Author Topic: Pressure Testing  (Read 1734 times)

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Offline brettl

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Pressure Testing
« on: April 04, 2014, 12:27:49 am »
I've worked on small engines for years but have never had to do pressure or vacuum tests. I now have quite a number of non-running saws that I've picked up and I would like to learn how to do these tests and the tools required. I would really appreciate any direct advice and/or referrals to good referrance material. Thank you.
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Online 660magnum

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Re: Pressure Testing
« Reply #1 on: April 04, 2014, 03:49:49 am »
There are many variations on how to go about pressure testing a chainsaw crankcase.

If you have a repair manual for your chainsaw, there is likely a crankcase pressure testing tutorial in the repair manual. If you have several different manuals, you may find several different ways to perform the pressure test?

The basic factors are:

The chainsaw must have the cylinder sealed to the crankcase.

The muffler and carburetor must be sealed off. This can be done with solid gaskets, inner tube, plastic or metal pieces, or combinations of these.

The pressure used is 1/2 atmosphere which is .5 KPa or 7 lbs pressure or 14" of vacuum.

Pressure is easiest to do as any leaks can be found with dish washing soap bubbles.

Crankshaft lip seals should also be checked with a vacuum due to their design. A chainsaw can check OK under pressure but absolutely fail the vacuum test due to the crankshaft seals.

The pass/fail parameter is that the crankcase should not loose more than 1/3 of the test pressure within 30 seconds.

There are a lot of ways to create the pressure or vacuum.

The most accepted way is to use the automotive tester for bleeding brakes and checking vacuum switches.

The Mity-Vac 8500 is the definitive tool (Ebay?)



But there are many cheaper ways to make 1/2 atmosphere pressure or vacuum. One of the main special things needed is a newer style automotive vacuum/pressure gauge which can be found rather cheaply like this oversize Chinese combination gauge (Ebay?).



You can use the $25 plastic Mity-Vac to make the vacuum.

BTW you can use the plastic Mity-Vac to make pressure too but you will have to use that separate gauge mentioned above. On the plastic Mity-Vac, the pressure port is on top in front of the built on gauge.

You can also make 7 lbs pressure with a squeeze bulb from a blood pressure tester too. (Ebay?)



Most people can easily suck 14" of vacuum but you would need a check valve and a vacuum gauge tee'd into the line.  Check valve . . .  http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXG869&P=ML



On many saws you can use the impulse line as the pressure/vacuum test point. But some saws have a internal impulse path for the carburetor fuel pump.

You can take an old spark plug and bust the ceramic out and epoxy a tube in the spark plug body for a test point but the piston must be down below the upper transfers to make the test.

I've also seen test points in a block off plate for the carb or muffler but remember the piston can not be covering the associated entrance to the crankcase..

Besides the crankshaft seals, other typical leak places are between the cylinder and carburetor, Cylinder base gasket, Decompression valve, spark plug, or your block off plates.

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Offline Cut4fun .

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Re: Pressure Testing
« Reply #2 on: August 05, 2014, 02:35:12 pm »
Just looking for the shiney mityvac you were all talking about. 

This one from winland sent was plastic I think and in $40's he said.



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We should share what we know... someone may learn...
That knowledge can live after us... and that "Pays It Forward".
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Online 660magnum

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Offline gregh

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Re: Pressure Testing
« Reply #5 on: January 28, 2016, 10:06:41 pm »
Harbor freight sells a mighty vac, about $40


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Re: Pressure Testing
« Reply #6 on: January 29, 2016, 12:22:37 am »
The Harbor Freight Mityvac for $40 is a vacuum only MITYVAC - Item#39522

http://www.harborfreight.com/mityvac-vacuum-pump-39522.html

With this version, there's no way to make pressure or to measure it.

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Offline gregh

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Re: Pressure Testing
« Reply #7 on: March 27, 2016, 01:51:49 pm »
Why pressure?


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Offline jmester

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Re: Pressure Testing
« Reply #8 on: March 28, 2016, 08:44:18 pm »
Two strokes make both pressure and vacuum in the bottom end / crankcase. The crankshaft seals have to hold air either way. Seen just as many seals pass a vac test and fail a pressure test. As I have seen the other way around.
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Re: Pressure Testing
« Reply #9 on: March 28, 2016, 10:51:30 pm »
Two strokes make both pressure and vacuum in the bottom end / crankcase. The crankshaft seals have to hold air either way. Seen just as many seals pass a vac test and fail a pressure test. As I have seen the other way around.

+1 on that.
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