A A A A Author Topic: leak down test  (Read 303 times)


leak down test
« Started on: 08-Jan-20, 16:42 »
Hoping that someone has a good suggestion or maybe can confirm that it's all good and I don't have to worry...

Inlet & outlet valves on both cylinders all set to spec.
Bought a (cheap) compression tester + leak down tester on eBay:

1) Using the compression tester on a good running engine showed 125psi (throttle fully open) on both cylinders and on the engine I suspect has some issues showed the same. I guess my cheap tester is ok as a reference but not to know what the compression value actually is.

2) Leak down tester can only be set to 2 bar as that already gets a reading of 0 % leakage. Not connecting the leak tester to the cylinder shows a nice stable  0 % leakage, even after I switched off the compressor (see picture)
Setting the cylinder to TR (or TL) then connecting leak tester to cylinder, switching off noisy compressor (set to 2 bar as shown on the tester). The pressure + leakage gauge are going up then down to 0 within few seconds (on both cylinders). I guess that means a huge leak or does my compressor put a little extra pressure into the line and then releases the air? I only hear a faint noise of escaping air!

3) Putting compressor directly on the cylinder with 7 bar/100 psi (as all forums and you-tube videos suggest), then switching off the compressor, results in pressure loss to 0 bar in 7 seconds. Does this mean anything as the air is now leaking thru the cylinder as well as the compressor? I only hear a faint noise of escaping air!

Thanks in advance for the replies


Re: leak down test
« Reply #1 on: 08-Jan-20, 16:55 »
My leakdown tester has no gauges so I can't comment on your actual numbers. I just pressurise to 120 PSI.

Instead I use a piece of fuel line placed near {not in} my ear.

Put this into each intake and exhaust point near each valve. There should be no sound of leakage - which will be a hiss of varying volume depending on the size of the leak.

There should be no leakage at all from any of the valves.

Some degree of leakage past the rings is normal.

If the motor has been out of use for some time there is a possibility that the rings are gummed in their grooves and that compression will improve with some milage. My 500C had around 140 PSI when I got it. I found when troubleshooting a few years later it had 170.

The bike should run OK at 125 even though it is off spec.


Re: leak down test
« Reply #2 on: 08-Jan-20, 18:23 »
Forgot to say - was your compression test done with the throttle wide open? If not, your figure will actually be at least 15 - 20 PSI higher.

I also assume you had to do the test cold. If so, you can probably add around another 15 - 20 PSI.

That would take you to 150+ I would think.


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