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  1. #1
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    How to do a leak down test

    This will show how to perform a leak down test on your Fuji (blue) engine.

    An air leak can cause your engine to run lean, usually resulting in a holed piston, or an engine the races at idle. Also known as the "death rev" when running on the trailer.

    It can be done while inside the ski or on your workbench, and it can be done for under $30.

    All you need is a 3 pack of racquette balls ($4 at Wal Mart) and a vacuum pump. ($20 at Harbor Freight). Or you can rent them from an auto parts store if you don't want to buy one.

    1. Cut 2 racquette balls in half for 44mm carbs.(PRO785) Or a little bit smaller than half for 38mm carbs. (650/750/780)

    2. Lightly grease the halves and push them into the intake manifold. The grease is to help seal air-tight.

    3a. Lightly grease the whole ball and CAREFULLY push it into the exhaust manifold. DO NOT push it all the way into the manifold, where you can't get it back out.

    3b. A PRO785 will need to buy another can of racquette balls, and install them into the exhaust pipe adapters.

    4. Attach a piece of 1/4" hose to the pulse fitting on the lower engine case.

    5. Connect the vacuum pump to the pulse fitting via the 1/4" hose, and pump until you get 9 inches of vacuum.

    6. Note what time it is, and in 10 mins. come back and see what the reading is at.

    You shouldn't lose more than 3-4 inches of vacuum in 10 mins.

    If it does drop more, then you may have a air leak. Try spraying soapy water around the rear crank seals, intake manifold gaskets, cylinder base gaskets, head gaskets, exhaust manifold gasket, and engine case seam. Looking for any type of abnormality suggesting a leak.
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  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Here's my completed test.

    I performed my test on the rebuilt engine from "her ride".

    It only lost 2 inches of vacuum over 20 mins. time.

    This is a WELL sealed engine.
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  3. #3
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by xlint89 View Post
    ... Try spraying soapy water around the rear crank seals, intake manifold gaskets, cylinder base gaskets, head gaskets, exhaust manifold gasket, and engine case seam. Looking for any type of abnormality suggesting a leak.
    Note that you must switch from vacuum to mild air pressure before spraying the soapy water. Otherwise, the bubbles from the soapy water will get sucked inside the engine, where you cannot see them.

    If you want to see the bubbles from an air leak, use air mild pressure (keep it under 10PSI)

    Make sure the racquette balls and other seals you are using won't get ejected by the air pressure.

  4. #4
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Right ^^^^^^^

    And you can substitute the racquette balls for any type of block off plates. Aluminum, plexiglas, etc.....
    Last edited by xlint89; 05-22-2011 at 11:51 AM.

  5. #5
    thester71's Avatar
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    You could also use rubber stoppers, I am ordering the appropriate sizes for both of my skis, including the pipes for the pro.
    Here is an ebay link:
    http://cgi.ebay.com/SIZE-13-5-BLACK-...item359cade354

    Just scroll down for all the different sizes available.

  6. #6
    mr. happyfunbear tenjuna's Avatar
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    3 questions on this:

    1. how long does it take to create the required vacumn using the pump pictured?

    2. how do you change from vacumn to pressure for testing? is it the same pump somehow or should I find another one?

    3. what the heck do I do if I get no vacumn whatsoever? if I rebuilt this engine wrong I am using it as a paperweight, I will put a fork in it lol.
    '

  7. #7
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    If there are no leaks, it should show a vacuum rather quickly. If you can't get a vacuum, switch to pressuer and find the leak. I mavde block off plates and sealed them with rtv. That way I knew that wouldn't be the source of any leaks. Pressure and soapy water will show you what you need to do very quickly. Sometimes it takes some patience but its well worth the effort to ensure a reliable long lasting engine

  8. #8
    mr. happyfunbear tenjuna's Avatar
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    err what do I use for pressure then?

  9. #9
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    Some vacuum pumps can be reversed. Otherwise a bicycle pump with a schrader valve inline with the 1/4 inch hose going to the impulse fitting

  10. #10
    jbizub's Avatar
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    I will be doing this soon. What should the normal rpm by out of water at idle?

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