Leak down test

5 replies to this topic
  • Slowmotion426

Posted March 15, 2005 - 01:06 AM


Can anybody tell me the proper procedure to perform a leak down test on my 2001 YZ426F and what gauge/tools are needed? Thanks for any help. :)

  • Frostbite

Posted March 15, 2005 - 01:22 PM


I don’t know the proper procedure but I did a head leakdown test on a 4 cylinder streetbike engine with the head off. I took a piece of clear plexiglass big enough to cover the squishband area of the head, drilled a hole in the center of it and installed a brake bleeder nipple. Then I used a large rubber O ring, (like off a spin on type auto oil filter) and greased it up to make a good seal between the plexiglass and the mating surface of the head. Then I connected a vacuum pump to the nipple and pumped out the air and let it sit overnight to see if it held the vacuum. It held.
Most automotive stores sell the vacuum pump as part of a brake line bleeding kit.

  • rasta

Posted March 15, 2005 - 04:31 PM


you will need special leak down tool for the job , it is expensive tool, best bet is to let dealer do it for you, also for the top end you can just pull of the head and fill up intake and exhaust port with carb cleaner, hold it upside down and see if it leaks through the valves, that is all i know

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  • sparks426

Posted March 15, 2005 - 06:04 PM


I got my leak down tester from Summit racing. Only problem was I had to make an adaptor out of an old spark plug, since I could not find a 10mm adaptor anywhere. Not too hard to do, and the thing was $65+/- from Summit. Much easier than pulling apart your engine and dumping solvent in the ports. Only problem is it can be difficult to get it right on top dead center, so to keep it from pushing down I usually use a breaker bar on the flywheel nut and hold it that way.

Whenever I do it, I pull the spark plug. Pull the ignition cover plastic plugs out and turn the engine over to TDC. I like to use a breaker bar to do this, and position it right on top of the footpeg once I've found top dead center, so I can stand on the handle once air is applied. From there, thread the leakdown tester into the spark plug hole, and start the air flowing. The tester should have a regulator, use that to bring the pressure up to 100 psi on the first gauge, and watch what the second gauge says. You should be in the mid to high 90's on the second one. If not, you may have to pull the carb/exhaust so you can listen and see if the valves are leaking, or if it is coming out through the crankcase indicating a cylinder problem.

  • MNellis

Posted March 15, 2005 - 08:37 PM


All this talk of pulling the head, greasing things up or pouring solvents in the head is NUTS.

The idea of a leakdown test is to determine how efficient your rings and valves are sealing. A compression test is a good start but a leak down test is the best way to go.

First get yourself a leak down tester.


As the previous poster mentioned, be sure you get one with the proper size fitting for your spark plug hole.

Once you pull the plug, attach the hose and fitting to the spark plug hole but do not attach it to the tester at this point. You need to get the piston near top dead center. You can do this by using a socket on the end of the flywheel and hold your thumb over the end of the hose. You'll feel the pressure build on your thumb as the piston reaches top dead center (do not go past TDC). Next, put the bike in gear and hold the rear brake to keep the piston from moving. Set the first gage to some pressure (100 psi is fine but we use 80 psi on our airplane engine). Remember to hold the brake on. You can actually grab the rear wheel and move the piston up or down to get even closer to top dead center. You'll see the gage move.

With the pressure set, open the valve on the tester to supply pressure to the cylinder. Compare the supply pressure to the pressure the cylinder will hold as shown on the second gage. Subtract the cylinder pressure from the supply pressure and divide by the supply pressure to get a percent leakdown.

(supply pressure - cyl pressure)/supply pressure = % leakdown
example - (100-93)/100 = 7% leakdown

If you are holding at least 5% leakdown then I'd say things are in pretty good shape. 10% is not bad but 15%-20% indicates it's time to start looking for problems.

Again, as the other poster mentioned, you can listen through the carb or the exhaust to see where the leakage is coming from if it's excessive.

Hope this helps.

  • Slowmotion426

Posted March 15, 2005 - 09:46 PM


Hey thanks a lot guys for all your help! I'm on it now! :)

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