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  1. #1
    PWCToday Guru E350's Avatar
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    How to Do 2-Stroke Leak Down Test

    So, I was riding the '88 X2 650cc this afternoon WOT and it was doing what I have been complaining about, mainly it loses rpms when I pull the throttle the last 1/8 or so to the throttle stop (so I back off on the throttle some and the rpm's increase).

    Then all of a sudden, wham the ski loses power and I almost go over bars, so I kill it. And feel in the intake grate for debris and there is none. Start back up and it starts fine but dies if I try to accelerate at all.

    I just did a compression test. Spot on: 160psi - 160psi (both cylinders)

    Spinning the engine with the starter reveals no bangy-bangy noises, rather it spins smooth as silk.

    So a while ago I bought this OTC 5609 Cylinder Leakage Tester Kit. So, I rotated piston #1 until it looked like it was top dead center, screwed in the hose adapter, attached the leak down gauges, hooked up my compressor and

    Dang the compressed air just pushed the piston down and I can hear air leaking probably out of the back cylinder because the spark plug is out of #2 cylinder too.

    The leak down tester instructions say remove all spark plugs. But I am wondering is that the way to do it on a 2-stroke?

    In other friggin' words, would you real gurus, clue me in on how to do a leak down test on a 2-cylinder 2-stroke engine like my wife's 650cc?
    Last edited by E350; 09-16-2017 at 11:26 PM.
    Learning Slowly...

  2. #2
    PWCToday Regular IamMikeM's Avatar
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    Re: How to Do 2-Stroke Leak Down Test

    Have a look at this. Good info here.
    http://www.klemmvintage.com/airleaks.htm
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  3. #3
    PWCToday Guru E350's Avatar
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    Re: How to Do 2-Stroke Leak Down Test

    IamMikeM: Perfect! Thank you very much buddy!

    The entire article is great but here is what I was missing. A pressure test is a little more involved than a compression test. Some disassembly is required:

    "The Pressure Test

    The step by step is :

    A) Seal off the intake manifold with a plug or rubber gasket

    B) Seal off the exhaust port with an automotive expanding rubber “freeze plug”.

    C) Attach an air fitting that lets you pressurize the motor (usually thru the inlet plug or the spark plug hole

    D) Pressurize the motor to 6-9 psi via a hose that has an in-line gauge.

    E) Watch your in-line gauge for 5-10 minutes to confirm you have “absolutely no loss” of air pressure. A loss of even 2-3 psi is not acceptable.

    F) If your gauge shows a leak, spray the gasket seams and seals on the pressurized motor with WD-40. The WD40 will make visible bubbles, and a leaking sound, when you spray the leak point."

    My guess is that you also have to leave the spark plug in the cylinder which you do not have the leak test adapter screwed into. It is also noteworthy that the piston in the cylinder you are testing does not need to be at top dead center on the compression stroke. (Because you are not testing the piston rings sealing ability with the valves closed on a 4-stroke engine. Rather, you are testing the entire crank case's ability to seal in pressure in a 2-stroke engine, which has no valves.

    My plugs are carbon/fuel/oil fouled (shiny black). I would think that an air leak would cause a hot whiteish/gray plug at least in the rear cylinder. So, I will investigate further.
    Last edited by E350; 09-17-2017 at 03:49 AM.
    Learning Slowly...

  4. #4
    PWCToday.com Is My Home Away From Home delagem's Avatar
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    Re: How to Do 2-Stroke Leak Down Test

    The OTC 5609 is a 4-stroke leakdown tester. I'm not going to say it won't work on your 2T motor, but it isn't the right piece of equipment for that job. Gauge doesn't have the resolution in the 0-10 psi range we're looking for.

    Additionally, I know nothing of the OTC brand, but I have tried off-brand leakdown testers, and they were terribly inaccurate.

    You need a proper 2T leakdown tester. Honestly, they're very easy to build. Hardest part is finding a gauge that's only got 0-20 or 0-30psi range. Yes, the other spark plug must be left in. Or better yet, leave both in, and use your fuel pump pulse line as the point where you add the 10psi air pressure.

    One step is missing from your instructions, too. After Step D, turn the air off. It's obvious if you're using a hand pump to raise the pressure, but might not be obvious to someone using a 4T tester that uses a compressor and a regulator.
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  5. #5
    PWCToday Guru E350's Avatar
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    Re: How to Do 2-Stroke Leak Down Test

    delagem: 10-4. Everything you said rings true.

    And, yes I used a compressor. And now that I understand the purpose and function I think I could build one for our 2-strokes.

    I have an 1-15 psi Accugauge for ATV tires which I could use. I would have to get fittings to make a simple manifold with a shutoff, or possibly I could unscrew the current gauge and screw on the Accugauge on instead.

    https://www.ghmeiser.com/dial-gauges.htm

    Thanks for your help.
    Last edited by E350; 09-17-2017 at 12:30 PM.
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  6. #6
    PWCToday.com Is My Home Away From Home Myself's Avatar
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    Re: How to Do 2-Stroke Leak Down Test

    I've had 2 different Keihin CDK carbs do this in the past. No amount of cleaning or adjustment would straighten them out. They were obviously running out of fuel but would only do it after a wot run. I used a different carb on each and solved the problem. I strongly believe they had developed a hairline crack somewhere in the carb body that could actually pull air in. Add a piece of clear tubing just before the carb and run it strapped to the trailer to just make sure you're not sucking air into the fuel supply through the switch or pickup assembly.
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  7. #7
    PWCToday Guru E350's Avatar
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    Re: How to Do 2-Stroke Leak Down Test

    Myself: If you get a chance, take a look at my thread here:

    http://www.pwctoday.com/showthread.p...28#post4448928
    Learning Slowly...

  8. #8
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: How to Do 2-Stroke Leak Down Test

    Quote Originally Posted by E350 View Post
    IamMikeM: Perfect! Thank you very much buddy!

    The entire article is great but here is what I was missing. A pressure test is a little more involved than a compression test. Some disassembly is required:

    "The Pressure Test

    The step by step is :

    A) Seal off the intake manifold with a plug or rubber gasket

    B) Seal off the exhaust port with an automotive expanding rubber “freeze plug”.

    C) Attach an air fitting that lets you pressurize the motor (usually thru the inlet plug or the spark plug hole

    D) Pressurize the motor to 6-9 psi via a hose that has an in-line gauge.

    E) Watch your in-line gauge for 5-10 minutes to confirm you have “absolutely no loss” of air pressure. A loss of even 2-3 psi is not acceptable.

    F) If your gauge shows a leak, spray the gasket seams and seals on the pressurized motor with WD-40. The WD40 will make visible bubbles, and a leaking sound, when you spray the leak point."

    My guess is that you also have to leave the spark plug in the cylinder which you do not have the leak test adapter screwed into. It is also noteworthy that the piston in the cylinder you are testing does not need to be at top dead center on the compression stroke. (Because you are not testing the piston rings sealing ability with the valves closed on a 4-stroke engine. Rather, you are testing the entire crank case's ability to seal in pressure in a 2-stroke engine, which has no valves.
    Pressure Test:
    If you didn't remove the inspection plug on the front (flywheel) cover, a pressure leak
    on the front of the engine has no where to go, and so the front cover will seal the
    leak (it will act like there is no leak, even if a leak exists).

    No pressure test is complete without performing a vacuum leak test. A pressure/vacuum
    test is the standard test for all 2-strokes because main seals will start to leak vacuum
    before they leak pressure.


    Bill M.

    Quote Originally Posted by E350 View Post
    My plugs are carbon/fuel/oil fouled (shiny black). I would think that an air leak would cause a hot whiteish/gray plug at least in the rear cylinder. So, I will investigate further.
    One of your plugs looked like it was wet fouled (fuel was actually filled the plug gap,
    and caused a cylinder to drop), or the gap is way too tight?

    It would certainly cause your problem. Look at my suggestions for a rich condition
    on your other post. BM.
    Last edited by wmazz; 09-17-2017 at 06:10 PM.
    Horsepower == Speed, RPM != Speed



  9. #9
    PWCToday Guru E350's Avatar
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    Re: How to Do 2-Stroke Leak Down Test

    Raise your hand if you thought that our 1988 650cc X2 had an engine air leak...

    DSC03717- lq.jpgDSC03738- lq.jpgDSC03731- lq.jpg

    First I blocked off the where the exhaust meets the pipe, and then where the carburetor meets the intake, then I disconnected the fuel pump "pulse" line to the carburetor and hooked it up to the Mitivac and then turned the Mitivac to "vacuum" and ...

    Nothing. I couldn't get the Mitivac to move off zero. Maybe I really needed to keep pumping the lever on the Mitivac to create vacuum but I just couldn't get ANY vacuum. (I know the Mitivac can create vacuum because I used it earlier in the day and created 15Hg to remove air bubbles from the power steering pump on the Bronco.)

    So, I then turned the Mitivac to "pressure" and ...

    Nothing. Again I couldn't get the Mitivac to move off zero. I couldn't create ANY pressure at all.

    So, I am going to modify the 4-cycle leak down tester I have to hook up to the "pulse" line and hook it up to my air compressor and see if I can pressurize the crankcase enough to find the leak(s) with a spray bottle with dish washing detergent.

    When I rode it last, last week, I noticed that the engine was hotter than usual, that it had a little trouble turning over to start, and saw a wisp of exhaust near the back of the engine compartment when I inspected to determine what was going on.

    Since nothing has been done to this ski since at least 2000, I assume that it needs at least forward and aft crank seals.

    Any questions, comments, suggestions will be appreciated.
    Last edited by E350; 10-07-2017 at 10:50 PM.
    Learning Slowly...

  10. #10
    PWCToday.com Is My Home Away From Home Myself's Avatar
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    Re: How to Do 2-Stroke Leak Down Test

    The way you have it blocked should be just fine. I use the air compressor with regulator turned down for 10psi. Im guessing you have a blown head gasket. Take off the cooling line from the head and smear soap bubbles over it then put some air into the pulse fitting. If the bubbles start blowing up then you have air leaking over to the cooling side. Normally from the head gasket.
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    '99 Yama GP1200 65U 61.8 GPS
    ported cylinders, matched cases, milled head, blueprinted pump, long ride plate

    '96 Seadoo HX 717 53.13 GPS
    light porting, massaged cases, lightened flywheel, port matched manifolds, Rossier pipe, stubby pump cone

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