hose between plug adapter and the release valve?  Whatever volume present there adds to the chamber volume and can effect compression ratio AND the compression readings.

I'd like to see wet vs dry test results with less insertion of oil added.  A teaspoon or less... after the engine has been run a bit to clear the chambers.

Another factor is the cylinder temp.  Cold cylinders pistons and rings will not seal as well as ones at operating temperature due to expansion characteristics.  I would expect a cold wet and dry test to show higher numbers with oil as that will improve the ring seal.

Summary:
The dry numbers look pretty even for a cold engine.  I might recheck the valve adjustment as that has a direct effect on compression ratio.  (The intake valve closes 35 degrees AFTER bottom dead center while the piston is already moving up on the compression stroke.  Tappet clearances can easily effect just when actual close occurs.)

This engine should run smoothly if ignition and carburation are also in good condition. 

You want to replace what?

[OldSchool_IsCool]

Thanks for the reply TT.  I'm re gasketting the top of the engine due to a leaky head o-ring.  See my other link

http://www.sohc4.us/forums/index.php?topic=17132.0

I'm wanting to get a compression test done so that I'm forwarned of other issues I need to take a close look at once I got the engine in bits.

I'll go ahead and recheck/adjust the tappets then run the engine to clear the cylinders.  I'll then get dry and LESS wet pressure readings on a warm engine and post them here.

[OldSchool_IsCool]

More results.

I let the bike rest for two days to let the oil drain down.  Reassembled and fired her up!  DAMN! it was nice to hear her roar again!  Gota say, adjusting the valves seems to have had a positive affect, she sounded a little pissed off at me for not taking her out on the town for over a month!    Once warm, she held a smooth idle at 1000 RPM.  Previously, she couldn't hold idle below about 1200-1400.

Anyway, when I shut her down, I did a quick disassembly, re-adjusted the tappets and ran new compression tests.  By the time I got to the tests, the engine was still warm.  I could rest my hand on the head without discomfort.  I ran a series of dry and wet tests.  Here are the numbers:

115, 105, 110, 110 dry
115, 100, 110, 110 wet

I used much less oil for the wet test this time.  The photo is an attempt to give you an idea on how much.  The wrench is a 10mm (tappet lock nut size).  I tried to get the picture before the oil wicked into the paper too much.  Sorry if its a little fuzzy.  With the wet/dry numbers being so close, I wonder if I didn't us enough... sigh

[750goes]

Results show that all cylinders are wearing at the same rate (great comfort in that isn't it)...but overall from where you started and with the several tests and adjustments you have completed - there is nothing disastrous to see...

If you are going to do a top end rebuild - I would probably go for new rings, camchain, inspect the valves for leakage when you bench test it and get them cleaned up if necessary, and if everything else looks fine, put it back together...(new seals of course - plus new head gasket/base gasket)...

[TwoTired]

As far as I am concerned, there is nothing wrong with your engine.
You will have to go back and readjust your valves, though.  The clearances in the book are for a stone cold engine.  These clearances change when the engine warms up and metal bits expand with the heat.  You can expect a noisy valve train next time you run it, unless you readjust the valves cold. And, there may be some concern about valve tip or adjuster tip battering over the long haul with excessive clearances.

I don't know why you are so intent on finding more problems.

If the rings are leaking cylinder pressure, the oil will seal the rings to make the pressures temporarily higher.  An engine properly worn in at operating temperature will have the best piston-ring-cylinder fit.  Adding a temporary oil seal won't improve it, as your tests clearly show.  If you add enough incompressible fluid in the cylinder, the pressure will, of course, rise higher.

If you can find standard sized rings for the 550, you would also have to hone the cylinder walls into a cross hatch pattern, and follow a break in procedure to get the new rings to seat correctly.  Both honing and the break in process removes metal from the cylinder walls and accelerates the cylinder wear.  It's not hard to imagine 5-10K miles being removed from your engine life with just this process.  You would have to measure the cylinder wall wear before and after the hone to to see how close to wear limits you are before returning to service.

Most likely you can't find Std sized rings.  And, overbore pistons and rings aren't too easy to find either.  Now you will have to bore and hone four cylinders to match the new pistons size.  You also have to find a machine shop that can bore straight and accurate holes without removing too much metal to fit your new pistons properly, too.

You can go ahead and do a top overhaul on your motor if you want to, of course.  But, unless that oil weep is severe, I'd leave it alone until you decided the bike is worth a $500 top overhaul.

If you absolutely HAVE to have a bike with no oil weeps, then disturb as little as possible with your currently fine running engine, in order to get it oil tight.  However, while essential, new gasket installation is NOT a guarantee it will become oil tight.  Cleaning and assembly techniques play a large role in making an oil tight engine.

It's not rocket science.  Lots of people attentive to detail can perform this task successfully.  Only you can judge your own skills and prowess.  However, I would hate to see you trade the misery you have now for a bigger misery afterwards.

Good luck!

[OldSchool_IsCool]

As far as I am concerned, there is nothing wrong with your engine.
...
It's not rocket science.  Lots of people attentive to detail can perform this task successfully.  Only you can judge your own skills and prowess.  However, I would hate to see you trade the misery you have now for a bigger misery afterwards.

OK, I've read a number of posts and want to make sure I got the compression test procedures correct.

1- charge battery for consistant cranking or eat my Wheaties and plan to kick start
2- check/set valve gaps
3- warm engine
4- pull all 4 plugs
5- connect compression tester to first plug hole
6- crank or kick to get highest reading
7- repeat 5&6 on remaining three cylinders
8- repeat 5,6 & 7 this time with a squirt of motor oil in the hole first
9- post results here for some wildly contradictory analysis
10- pick the results that I wanna hear.

Sound about right?

BINGO!! That's the "results I wanna hear"!  I have the OEM gasket kit in hand, with no expiration date, by the way, so a future rebuild is partially funded.  Reading between the lines to TT's post, "better the devil you know then the devil you don't".  I'll hold off on a top-end rebuild until it is REALLY needed.  At this point, I've done nothing to the engine that wouldn't need to be done for a tune-up.  Front brakes then become my winter project.

I can put up with a weepy seal.  Already have the water(oil)-proof pants! 

As far as my own skills and prowess, well. lets just say adjusting the tappets stretch them both!  I've read other post about using wadded paper or maybe lengths of rope to soak up the oil.  Sounds like my kinda fix!

[Dennis]

Now you're getting it!
Don't go looking for trouble.
More time ridin'   .........  less time wrenchin'

Good luck with the brakes.

[TwoTired]

My 74 CB550 has been weeping at the head for 20 years.  I have a NOS top end gasket set, also 20 years old, waiting for the leak to get bad enough or something else to go wrong and give me a good reason to open it up and fix that leak in the process.
I like to fix things that are broken.  But, the danged thing just ran too good to rip it apart.  "If it works, don't fix it!"

However, "Pig Pen" is just about ready for its "new" gaskets, I'm afraid.  Maybe this spring...

Cheers,