Author Topic: Leak down test (Read 9000 times)

hub97119 · « **on:** January 20, 2014, 11:30:22 PM »

Can someone tell me if it is fine to pressurize the cyl. using the kick starter vs. an outside air source ( pump ) for leak detection. I have tried using a pump but can't seem to get it to pressurize. I have checked all my fittings, no bubbles anywhere so i thought what the hell, I will just pressurize it internally. Holds pressure fine, no loss. Pressure is pressure right ? I have intake and exhaust blocked off with a gauge from the head. I am familiar with the need and the method for a leak down test and as long as we are looking at just static pressure loss it just seems logical that it wouldn't matter how the pressure is introduced...were am I going wrong here? Thanks Ron

rsss396 · « **Reply #1 on:** January 21, 2014, 02:25:24 PM »

You are pressurizing the whole crankcase and cylinder topend, you have to plug the intake boot and exhaust port, this checks for leaks at the head gasket, base gasket,reed gaskets, and crankseals.
So using the kick starter is not going to do anything

hub97119 · « **Reply #2 on:** January 21, 2014, 08:58:46 PM »

As I said I am familiar with the method for leak down tests, so that is not the question. I'm assuming that when I pressurize the cyl. the crankcase will also be pressurized. Now does it matter how that pressure is intoduced. I can run the piston to BDC and put 8 lbs. in the cyl. with the kick starter or I can introduce pressure through to head via a pump ( with intake and exhaust blocked )....either way I have a pressurized cyl. I know it has always been done with a pump but I'm really trying to see if my rational is skewed here.

Tbone07 · « **Reply #3 on:** January 21, 2014, 09:07:23 PM »

I think I see what you're saying. Instead of plugging the intake and exhaust ports and pressurizing the engine, you want to use the compression from the engine itself (via kickstarting it) to see if you have any leaks?

Do I follow correctly?

hub97119 · « **Reply #4 on:** January 21, 2014, 09:16:05 PM »

correct

hub97119 · « **Reply #5 on:** January 21, 2014, 09:16:53 PM »

but with the intake and exhaust plugged

traxman · « **Reply #6 on:** January 21, 2014, 09:36:32 PM »

With the intake and the exhaust close you have no way to introduce additional air into the engine to create pressure. Kicking over the engine will do nothing but move the piston up and down. That's why 2 strokes have a reed system to create a one way valve (doesn't work perfectly) to introduce a fresh charge of fuel and air into the engine. With it blocked the motor doesn't pull or push anything but the air already trapped inside.

Tbone07 · « **Reply #7 on:** January 21, 2014, 10:07:30 PM »

Quote from: hub97119;22986

correct

I see what you're saying but that won't have the same effect as a true leak down test.

The motor gets compression when the sings seal past the last port, otherwise the air will just leak out one of the ports. At that point you aren't pressurizing the entire engine, just the combustion chamber. That leaves out the crank seals, base gasket, exhaust flange gasket, etc.

4WHLr · « **Reply #8 on:** January 21, 2014, 11:44:27 PM »

You may be confusing a 4 stroke Leak Down test with a 2 stroke Pressure test (which checks for air leaks). Like these guys have said what youre doing wont work. Duncan racing has some tech sheets on thier website that show the process in detail and explains it better and faster than I can type:onthego:

hub97119 · « **Reply #9 on:** January 21, 2014, 11:48:15 PM »

But I have the ports sealed, and pressure being what it is will take the path of least resistance. If I had a leak the pressure would find it regardless of how it was pressurized, even if it was just air trapped inside. It would equalize to the ambient ( outside pressure ) If I am pressurizing the the cyl. with an outside pump the air will seep past the rings to the crankcase, same as if I pressurize the cyl. with the kick starter. Still looking for someone to blow holes in this theory. Right now it is just a mental exercise as I was able to pressurize with an air pump and the kick starter method; with no leaks finally!

hub97119 · « **Reply #10 on:** January 21, 2014, 11:54:56 PM »

I think I should have omitted the " leak down test " as a description to get the thinking solely on "pressure"

hub97119 · « **Reply #11 on:** January 21, 2014, 11:58:55 PM »

This is what I am thinking - pressure inside a system will dissipate if allowed to. ( ie: air leak ) it shouldn't matter if that pressure is built up externally or internally

4WHLr · « **Reply #12 on:** January 22, 2014, 12:02:20 AM »

You want the whole crankcase pressurized. This test isnt just for checking ring seal. The crankcase is all part of the induction system of a 2 stroke. The only pressure you are making is in the combustion chamber. Which will equalize back to nothing when the piston goes back down. You arent introducing any more air volume to have anymore pressure in there. Like stated above youll just be moving the same air around. Check that tech sheet out.

4WHLr · « **Reply #13 on:** January 22, 2014, 12:10:36 AM »

I understand your thinking with the piston used as a pump but not in a sealed engine.

hub97119 · « **Reply #14 on:** January 22, 2014, 12:12:43 AM »

I looked at the tech sheet...good explanation, but I'm thinking you may have found the weak link in my theory more than anything. "You arent introducing any more air volume to have anymore pressure in there." So simple pressure in the cyl. without additional volume won't give a true leak test ? correct ?