Leak down test?

Iv'e never heard of a leak down test before, so I looked to google and kinda found out what it was. My question is on a 1995 yamaha yz 125 2 stroke liquid cooled dirt bike. When I do a leak down test and spray soapy water around the outside of my engine and the bubbles don't appear but the pressure goes down what would my problem be? thanks for reading hope you can help..

You have several points of leaking possible

 

rings

Head gasket

base gasket

case gasket

right crank seal

left crank seal

 

If you are just doing the combustion chamber, on a cold engine, you are learning nothing.

 

Look for 'YZ leak down' test on Youtube

You have several points of leaking possible

rings

Head gasket

base gasket

case gasket

right crank seal

left crank seal

If you are just doing the combustion chamber, on a cold engine, you are learning nothing.

Look for 'YZ leak down' test on Youtube

Add in your intake gasket too...

Power valve also, not that you need to fix it but it may be the source of the leak

Edited by Kinger317

You have several points of leaking possible

 

rings

Head gasket

base gasket

case gasket

right crank seal

left crank seal

 

If you are just doing the combustion chamber, on a cold engine, you are learning nothing.

 

Look for 'YZ leak down' test on Youtube

Rings have nothing to do with a leakdown test and neither does a cold engine. You are pressurising the entire intake, crank and cylinder

You will have to remove the ignition cover and probably the flywheel to see a left side crank seal leak. Remove the right side engine cover to see the right crank seal leak.

ok so I started the engine and took carb cleaner around all the seals head gasket, base gasket, case gasket, left and right crank seal, power valve gasket, carb boot gasket, and nothing changed the idle so Im guessing they are not leaking phew thats good right just needs a top end rebuild cause it dont run like it should and fouls plugs quite often, tried running 40:1 and still no change in performance wants to cut out and sputter on high throttle.. lwt me know what you think. thanks for replying to my post..

Rings have nothing to do with a leakdown test and neither does a cold engine. You are pressurising the entire intake, crank and cylinder

 

So, you test the combustion chamber with a cold motor?  That must be helpful.

 

How do you pressurize a crank again? I forgot.

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  • KRANNIE are you talking to me bud. uhm I checked around the outside of the engine while it was running with carb cleaner and the motor did not idle up or die out. so Im guessing no leaks, right! The compression test that I did when I first go the bike was around 90 psi. I know right not good. It was a cold engine when I took the compression test. Is that not the right way to do it.. let me know what you think. thanks

So, you test the combustion chamber with a cold motor? That must be helpful.

How do you pressurize a crank again? I forgot.

Leak down test on 2 stroke has nothing to do with how well the rings seat and how much compression the motor has. A leak down test on a 2 stroke is a method of pressurizing the entire combustion chamber, transfer ports, intake and exhaust ports, and crankshaft sealed area to determine if it is all air tight. Air leaks are major problem on 2 stroke...much more problematic than on 4t motors.

And to the OP...spraying carb cleaner or other liquids around the engine while it's running to see if there is a change in rpm is really not affective 90% of the time.

A proper home depot leak down testing tool can be made for about 25 bucks and is an invaluable tool after a complete rebuild. I completely rebuilt my motor with almost every new part known to yz250 motors. Still had leaks that drove me nuts.

My .02

E

I have always done a leak down test on the combustion chamber to see if the head gasket is working, and if the rings are leaking more than 10% per second, on a hot motor.

 

No, this has nothing to do with the intake tract sealing, but I still do it, as if it is leaking, it still needs attention.

Go to Ken O'Connor racing on U Tube and do a vacuum test instead. Much safer. Don't spray anything on the Stinking bike,its not a car. You can do damage to your motor injecting contact cleaner in to to it. Play it safe, watch O'Connor do it. It should only need to hold a 6 to 7 psig vacuum .Unsure of how long but my rebuild held 7 inches of vacuum over night,Good Luck  

http://www.kenoconnorracing.com/Vacuum%20Leaks.html

Edited by nicko-31

If you had 90 psi cold or not you need to rebuild the top end. Would say Don't bother doing much of anything else until you do that.

right on I really appreciate the help guys. You all gave me some valuable information. So what am I going to need to build this leak down test tool. Could I use a blood pressure gauge and the pump that comes with it and just build the boss that will go into my carb boot. pressure it up watch for a few minutes and go from there. Do you think that blood pressure cuff tool will work? thanks for helping me out. 

Go to the FAQ thread at the top of this forum. There are instructions to build a leak down tester and how to use it in there. Plus you can get other questions you might have answered there too. Lots of good info in there.

Edited by cman962

You could....assuming you can convert the fittings and keep them from leaking and remember, BP is measured in mmHg and youll need to convert to psi.

IMO buy a gas pressure test gauge for ~$8 and screw on some fittings. Dont forget to plug the exhaust port with a freeze plug and crank breather tube with a bolt.

Heres mine:

ImageUploadedByThumper Talk1439376019.173727.jpg

Hi,

Bringing this thread up for a question.. What are you guys sealing the exhaust port with? I see some people use a expansion plug but will it seal a YZ250 port? it's more oval than circular.. Thanks for any info..

I taped it over good and through a hose clamp on it! Crude but it worked for me once in a pinch! I will be curious also what others have to say

I had to do the same thing but my motor was on a bench and it was much easier that way.

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