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Chainsaw Repair
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Return to: Stickys of How to Basics commonly ask questions
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#Post#: 38037--------------------------------------------------
Pressure Testing
By: brettl Date: April 3, 2014, 11:27 pm
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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.
#Post#: 38039--------------------------------------------------
Re: Pressure Testing
By: 660magnum Date: April 4, 2014, 2:49 am
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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?)
[URL=
http://smg.photobucket.com/user/jamesirl/media/KGrHqZHJBwFHnPmRwlEBR7sH-3zj60_5…
/>width=700
height=557]
http://img.photobucket.com/albums/v11/jamesirl/KGrHqZHJBwFHnPmRwlEBR7sH-3zj60_5…
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?).
[URL=
http://smg.photobucket.com/user/jamesirl/media/T2eC16VHJH8E9qSEWJvfBRVLdwsZyw60…
/>width=700
height=700]
http://img.photobucket.com/albums/v11/jamesirl/T2eC16VHJH8E9qSEWJvfBRVLdwsZyw60…
/>
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?)
[URL=
http://smg.photobucket.com/user/jamesirl/media/T2eC16dHJHIFFhsdpC1FBRwiHTUUo60_…
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
[URL=
http://smg.photobucket.com/user/jamesirl/media/lforq2050.jpg.html][IMG]http://i…
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.
#Post#: 44138--------------------------------------------------
Re: Pressure Testing
By: Chainsawrepair Date: August 5, 2014, 1:35 pm
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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.
http://i59.photobucket.com/albums/g289/doemaster789/166/plates001_zpse56b5287.j…
#Post#: 44143--------------------------------------------------
Re: Pressure Testing
By: 660magnum Date: August 5, 2014, 2:38 pm
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Silverline Elite
http://www.ebay.com/itm/Mityvac-8500-Silverline-Elite-Hand-Vacuum-PumpTest-Kit-…
#Post#: 44145--------------------------------------------------
Re: Pressure Testing
By: 660magnum Date: August 5, 2014, 3:05 pm
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This is the only other one that does pressure and vacuum #8255
for the pump only
http://www.ebay.com/itm/Mityvac-MV8255-Selectline-Dual-Vacuum-Pressure-Pump-/39…
$43.59
http://www.amazon.com/dp/B002YKGZBW/?tag=googhydr-20&hvadid=43843742002&hvpos=1…
#Post#: 68707--------------------------------------------------
Re: Pressure Testing
By: gregh Date: January 28, 2016, 9:06 pm
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Harbor freight sells a mighty vac, about $40
Sent from my iPhone using Tapatalk
#Post#: 68711--------------------------------------------------
Re: Pressure Testing
By: 660magnum Date: January 28, 2016, 11:22 pm
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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.
#Post#: 70684--------------------------------------------------
Re: Pressure Testing
By: gregh Date: March 27, 2016, 12:51 pm
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Why pressure?
Mower Tech small engine repair
Joplin, mo
#Post#: 70713--------------------------------------------------
Re: Pressure Testing
By: jmester Date: March 28, 2016, 7:44 pm
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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.
#Post#: 70718--------------------------------------------------
Re: Pressure Testing
By: 3000 FPS Date: March 28, 2016, 9:51 pm
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[quote author=jmester link=topic=3375.msg70713#msg70713
date=1459212258]
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.
[/quote]
+1 on that.
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