Borrowed a mate's compression test thingy the other day to get reference set of compression readings. Anyway...aaaagh!!...the four readings I got were all in the 65 - 70 psi range, not the 180 odd that I was expecting. I didn't do the tests with the throttle wide open. Would this have made any difference? She starts and runs great. What does it all mean?

BTW, bit the bullet the other day to flush the brake fluid through the system, both front and rear, inc the ABS modulators. Used my home developed "sucking" machine and it worked great!! As TWB described in an earlier post "tighter than a pole dancer's tummy" since the flush!

Thanx, Rick G.

The throttle needs to be WFO so the maximum amount of air goes into the cylinders to get a proper reading.

If the readings are low, you inject a bit of oil into the spark plug holes. If the readings go up, it could be the rings are not sealing well. If the readings remain the same, it could be valves not sealing as well as they should.

"Used my home developed "sucking" machine"

Please explain!?!

I found vacuum pump, as used by airconditioning people, on the side of the road near my joint. Literally fell off the back of a truck. Bought some brass spigots at Harris's (local shop that sells everything for Gyro Gearloose type jobs). Fitted three to the lid of a glass jar. Connected one via 6mm plastic tube to the vacuum pump, another length of 6mm plastic tube to go the the bleed nipple and the third I fitted a tap or valve so control of the suction could happen. (Valve fully open=minimum suction, valve fully closed= max suction.)

Connect it up, start the vacuum pump, open the bleed nipple and out comes the fluid. An assistant keeps the reservoir from going empty. Worked great!

People living in the Maitland, NSW area would know Harris's at East Maitland. Greatest shop on Earth!

Will borrow the gauge again and re-do the tests and report back.

Thank, Rick G.

im going to have to find one of them brake fluid sucker things soon also ..ulrich

im shocked at the retail prices for something so simple ....but helpfull

about the compression test mate...wow that was low ......i would be expecting double that ....maybe 165 if the bores and rings and valves were good ,,,sounds like you should definatly retest .....although if your valve clearances were out ...that might give you that low reading

If you want a vacuum pump just go to an auto wrecker and get a cruise control vacuum pump of a EA ford Fairmont or later they all had a vacuum pump to run the cruise control. You find it behind the washer bottle on any of the Fords of the era that has Cruise control. It works a treat.

cheers k freak good advice ..heading for thr wreckers ....wooohoo

Rick G. I think a compression test is maybe good for something like a lawn-mower.

A real accurate tool is a leak-down test.
That test does not rely on the piston to try & pump up pressure in the cylinder.
Compression tests can be very inaccurate, even a less than fully charged battery will affect the results.
A leak down test basically pressurizes the combustion chamber @TDC through a regulated guage.
A reading from the guage will register the pressure loss %.
The actual sound of the pressurised escaping either from the intake valves, exhaust valves, or the piston rings makes trouble-shooting the problem easy.
I use this method on any engine I want to "inspect" .

I use this particular tool: http://buy1.snapon.com/products/diagnostics/eepv309a.asp

Best,
RL

While I agree a leakdown test is theoretically better, a competently executed compression test is more than accurate enough. To get wildly different results due to a low battery, the engine has to be cranking noticeably slowly. WOT versus closed throttle does make a big difference though (as already menitoned, tests should always be done WOT), as does a hot engine versus a cold engine. I perform both hot & cold compression tests, as they tell you different things. If I want to check individual valves in response to a low compression test result on one cylinder, I set up a (vacuum) leak down test rig on the individual port. Yes, this requires removal of the carbs / exhaust, but I don't do this second test lightly and find it a useful final check before pulling the head.

You really shouldn't bother with a compression test before ensuring the valve clearances are correct. To a lesser extent, the same is true of throttle body balance - it all depends on what kind of decisions you intend to use the results in support of.

I'm with Stout on this one!

The benefit of a compression test is it is a quick relatively low cost diagnostic test that tells you if you have compression and is it balanced across the cylinders.

The old adage that to run an engine you need ignition fuel and air needs to be modified to include compression, If many cylinders are low on compression starting can be a real problem.

So to get a quick snapshot of how the engine is I would always recommend a wide open throttle compression test. If that shows low compression on one or more cylinders then further investigation is required. Throttle body balance should not significantly affect compression if the test is done with wide open throttle.
Regards,
K-BIKE

Rene Z, I'm sure in a pinch, when no other diagnostic tool is available, ye-old compression tester will shed some light on an engines viability.
I was reffering to the level of accuracy in the diagnostic process when I advocated using the leak-down tester.
Have you ever used a leak-down tester?

RL

RL, yes, I have and I agree with your comments regarding the more complete info obtained when using it. However not very many have access to the equipment and you buy a compression test set for £20 of Fleabay. I see it more as a next step after you have confirmed with a compression gauge that there is a problem with the compression. For me than the next step would be a leak-down test, as it involves more work/time.

Ulrich_Grabau wrote:Borrowed a mate's compression test thingy the other day to get reference set of compression readings. Anyway...aaaagh!!...the four readings I got were all in the 65 - 70 psi range, not the 180 odd that I was expecting. I didn't do the tests with the throttle wide open. Would this have made any difference? She starts and runs great. What does it all mean?

BTW, bit the bullet the other day to flush the brake fluid through the system, both front and rear, inc the ABS modulators. Used my home developed "sucking" machine and it worked great!! As TWB described in an earlier post "tighter than a pole dancer's tummy" since the flush!

Thanx, Rick G.

Throttle wide open or not it is the same, after the members are talking about a vacuum pump ... I don't understand ...

Segafredo, it is a side issue, nothing to do with compression testing. I made a device, using a vacuum pump, to flush the brake fluid out of the braking system, and replace it with new fluid. It worked great and I got all excited that it did work and had to make mention of it.

Cheers, Rick G.

Compression test always with the throttle full open. You're testing the cylinder, not the stuff around it (and yes it does make a difference!). Make sure you take out the fuse of the fuel pump first. If the bike has stood for a long time a bit of oil in the cylinder might be in order to get a representative reading.

ReneZ wrote:Compression test always with the throttle full open. You're testing the cylinder, not the stuff around it (and yes it does make a difference!). Make sure you take out the fuse of the fuel pump first. If the bike has stood for a long time a bit of oil in the cylinder might be in order to get a representative reading.

I have found that putting oil in before a test is not such a good idea as it can hide bad rings. If you get an even reading on all then put oil in and do it again and if it comes up more than about 3 - 5% then a leak down is the only way to get any more info. If the pressure is low to start with and the oil does nothing then it is probably a bad valve or possibly a bad piston but if the pressure comes up significantly then probably rings. Only a leak down test will tell any more. If you cant do a leak down then it's time to remove the head or as I did just find another K100 engine which was heaps cheaper.