scoring in 046....how bad is this

[cedarshark]

Pressure/vacumn test that saw before you ever fire it up again. If it flunks the test, pressure it up to 7-10 psi with the exhaust/intake ports sealed and dunk it in a full tub of water. The bubbles will tell you where its leaking. My money is on a crankshaft seal.

 

[PaulB84]

Pressure/vacumn test that saw before you ever fire it up again. If it flunks the test, pressure it up to 7-10 psi with the exhaust/intake ports sealed and dunk it in a full tub of water. The bubbles will tell you where its leaking. My money is on a crankshaft seal.

You're saying dunk the entire reassembled saw if the leakdown test is bad?

Doesn't the leakdown on a 2 stroke only tell you whether the rings are not holding pressure?

I'm totally unfamiliar with the bottom end of these things but crank seals seem like a real bear, particularly flywheel side

 

[cedarshark]

Paul, the intent is to systematically eliminate the possibility of an air leak that might have caused the lean condition in the saw. You previously mentioned bad gas but until you determine if/where you have an air leak, it is suspect but unconfirmed. Air leaking past the rings is irrevalent at this point. The objective is to seal the crankcase, pull a vacumn on it and track the time it takes for the vacumn to bleed off. if the saw will not hold -7 psi for approx 10-20 seconds, pressure it up to + 7-10 psi and dunk it. I use this method, as it clearly tells me where the leak is and eliminates the possibility of a false reading of leaks from the plugs or blankoff plates(sheet rubber) behind the carb and muffler(to seal the crankcase). Like cleaning the cylinder, pressure/vac testing can be done different ways. Some people use soapy water in a spray bottle and watch for bubbles around the seals.

If you do not pressure/vac test the saw, and you do have a leak, all your work will be for naught, you will be unable to properly tune the carb(because of excess air) and you will re-score the jug. A search on pressure/vac testing will reveal hundreds of threads which will give you the exact procedure and tools you need. I use a MityVac which will pressure or vac test at the flip of a switch.

Yes, seals can be contentious, especially small ones. They can be frustrating but most can be done with common tools. I got tired of fighting them and splurged on a Stihl seal puller with jaws to fit all Stihl saws(and most other brands) so now pulling seals is very easy. Again, search is your friend and will reveal dozens of homemade and commercial pullers that have been adapted for saws.

 

[PaulB84]

Paul, the intent is to systematically eliminate the possibility of an air leak that might have caused the lean condition in the saw. You previously mentioned bad gas but until you determine if/where you have an air leak, it is suspect but unconfirmed. Air leaking past the rings is irrevalent at this point. The objective is to seal the crankcase, pull a vacumn on it and track the time it takes for the vacumn to bleed off. if the saw will not hold -7 psi for approx 10-20 seconds, pressure it up to + 7-10 psi and dunk it. I use this method, as it clearly tells me where the leak is and eliminates the possibility of a false reading of leaks from the plugs or blankoff plates(sheet rubber) behind the carb and muffler(to seal the crankcase). Like cleaning the cylinder, pressure/vac testing can be done different ways. Some people use soapy water in a spray bottle and watch for bubbles around the seals.

If you do not pressure/vac test the saw, and you do have a leak, all your work will be for naught, you will be unable to properly tune the carb(because of excess air) and you will re-score the jug. A search on pressure/vac testing will reveal hundreds of threads which will give you the exact procedure and tools you need. I use a MityVac which will pressure or vac test at the flip of a switch.

Yes, seals can be contentious, especially small ones. They can be frustrating but most can be done with common tools. I got tired of fighting them and splurged on a Stihl seal puller with jaws to fit all Stihl saws(and most other brands) so now pulling seals is very easy. Again, search is your friend and will reveal dozens of homemade and commercial pullers that have been adapted for saws.

Understood. So it would seem pressure/vac testing could only be done prior to final reassembly? reinstalling the new piston and rings would give you a false positive if the rings we in fact sealing perfectly (not sure if this is possible or not, do the rings hold pressure over time or ?)

Sorry to baby step through this but this test would be done simply by replacing jug on new cylinder gasket without piston installed, block off ports and pressure test?

Also, why if everything was working as it should would it only hold vacuum for 10-20 seconds? I woulda thought it would hold vacuum/pressure a lot longer than that?

 

[huskihl]

Replacing the seals only requires removing the recoil and flywheel on the left, and the clutch and oil pump (or whatever else is there) on the right. They're replaced from the outside.

 

[PaulB84]

Never mind my question regarding piston and rings in or out, I'm an idiot. Piston below transfer ports would allow the test to work....duh.....

But since I have the upper end apart anyway would I wanna just slap the jug back on the lower end and do the test before installing new piston and rings, just so if it fails I have less teardown to do? Or does the piston and rings being in somehow affect the test? (Can't imagine it would....)

 

[PES+]

When you do a pressure and vacuum test you seal off the intake and exhaust ports so it doesn't metter where the piston sits.

If you do it before tear down you know if you need more parts and can order them.

Saves time and trips.

 

[huskihl]

Piston and rings aren't really in the equation. Install spark plug, seal off intake with inner tube rubber with bolt holes in it and install intake over it to seal the jug off (or a zip lock bag with a hose clamp where your boot goes if it's that style), and a piece of inner tube where your exhaust gasket goes and bolt the muffler on.

 

[PaulB84]

Piston and rings aren't really in the equation.

This is the last concept I'm having trouble with. Are they not in the equation BC the rings simply don't hold pressure/vacuum over time? So piston installed or not the test should work the same?

 

[PES+]

This is the last concept I'm having trouble with. Are they not in the equation BC the rings simply don't hold pressure/vacuum over time? So piston installed or not the test should work the same?

Correct

Do not confuse this test with a 4 stroke leakdown test as it is very different.

 

[huskihl]

This is the last concept I'm having trouble with. Are they not in the equation BC the rings simply don't hold pressure/vacuum over time? So piston installed or not the test should work the same?

Correct. Piston and rings don't have to be there. But the jug needs to be affixed permanently ie base gasket and/or case sealant to make sure it doesn't leak. So IF the bearings aren't bad (so you won't have to tear it down again), might as well install gasket, piston and rings, and cylinder one time and be done with it.

 

[PES+]

http://www.arboristsite.com/community/threads/leakdown-vs-pressure-vs-vacuum.82853/

 

[PaulB84]

Correct

Do not confuse this test with a 4 stroke leakdown test as it is very different.

Understood but since a 2 stroke leakdown test is a fairly useful diagnostic tool I was assuming a well built 2 stroke warmed up would have a very small leakdown % or time depending which test you're running. It seems strange to me that the vacuum test can be performed with rings and piston in the cylinder but I defer to the fantastic advice available on this site. Doesnt a well built 2 stroke leak like 1%? Seems like that would make it harder to pick up on the vacuum test, esp if you're specifically looking at the crank seals. Maybe these saws are nowhere near that tho.....

 

[PaulB84]

Correct. Piston and rings don't have to be there. But the jug needs to be affixed permanently ie base gasket and/or case sealant to make sure it doesn't leak. So IF the bearings aren't bad (so you won't have to tear it down again), might as well install gasket, piston and rings, and cylinder one time and be done with it.

Since tearing down only involves removing the 4 cylinder bolts, I'd rather do that twice than seat the piston in the cylinder w new rings....unless I'm missing yet another important concept. Thanks for holding my hand guys.

http://m.harborfreight.com/mityvac-vacuum-pump-39522.html?utm_referrer=direct/not provided

Gonna pick this up tomorrow unless there's a better option I should be looking at

 

[LegDeLimber]

You'll need to test it with the piston & rings installed.
Each time you pull the cylinder, you break the seal between it and the crankcase.
Thus You have a repete potential for a leak at the base of the cylinder even though you have checked the entire saw beforehand.

 

[PaulB84]

You'll need to test it with the piston & rings installed.
Each time you pull the cylinder, you break the seal between it and the crankcase.
Thus You have a repete potential for a leak at the base of the cylinder even though you have checked the entire saw beforehand.

That's a damn good point. Thanks!!

 

[PES+]

Usually in a case like yours the pressure/leakdown test is done as soon as the scoring is seen with the saw still assembled so that you can see why it scored and get a better idea of any additional parts that may be needed.

In your case you can still do the test if your base gasket is good even without the piston installed just prop up the rod and or pad the small end and install the cylinder and test as usual.

True leakdown testing is not that common in 2 strokes but can be done but is little different in use from a compression test with some exceptions.

Always a good idea to test after you install everything too as it can find any mistakes that may be made on reassembly.

 

[PaulB84]

Usually in a case like yours the pressure/leakdown test is done as soon as the scoring is seen with the saw still assembled so that you can see why it scored and get a better idea of any additional parts that may be needed.

In your case you can still do the test if your base gasket is good even without the piston installed just prop up the rod and or pad the small end and install the cylinder and test as usual.

True leakdown testing is not that common in 2 strokes but can be done but is little different in use from a compression test with some exceptions.

Always a good idea to test after you install everything too as it can find any mistakes that may be made on reassembly.

Since im gonna replace fuel and impulse lines, change out decomp valve w delete kit, new cylinder gasket and check everything w the vac/pressure test (the crank seals are the only thing I'm not changing out before testing?) , can I call that good assuming it passes the vacuum test?

 

[PES+]

Won't know until you test it may need seals or an intake boot if leaky

 

[PaulB84]

Won't know until you test it may need seals or an intake boot if leaky

Seals would suck, hoping that's not gonna be necessary but yup def a possibility. How would I test the boot since it won't be attached during vac test? (Or should it be?)