Leakdown test on OB??

[xjma]

I have a leakdown tester that I have used for 2-stroke dirt bikes, ATVs, and snowmobiles. You have to block off the exhaust port with an old bicycle tube under the flange and then you put the adapter into the rubber boot for the carb, clamp it down and pressurize/vacuum test the crankcase. It has a gauge on it of course and you see how long it can hold 7 psi without any pressure drop.

I'm interested in performing this test on my Johnson 6hp 2stroke, it's a 2000. No rubber carb boot....but I guess I could just tap a fitting into an aluminum plate, drill two holes where the carb studs are and seal it up to the intake for that part....but how to block off the exhaust????? Would I basically need to find some rubber expanding plug to go into the exhaust port that heads down out of the exhaust chamber on the side of the motor?? Need to pull the power head?? Is this a pita or quick/easy job on this motor?? Do I need to replace any seals if/when I remove the powerhead?? I don't even know what the shape of the exhaust port is coming out of the chamber on this motor.....is it sealed up enough that I could just plug it where it comes out down above the prop?? How tight are the connections from the motor down to the exhaust exit port??

I searched and didn't find anything on here about a leakdown test for 2-strokes, only for I/O 4strokes....could the same/similar test be called something different in the world of outboard motors?? Thanx!

 

[boobie]

Re: Leakdown test on OB??

You normally use a compression gauge for testing 2 strokes. You can use a leak down tester on them, but it is done through the spark plug hole. I happen to have one but only use it on 4 strokes. I have never heard of your procedure.

 

[seahorse5]

Re: Leakdown test on OB??

xjma,

what you are describing is not a leakdown tester but simply a pressure check of a crankcase. You would have to take your powerhead off, fabricate plates to block off the intake and the powerhead base. Sure ain't worth the time or the trouble.

A true leakdown tester is used for aircraft engines and 4-stroke outboards and consists of one or two pressure gauges hooked up to an air compressor. You hold the flywheel in positon at TDC, turn on the air and read the difference in the gauges.

 

[xjma]

Re: Leakdown test on OB??

Well I guess I got my answer, doesn't seem like an easy test to perform on a 2stroke outboard!

Compression tester is good for testing the seal between the piston/rings and the cylinder walls, but that's only one aspect of the compression necessary for a 2stroke to run properly. Here is a store bought version of what I'm talking about http://www.motosport.com/dirtbike/MOTION-PRO-2STROKE-LEAK-DOWN-TESTER
You could do this procedure from the spark plug I guess but the leak down test I'm referring to does not care about piston/rings to cylinder sealing, it's to test if there are any leaks in the crankcase. You know the fuel/oil mix is pressurized in the crankcase and theoretically the oil drops out of suspension at this point (a good amount of it anyways, which is why there is normally a puddle of oil in your 2stroke crankcase when you open it up). If/when there is a leak in the crankcase, which often times can occur from a worn crank seal, air will leak into the crank. The crank pulls and pushes the mix, so it can leak air out, and pull air in. When it pulls air in, it leans out the mixture and well I think we all know what happens then.

 

[metriccrescentwrench]

Re: Leakdown test on OB??

but the leak down test I'm referring to does not care about piston/rings to cylinder sealing, it's to test if there are any leaks in the crankcase..

As seahorse mentioned, that's not a leakdown test but rather a crankcase pressure test. A leakdown tester uses compressed air at a regulated pressure, an orifice of a specified diameter, the tester is connected at the spark plug hole and you read the gauge to determine leakdown percentage. I've watched it being performed on aircraft engines, a good mechanic can move the piston up and down in the cylinder and look for sticking rings or bad spots in the cylinder. They're much more informative than a simple compression test.

 

[xjma]

Re: Leakdown test on OB??

so can it be done on an outboard without taking the whole thing apart?

 

[boobie]

Re: Leakdown test on OB??

No. You have to block off the exhaust some how.