Husqvarna 394 low compression

[andrewhanson]

So I recently picked up a used husqvarna 394 that is in excellent cosmetic condition for a good price. I ran a comression test on the saw and got 150psi. From what I know a husky 394 should have about 180-190 psi if compression. I checked out the piston from the muffler side and it looked practically new. I was thinking of replacing the rings but I don't think this saw has been used enough to wear out rings. Any ideas about what's wrong?

 

[pioneerguy600]

What type of compression tester are you using? The automotive type can give you a lower reading than the actual compression of a small chainsaw cylinder puts out.

 

[andrewhanson]

What type of compression tester are you using? The automotive type can give you a lower reading than the actual compression of a small chainsaw cylinder puts out.

I'm using one I got from Baileys.

 

[pioneerguy600]

I'm using one I got from Baileys.

So it has a short hose with a scrader valve in the end that screws into the cylinder head?

 

[andrewhanson]

So it has a short hose with a scrader valve in the end that screws into the cylinder head?

Yep

 

[pioneerguy600]

In that case then at 150 lbs it is not that bad, a new set of rings may bring the comp up 20-25 lbs. Lowering the jug by leaving the gasket out may add anothey 15 or so. I have found the 394`s to have excessive squish, the space between the piston top and the cylinder head. Check yours before final assembly.

 

[andrewhanson]

In that case then at 150 lbs it is not that bad, a new set of rings may bring the comp up 20-25 lbs. Lowering the jug by leaving the gasket out may add anothey 15 or so. I have found the 394`s to have excessive squish, the space between the piston top and the cylinder head. Check yours before final assembly.

Won't removing the gasket cause problems like a air leak? Or am I talking nonsense here? Also this is my new milling saw if that changes anything.

 

[MechanicMatt]

When you put the jug back on give it a coat of sealant like yamabond. That'll keep the air from leaking in. Or you know just give up and put it in a box and send it to me, 150 psi is REALLY REALLY low Ill dispose of that saw properly for ya:msp_razz:

 

[pioneerguy600]

Won't removing the gasket cause problems like a air leak? Or am I talking nonsense here? Also this is my new milling saw if that changes anything.

You can seal the cylinder to the base with with products suitable to be used with gasoline, I use Permatex Copper and have so for 10 years or more with no problems at all.

 

[Big_Wood]

do you have any idea what the history is of that saw? if the saw was used for milling before that would explain it looking pretty and having low compression. I've seen mill saws look like they just came off the dealers shelf but their crank and top end are just cooked

 

[parrisw]

There is no way a stock 394 is making 190psi.

 

[nmurph]

I think a more realistic compression reading would be 170-ish. I would pull it down and replace the piston/re-ring it. To me, if I have gone to the trouble of breaking the saw apart, the $20-25 more that a piston costs over a set of rings is money well spent. If the machining on the piston is still very obvious, then I might just re-ring. But if compression has dropped that much it is likely the piston will have lost enough diameter on the skirts that it will probably start to slap before a new set of rings wears out. Then you will be back in it for a piston. Spend the money for a new piston and you will be money ahead in the long run.

 

[mdavlee]

I had a good running 394 that had 175 lbs of compression at around 1100 ft of elevation. I don't think you'll see much higher than that without a thinner gasket or machine work.