Pressure/Vacuum testing follies.

[TJ the Chainsaw Mechanic]

Ok, SO I have pressure tested several saws that Hold pressure....confirmed by putting the whole saw under water and no bubbles... but they don't hold a vacuum. Any ideas?

 

[Mark71gtx]

Does it have a decomp? If it does, plug it and try again. I find most of my leaks wind up being a spark plug or decomp...

 

[TJ the Chainsaw Mechanic]

Does it have a decomp? If it does, plug it and try again. I find most of my leaks wind up being a spark plug or decomp...

I never thought of that! Thanks, yes this recently was an MS362 with a decomp valve. Held pressure just fine but leak vacuum almost as fast as I could pump it. Saw runs good though.

 

[Mark71gtx]

I always vac test first. If it fails, I then pressure test while spraying soapy water. When I find the bubbles, I find the leak. Then I repair that leak and start again. The weakest point will always leak first. Just because it didn't bubble before you repaired the first leak, doesn't mean it will not leak now.

 

[Evanrude]

Stihl's service manuals actually specify to pull the decomp and replace with a plug. I've had weak decomps throw me before.

If you plug it and still have a leak on vacuum, it can be a bad seal that holds pressure.

Sent from my SM-T350 using Tapatalk

 

[racerjohnbf]

I don't know if it's right or wrong, but I always take out the decomp and plug the hole with a bolt with an o-ring on it when I pressure/vacuum test. I think the decomps will only hold pressure and not vacuum, I might be wrong though.
Edit: Missed Evanrude's post above when I posted, I thought I had read to do it that way somewhere.

 

[TJ the Chainsaw Mechanic]

Thanks for the info guys! Know what to look for.

 

[panteliss]

The decomp valve is there because is must be there , and must keep pressure and vacuum . its rediculus to take of the decomp for testing the saw and then put again a decomp valve that maybe have problem and will give you leaks with results of scored piston or other problems

 

[Evanrude]

The decomp valve is there because is must be there , and must keep pressure and vacuum . its rediculus to take of the decomp for testing the saw and then put again a decomp valve that maybe have problem and will give you leaks with results of scored piston or other problems

Very true. Safest way would be test with decomp then plug it and retest. I was just stating what the service manual says, which we know isn't always best in the real world.

 

[Evanrude]

For those of you work on a lot of Stihls... If you do not have one, get the pres/vac testing flange for Stihl. I grabbed one from Dave's ebay store recently and it's made testing much easier. PN 5910 850 4200

Anyone konw what the difference is between PN 1128 850 4200 and 5910 850 4200? I'm guessing the latter is an updated number???

 

[racerjohnbf]

So what I should be doing is testing first with the decomp in and then using the plug if I have a problem in order to rule out the decomp. Would that be correct?
And Evanrude, I will deffinatly be picking up one of those test flanges. How much did it cost?

 

[Evanrude]

So what I should be doing is testing first with the decomp in and then using the plug if I have a problem in order to rule out the decomp. Would that be correct?
And Evanrude, I will deffinatly be picking up one of those test flanges. How much did it cost?

That's what I do. Start with the simplest fix first.

Look around for that part number, maybe even ask your dealer to order if you're tight with them.

 

[angelo c]

Ok, SO I have pressure tested several saws that Hold pressure....confirmed by putting the whole saw under water and no bubbles... but they don't hold a vacuum. Any ideas?

sometimes seals will seal on pressure and NOT on vac. sometimes you need to rotate crank a few times to check for "flat spots" in seals. think about it, pressure will expand things, negative pressure( vacuum) will decrease stuff...in theory at least I think....

 

[TJ the Chainsaw Mechanic]

Makes perfect sense. I'll be putting new crankseals in a 192 whenever my parts show up so I'll be sure to try some of this out.....even though it doesn't have a decomp valve.

 

[panteliss]

Very true. Safest way would be test with decomp then plug it and retest. I was just stating what the service manual says, which we know isn't always best in the real world.

You can test the decomp valve alone .

 

[panteliss]

For those of you work on a lot of Stihls... If you do not have one, get the pres/vac testing flange for Stihl. I grabbed one from Dave's ebay store recently and it's made testing much easier. PN 5910 850 4200

Anyone konw what the difference is between PN 1128 850 4200 and 5910 850 4200? I'm guessing the latter is an updated number???

the 5910 850 420 can fit to the olders models but also can fit and to some new models but not big diference

 

[panteliss]

So what I should be doing is testing first with the decomp in and then using the plug if I have a problem in order to rule out the decomp. Would that be correct?
And Evanrude, I will deffinatly be picking up one of those test flanges. How much did it cost?

all the flanges are around 15 to 25 $ depend on wich flange

 

[panteliss]

sometimes seals will seal on pressure and NOT on vac. sometimes you need to rotate crank a few times to check for "flat spots" in seals. think about it, pressure will expand things, negative pressure( vacuum) will decrease stuff...in theory at least I think....

 

[panteliss]

Yeah but how we can spot from where we have vacuum leaks ? for the pressure we have the soapy water what we have for the vacuum ?

 

[Definitive Dave]

Makes perfect sense. I'll be putting new crankseals in a 192 whenever my parts show up so I'll be sure to try some of this out.....even though it doesn't have a decomp valve.

the 192 has two different types of seals for replacements - one is used if you are splitting the crankcase and the other are the push in style for when the crankcase will not be split they will not work properly when used for the wrong application, check ahead to make sure you have the right seals ordered for your job
Dave