Leak down test

Yellow_Cad · Jun 25, 2010

Did a search but came up dry. Can anyone please describe some methods of doing a leak down test on a twin (650 - 850) primarily where it comes to keeping everything at TDC while the test is preformed?

L.A.B. · Jun 25, 2010

Yellow_Cad said:
Can anyone please describe some methods of doing a leak down test on a twin (650 - 850) primarily where it comes to keeping everything at TDC while the test is preformed?

http://www.carcraft.com/techarticles/11 ... index.html

Often, the cylinder pressure applied to the piston may turn the engine over. The key is to have the piston as close to TDC as you can so the crank, rod, and piston are all as vertical as possible.

http://www.type2.com/library/engineg/leaktst.htm

http://en.wikipedia.org/wiki/Leak-down_tester

maylar · Jun 25, 2010

It's hard to keep a Commando motor from turning under the pressure of the test. Holding the engine with the rear brake requires dealing with chain slack. The "dead" space at TDC is rather narrow. In my experience, it's best done with a wrench holding the primary sprocket nut.

Both valves need to be closed.

Rich_j · Jun 25, 2010

I put a leakdown rig together a while ago. The piston seemed to sit quite happily at TDC without any help.

If you are building your own, bear in mind that it's a purely comparative tool.
0% is zero leakage and all testers will read the same at that point, the problem is that the other end of the scale (100%) is completely arbitrary and depends on the size of the orifice you use.
It's a handy tool to compare one cylinder with the other or if you regularly test your engine over its life to build a picture of the wear rate.

I've often thought it would be much better to dump the pressure gauge and orifice set up and just set a cylinder pressure via regulator and put a flow meter inline, then you get an absolute measure of leak rate at a fixed and repeatable pressure.

Yellow_Cad · Jun 25, 2010

Thanks for the posts. A couple of questions: Should both plugs be pulled? One source says the other plug helps hold the motor and to remove it would make it easier to hear if any air is going into the next cylinder. How about first gear and the brake? Maylar, if I use the primary sproket do I hold the crank end where the altenator is? Thanks again guys.

Yellow_Cad · Jun 25, 2010

One more question: Can the leak down test be done successfully with a cold motor? My bike is not running and yet I would like to do a LDT due to a low compression test reading.

geo46er · Jun 25, 2010

Hi YC,
I have a compression gauge with changable hoses (IE one for each size, 14mm, 12mm, and 10mm ). The gauge snaps on the end of any of the hoses with a typical air hose connector. I found that if I remove the valve core from the hose (just like a tire valve), screw it into the plug hole I can hook the air hose from my compressor to it. this will pressurize the cylinder and you can hear/feel any leakage past the valves or rings. I don't use full pressure, I turn on the compressor and let it build up to about 30lbs then turn it off. When I get the crank positioned where I want it I connect the airhose. Using only 30 or so lbs its easy to hold the crank from turning. While it will not provide you with a percentage of leak reading it will quickly reveal any leak problems.

GB

maylar · Jun 26, 2010

Hot or cold can give you different results. Doing both will give you more information. Yes, the alternator nut is the best thig to grab.

Both plugs out makes it easier to find TDC. Leaving one in won't help hold the pistons on a twin, as the "other" side won't be on a compression stroke.

Take the rocker covers off and rotate the crankshaft CCW until both rockers on the side you're testing have clearance. Hold it there with a big ratchet and do the test. Best done with a helper who can go around listening for air hissing from intake, exhaust, and crankcase breather.

Leak down test

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