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Crankcase Pressure Testing

Discussion in 'Chainsaw' started by Stihl #1, Nov 25, 2007.

  1. Stihl #1

    Stihl #1 ArboristSite Operative

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    The other thread about the MightyVac got me thinking about sharing a bench tester I built. Doing a pressure and vacuum test on a a two-stroke crankcase is a pain to do but it is so necessary to verify that the crankcase integrity is good. A small leak, or even a big one, is easy to mask by adjusting the carb, but it is still there and will require constant tuning to try to keep the engine running right, since the leak is affected by barometric pressure, humidity, and temp.
    I have had engines that were leaking so bad that it was hard to pump the hand tester fast enough to find the leak. Another tech. gave me the idea for this so I built one.
    [​IMG]

    The idea here is to have this on the bench so that there is no excuse not to do the pressure and vacuum test. The gauge is an automotive vac-pressure gauge from back in the carb days when we checked fuel pump pressure and manifold vacuum. The silver block in the middle comes from an air driven evacuator for air conditioning that I bought from Harbor Freight on sale for $7.
    To use this, block off the engine in the usual manner, and attach the hose from the tester, and for pressure testing, open the bottom valve while leaving the other two valves closed, then slowly turn in the regulator to bring the static pressure up to about 10 PSI. Then close the bottom valve and the pressure should hold. If not use soapy water to find the leak.
    For the vacuum test, leave the bottom valve closed and open the top two valves and slowly turn in the regulator to indicate about 8 to 10 negative PSI. Then close both top valves and the vacuum should hold.
    ALWAYS BACK OUT THE REGULATOR BEFORE USE SO YOU DON'T DUMP PRESSURE INTO THE ENGINE.
    The silver block is a venturi vacuum so when air is going through it it builds negative pressure, but it is noisy, so that is why I put a valve on the regulator side, so I can keep it turned off.
    Here are a coule of Stihl muffler spacers I use for blocking off the exhaust, which is much easier than using the muffler shell to hold the rubber flap in place:

    [​IMG]

    This is the 1123 block off plate, which I modified to use on both the 4180 and 4282 Stihl 4-MIX engines when testing them:

    [​IMG]

    I use dish detergent in a squirt bottle for finding leaks. I haven't tried it but someone said the bubble mix the kids use works good, so I am going to try that next time.
    I also use this tester to check fuel tanks on hand held equipment. They should always hold pressure, and always leak under vacuum.
    I use the vacuum feature to draw negative pressure on a 2 gallon plastic fuel can with 2 fittings I placed on the top of the can. Now I can evacuate the fuel from the tank for reuse or just to get it out for storage of the saw. I use a second can in the same way to draw out bar oil.

    QUESTONS FOR DISCUSSION:
    Which should you do, vacuum or pressure, first, and why?
     
  2. Lakeside53

    Lakeside53 Stihl Wrenching

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    Nice post!

    I did something similar for the same reason - a bad leak that's hard to find is a pain with bulb. Mine much cruder - I just put a 0-10psi regulator in series with my compressor regulator. Never found any real need for continous vac (yet).


    Continous pressure works great to dunk the entire engine in a 5 gal bucket of water. Best not to do with a points model unless the points are exposed and can be dried off with air/alcohol.

    Pressure or vac first? In my opinion, doesn't matter, but I usually do pressure as it finds all the easy stuff. Vac is pretty much only for seals as they are are only part that USUALLY fails vac after passing pressure.

    I always turn the crank while testing for vac - marginal seals often will only fail in one position.

    I don't use soapy water for vac leaks - I use 20-30wt oil. One drop on a seal will either stop the leak or get sucked inside. I don't like sucking water inside.
    I use the muffler mostly for the exhaust seal for speed. Just back out he screws a bit, slip in the rubber, tighten, test, then pop out the rubber and retighten.
     
    Last edited: Nov 26, 2007
  3. THALL10326

    THALL10326 The Champ

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    Well I'll be dayummm, now this is something. The two best Stihl techs in the country conversing with each other, way to go Stihl#1 and Lakeside,:cheers: :cheers: :cheers: :cheers: :cheers: :cheers:
     
  4. sugarbush

    sugarbush AboristSite Guru

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    interesting set up, sounds like it would work ok. I test about the same way as lakeside except use a rubber maid tub (easier to see the leak area). and oil on the vac leak. I remember a year or two ago whan i mentioned submerging a saw in water to find a leak wasn't to well thought of. a small leak can be missed with a squirt bottle. also use an air compressor for the bad one's. also use the muffler to seal with but remove it and have a bunch of differant rubber seals to fit about any bolt pattern. some saw's are hard to slide a block in behind the muffler.
     
  5. belgian

    belgian Addicted to ArboristSite

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    Thanks for this nice info.
    I recently got a Mityvac 4050 with adapters to perform the tests you just described. Can you tell me wich Stihl models suit those muffler spacers you mentioned ?


    .

    Another nice tip !


    This might come handy for all my collector saws !!!


    I intented to follow Lake's advice :) :)
     
  6. Stihl #1

    Stihl #1 ArboristSite Operative

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    Thanks for the comments, guys.
    The 1122 part is a muffler spacer from an old 066, but will fit a 036 up through an 066. Maybe even the 026.
    The little one, 4140, will work on the small Stihl trimmers and blowers, such as BG 55, FS 45, and others. Any 4140 series engine.
    Regarding the test procedure, all the older Stihl service manuals say to do pressure first and vacuum second, but then the latest manuals coming out of Germany have it reversed, so I did some asking around and the answer I got was that if you a had a seal that was marginal, and you did pressure first, that would push the lips of the seal tighter against the shaft, so that if you did vacuum second, it would pass. So now they say to do vacuum first, to make sure a marginal seal shows a leak, then pressure second, and if it holds then you know for sure you have a bad seal. So I guess that makes sense, but I have yet to see it proven one way or the other. I doubt it really matters.
     
  7. belgian

    belgian Addicted to ArboristSite

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    Well lookie here, nice to see you around, boss !! :cheers:

    Welp, I always thought you were the number two tech, after Lake and me of course :hmm3grin2orange: :hmm3grin2orange:
     
  8. belgian

    belgian Addicted to ArboristSite

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    Thanks again. You definately should post more !:clap:
     
  9. Roteiche

    Roteiche ArboristSite Operative

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    +1

    Roteiche:)
     
  10. RiverRat2

    RiverRat2 Addicted to ArboristSite

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    Yeah!!!!

    I'll say that again,,,,

    thats about right they are about as far apart geographically on the Lower 48 as they can be!!!!!! but right together on the methodology!!!!!

    Cool post!!!!!
     
  11. candlerslim

    candlerslim ArboristSite Lurker

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    Just two cents. My old man used to test for a bad leak in the field with tobacco smoke. He had a 3"-4" plastic tube that fit plug hole. He'd blow smoke down the hole and have someone else drag the chain and look to see if the smoke came out where it should not, like the crank seal.
     
  12. Supercharged86

    Supercharged86 ArboristSite Lurker

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    Hey Guys, Would a leaky crank seal affect all running conditions or just start, idle, WOT? Thanks. Steve
     
  13. splitpost

    splitpost *********

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    all
     
  14. Supercharged86

    Supercharged86 ArboristSite Lurker

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    Thanks Splitpost, Ok. So if the saw starts and idles well then I can rule out a problem with the crank seals? Thanks.
     
  15. splitpost

    splitpost *********

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  16. Supercharged86

    Supercharged86 ArboristSite Lurker

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    Your least favorite brand, :laugh:, Husky 141. I know a low end homeowner saw, just throw it in the trash. Anyway saw has 155 lbs compression, new plug, fuel filter and line, impluse tube, carb kit. thanks.
     
  17. splitpost

    splitpost *********

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    i dont dis-like huskys ,i just prefer my stihls:cheers:
     
  18. Dan_IN_MN

    Dan_IN_MN Addicted to ArboristSite

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    Mr. Mega poster here! :laugh: joined in 2002.....only 18 posts!
     
  19. Supercharged86

    Supercharged86 ArboristSite Lurker

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    Sorry I'm not "Mr. F#%@ing Expert" like you! But then again, if I went around and made ignorant comments like this for every 28th member, I'd have 2500 posts too.
     
    beelsr likes this.
  20. mifirewoodguy

    mifirewoodguy ArboristSite Operative

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    Why cant I see IMG pics? on any of my comps?
     

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