Stihl 880 magnum vacuum testing

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mkswmp

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I've got a 880 that's driving me nuts!! It holds 150 compression for about 10 minutes but when I do a vacuum test it has nothing. I've blocked the carb and exhaust just like every other saw and apply vacuum at the impulse line. I've even switched it around and plugged the impulse line and applied vacuum thru the carb adapter. Nothing. And I can hear any air being sucked thru anywhere. I've applied grease at both crankcase seals and nothing happens. I've even hooked up an actual electric vacuum pump trying to hear the air sucking from somewhere with nothing showing.
Am I missing something on this saw that needs to be plugged up? How can it hold that good of comprehension but no vacuum at all? Please help!!!

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mkswmp said:
I've got a 880 that's driving me nuts!! It holds 150 compression for about 10 minutes but when I do a vacuum test it has nothing. I've blocked the carb and exhaust just like every other saw and apply vacuum at the impulse line. I've even switched it around and plugged the impulse line and applied vacuum thru the carb adapter. Nothing. And I can hear any air being sucked thru anywhere. I've applied grease at both crankcase seals and nothing happens. I've even hooked up an actual electric vacuum pump trying to hear the air sucking from somewhere with nothing showing.
Am I missing something on this saw that needs to be plugged up? How can it hold that good of comprehension but no vacuum at all? Please help!!!

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Soapy water in a spray bottle spay untill you find the leak.
 
Yeah I've been doing that. Nothing shows. And under a vacuum soapy water don't really show anything as it would get sucked in and not blow bubbles. But thank you

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Reread my post. It holds 150 psi

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mkswmp said:
Reread my post. It holds 150 psi

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Here is how you do it! Get a Mitivac 8500 and apply pressure first if it dont hold spray soapy water to test for leaks if it still dont hold and you cant find the leak dip the whole thing underwater while applying pressure. Never mind the compression in the cyl. Fool prof method I have done this hundreds of times.
 
It does hold pressure. The only leak on the whole thing under pressure is my actual gauge and it takes about 10 minutes for it to even show on the gauge.

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I think I see what you are saying...I need to apply pressure with the carb and exhaust blocked off. So instead of using vacuum just apply pressure at the same port...which is the impulse line that I'm using. Is this right?

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mkswmp said:
Read the post... compression

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Compression has nothing to do with pressure held within the saw. It can have 150 psi compression and leak like a sieve. Also, compression is measured and held via shrader valves and measured in a gauge, not within the cylinder or engine.

Compression test = Shows the condition of the piston rings.

Pressure test = the condition of seals, o rings and gaskets.
 
And what does the vacuum test do?

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mkswmp said:
And what does the vacuum test do?

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Vacuum also tests the gaskets, o-rings and the validity of the oil seals. Old seals will lift off their seats (crank) under vacuum. I wouldn’t suggest submerging your saw in water. A little brush and some bubbles that form above the soapy water is all you need. It was a great tip shared with me and I use it all the time now.

Block off all air intake passages and pressurise your saw to 7psi. Then gently brush the bubbles over both oil seals while adding pressure at the same time. I think you’ll find they are going to be the issue, or at least one of.

Don’t use bubbles or water under vacuum, you don’t want that in your saw.
 
That's where I was getting confused. Whenever you Google pressure test for a saw it almost immediately shows you compression testing of saws. I've read so many times where pressure and compression have been using intermediately as the same thing. I'm getting the picture now since lone wolf explained it a little better.

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Well you're helped as well don't get me wrong. But lone wolf is the one that initially got the wheels spinning. But thanks just the same!!

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Plug the intake and exhaust, spark plug in and tight..apply a vacuum to the impulse and then 7psi to the impulse.

This will tell you if your case, seals, gaskets are sealing.


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I would have heard that much air being sucked thru when I put the electric vacuum pump on it!! And besides I'd already plugged it

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