False reading with compression tester

[andyd76]

I've just bought a draper compression tester with a 200mm extension and the reading on the back cylinder was down to about 100psi. After messing about for an hour i realised that it was the extension giving a lower reading, as it fouled the oil cooler I hadn't used it on the front cylinder! Has anyone else had this problem? Just a bit odd that draper appear to be selling an extension that affects the accuracy of the tester, I've repeated the test several times with and without the extension and the results are consistantly different by a long way.

Andy

[Wicky]

Try some teflon thread seal PTFE tape on the extension if it's still giving wonky readings take it back.

[sirch345]

Andy,
Perhaps this is just my odd way of thinking , but surely if you use an extension tube there is a bigger void to fill with compressed air and petrol, than if you do a test without it, consequently lowering the reading somewhat

I wouldn't worry too much about the amount its reading, the main thing with a compression test is that both cylinders are reading similar to each other.

[andyd76]

Chris,
My thoughts exactly, same volume into a larger area equals less pressure. So why make it? Am I missing something really silly? I will try some ptfe tape when i get time but i really dont think the problem is leakage.

Great site, my first twin and I'm hooked!

[turbo_billy]

Shouldn`t make any difference if you use an extension tube or not. When you crank the engine over you compress the air until the point where it eventually lets by the rings and valves and the gauge will not rise anymore. With a larger volume to fill you would just crank a little longer until the gauge stops rising. Think of a compressor with a 20 litre tank or a 100 litre tank. They will both give you, for example, 100psi but the larger tank will take longer to fill before the compressor cuts out.

[andyd76]

turbo_billy,
That would explain why the gauge rises for the first three or four strokes then settles on the final reading. I really cant think why both cylinders read about 100 psi with the extension and about 140 psi without, both measured with the engine cold.

[turbo_billy]

Choke off and throttle wide open both times?

[andyd76]

Yes thottle wide open and choke off

[turbo_billy]

Then IMHO I think you have a leak either at the plug thread or on the extension joint. Do what you said and try some PTFE and let us know. The gauge has a non return valve built in so the pressure there will not be released until you depress the button. The extension is normally simply a brass or rubber tube so the only "weak" points are the threads not sealing properly. Remember, not all compression testers come with the same length hose. Some are even just a rubber cone that you hold down over the spark plug hole to create a seal. I am interested to see what you find.
Regards
Tim

[Wicky]

Honda VTR service manual specs:

8-3 CYLINDER COMPRESSION

Warm up the engine to normal operating temperature. Stop the engine and remove the spark plug caps and spark plugs.
Install the compression gauge into the spark plug hole.
Shift the transmission in neutral.
Open the throttle all the way and crank the engine
with the starter motor until the gauge reading stops rising. The maximum reading is usually reached within 4 - 7 seconds.

COMPRESSION PRESSURE:

1,128 kPa (11.5 kgf/cm2 ,164 psi) at 350 min-1 (rpm)
Low compression can be caused by:
-blown cylinder head gasket
- improper valve adjustment
-valve leakage
-worn piston ring or cylinder
High compression can be caused by:
-carbon

[turbo_billy]

You are correct Wicky in that a proper compression check should be carried out with the engine hot. However, as we have said, even cold the compression readings should be the same with or without the extension. This still leads me to believe that there is a leak somewhere. After all, a small leak at 164psi in such a small "container" such as the barrel and gauge hose will take no time at all to be down on pressure.
Regards
Tim

[Wicky]

I agree Tim, - my recommendation comes from using my compressor to inflate bicycle tires with a tyre inflator with a pressure gauge. The weak point was the adapter connections to the valves hence my 1st tip to tape and seal the weak points. Assuming that's been done then we're getting closer to ballpark accuracy to manufacturer's spec using a correct standardised methodology.

[andyd76]

Wicky - Agreed the reading isn't accurate unless taken with a hot engine but I've found it useful in the past to compare the cylinders when cold. I'll play around with the tester on another engine as the Firestorm isn't the easiest to play around with, I've already lost the short 12mm thread adaptor down by the plug hole a couple of time. Thanks for the advice though.

[AMCQ46]

Has to be a leak in the adaptor.....also did you make sure to block off the vac pipe on the rear cyl, the one that operates the fuel tap?, if you don't do that the rear will always be low.


Edit...ignore this post, I was only half remembering the facts, it only corrupts the reading if you did the tests at closed throttle.......which is not the correct proceedure as it should be WOT.

[andyd76]

I didn't realise you had to block off a vacuum pipe, surely this connects to the carbs/inlet manifold somewhere? how does this affect a compression test? Looks like I have some reading to do.....