Topic: Compression test results

[famoussas1]

Mixed reviews on the inexpensive HF compression tester, so I went with it anyway.  Heated up engine, disconnected coils, opened up throttle and did test on each cylinder:  Here were my results:

Cylinder 1- 120 psi
Cylinder 2- 105
Cylinder 3- 110
Cylinder 4- 110

I guess I didnt know what to expect, but everything is closer than I thought. #2 is a little off the 10% everyone talks about, so what do you all think?  Still smoking oil, but I think I am gonna wait til the season is over then maybe rings?

[kslrr]

#2 is only off by 5/110 which is only 4.5%.  I wouldn't worry about it, like you said, until next season.  You also might recheck your valves.  A valve gap that is too small could effect this.

[CBR750RR]

What compression numbers were these motors putting down when new?

[kslrr]

If this is a 750, 150 to 170 is what I find with a quick search.  As long as all cylinders are within 10% of each other, it indicates even wear and no drastic issues.

[cmonSTART]

IF those numbers are accurate (and it's a big "if" with a HF tester) I would say that motor is pretty tired - even numbers or not. 

If the bike is running OK and not smoking too badly I might try to eek another season out of it and worry about it next winter. 

[TwoTired]

Don't know what bike/engine you have.
Don't know ANY engine history on this example.
Don't know what tester you have.
I don't believe you used the same tester Honda did when they printed 170 PSI as expected. (I believe they used a small engine tester rather than one for diesels and cars with 5-10 times the displacement.)

Further to diagnose rings with this method, requires a "wet test" compared to a "dry test".  Even better, use a leak down tester which allows you to learn just where the pressure is escaping.

[CBR750RR]

Spot on Twotired!

I really need to get around to getting a proper leakdown tester!

[cmonSTART]

All properly functioning compression testers will yield the same results, regardless of size.  The larger ones just take more cranks to get there.  There is a one-way valve built into them for this. 

TwoTired is correct about wet tests and dry tests to help diagnose, as well as a proper leak down test being helpful. 

[CBR750RR]

Can you guys educate me on what exactly is a wet test?

[cmonSTART]

Put a couple tablespoons of oil into the plug hole with an eyedropper.  The oil will coat the cylinder walls and rings and help the rings seal better.  If you see a large increase in compression after doing this it means your rings may be leaking.  If there are no real changes in compression your low compression issues are probably somewhere else (head gasket, valves!!) 

[TwoTired]

Can you guys educate me on what exactly is a wet test?

After taking the dry test numbers, put a few drops of oil into each spark plug hole, and repeat the test.  The oil will temporarily seal leaky rings and make the readings rise a bunch.
However, if a large volume of oil is added, this changes the compression ratio and even good sealing rings will show a large pressure boost.  I haven't calculated the 750 chamber volume, but you don't want to put in enough oil to change that volume significantly.
The 750 has about 20cc of squish chamber.

One Tablespoon is 15 cc, two tablespoons would be 30cc and hydro lock the piston at the top of the stroke, blowing gaskets or bending piston rods.

All properly functioning compression testers will yield the same results, regardless of size.  The larger ones just take more cranks to get there. 

Sorry not true at all.  There is a FAQ entry to explain why.

There is a one-way valve built into them for this. 

It matters where the check valve is places in the test aparatus.   If at the spark plug hole then the gauge can read true minus the offset of the check valve spring pressure.  If the check valve is at the end of a long hose, the hose volume adds to the squish chamber volume, and changes the compression ratio.  Addding volume lowers measured pressure in this case.

[CBR750RR]

Fantastic, glad I learned this, will try this out on my 78 cb750k with 20k miles.

so we are talking maybe 5-6 drops? enough to = less then a tablespoon or around 10ccs? should do it right?

[TwoTired]

Fantastic, glad I learned this, will try this out on my 78 cb750k with 20k miles.

so we are talking maybe 5-6 drops? enough to = less then a tablespoon or around 10ccs? should do it right?

3-4 drops.  10cc is half the combustion chamber volume and guaranteed to make compression higher.  A teaspoon is 5cc and too much imo, as it is 25% of the combustion chamber volume, which you don't want to change for a accurate diagnostic test.
If you could reach inside, you'd put a drop of oil at each 90 degree position of the cylinder wall and let it crawl into the crevices to take the place of a worn ring to piston/or ring to cylinder wall space, which ain't much volume at all.

If you put in too much oil, the compression numbers WILL increase.  How would you determine if it was a changed compression volume issue or a sealed ring issue?  If you aren't careful with what you are doing during this test, you can get a false positive and believe an unnecessary ring change was required.  This is the very reason why a leak down test is often viewed as a superior test of engine mechanical integrity, as this tool requires no inference or reasoning to interpret results.

Cheers,

[famoussas1]

its a 74 cb550.  I think theres about 34K on the engine.  That is all speculation though as only info from PO.  I wanted to get a better tester, however I was getting mixed results on the PSI value and the greater importance as the difference from cylinder to cylinder.  Could a valve gap issue cause burning oil in exhaust?

I suppose I could do a wet test.

[TwoTired]

Could a valve gap issue cause burning oil in exhaust?

You mean tappet clearance?  I don't see how.  Valve guide clearance, yes.

[anotherCB]

I also want to test the compression of my 750, however the compression tool I have did not come with the corrrect size adapter. Are those adapters available somewhere?

[fmctm1sw]

I have one of those quick disconnect HF jobs.  Works great on my truck but like TT said, it adds too much volume to the combustion chamber.  My Dad gave me a small one years ago with a rubber top that just snugs up to the top of the plug hole.  It works great on the small fours but you'll never get it into the no. 2 or 3 cylinder on the 550 or 750.  The difference in compression with that and my HF one is significant.  Around 120psi with the HF and around 150psi with the small one when I checked.  Before you panic, see if you can find a different compression tester.

Edit:  example of mine:  http://www.toolsource.com/compression-tester-05525-p-112461.html?sourceid=googleps