Chain saw woes update

BigDanT · Feb 23, 2018

Earlier I posted about an issue I was having with a Husqvarna 359 that wanted to rev up while at idle then die. I was advised that my problem may be leaking crank shaft seals or a bad carb so I decided to do a pressure test on the case and the carb. I tested the crank case with my new Mityvac 8500 and much to my surprise it held seven pounds for over 15 minutes! I also puled 7 inHG vacuum and was able to hold that as well. I then tested the carb with the same 7 lbs of pressure and it held as well. This is my first try at testing in this manner so I'm hoping I'm doing it right. I sealed off the muffler and intake, then attached the pressure line to the sparkie plug hole with the piston in BTC. I had pressure to both the intake and the muffler ports. So now I'm at a loss, what am I missing? Any Thoughts?
Thanks Dan

Adirondack case guy · Feb 23, 2018

Well, that tool appears to be pretty hi-tech, but how many cu.in of volume does it pump in each mode. May be well over what your saw engine generates and giving you false readings, if the saw uses less volume of air vacuum and pressure your test is invalid.
Loren

Bob Bancroft · Feb 23, 2018

How does your carb connect to the crankcase, or cylinder? Directly? Or is there a flexible sleeve in there?

Also, possibly by pressurizing the crankcase first, you "seated" the seals somewhat? It's a tiny SUCTION leak that will get you. And remember the crankshaft is spinning around in the seals when your trouble occurs.

Does your idle adjustment do anything?

steve1000 · Feb 23, 2018

A crack in the fuel line or filter above the tank fuel level might draw in extra air.

jim_uny · Feb 23, 2018

i follow donyboy73 on youtube. he does small engines & chainsaws in canada. his email is [email protected]. i would contact him.
donyboy73 working on a husqvarna

guido · Feb 23, 2018

Hello BigDanT,

Did you lock and position the piston in the right spot? A sure way to have the piston in the right spot, is to pressurized intake and exhaust ports individually. You would leave the spark plug in,

Guido.

T.R.K. · Feb 23, 2018

Well, its hard to tell from your picture how you blocked the intake, pulse hose and exhaust off, but your numbers indicate it passed that test. We use a Mityvac about like yours to test ours in the dealer shop.

Since you say you pressurized the carburetor, and I presume you mean through the fuel line, connecting to the fuel filter end of the fuel line, I think you can eliminate a hole in the fuel line as well. You can also put the cap on the tank, block the return if that saw has one, and pressurize the fuel line at the carburetor to test the vent. I don't have the spec. handy but it should relieve pressure around 5 psi. and not allow much vacuum if any to form in the tank at all.

You may have to take another look inside the carburetor.

Another thought is the sparkplug. You are running a resistor sparkplug in that saw, aren't you? Some Husqvarna ignition modules require a resistor plug to produce the spark at the correct time. I don't think I've ever witnessed this problem on one of their saws but I have 'fixed' several trimmers with strange misfiring by putting in a resistor sparkplug.

BigDanT · Feb 23, 2018

Thanks for the replies.
Guido...yes pretty sure piston stayed at bottom of stroke. I was able to release pressure from both muffler port and the intake side so I'm sure all of the crankcase in getting pressurized.
Jim...I've seen quite a few of donyboy73 videos while researching this issue, I might give him a try if I can't figure this out.
Steve...all new fuel lines, filter, impulse lines and vent line.
Bob Bancroft...carb connects via a soft bellows then a rigid intake that clamps to short tube coming off cylinder. New intake and all that is holding pressure too.
Loren...not sure about volume, this is a simple leak test, not much different than filling a tire and watching the gauge to see if it's leaking. All that I've read and seen indicates 7 lbs is the correct pressure to put into this crankcase.

Bob Bancroft · Feb 23, 2018

So there is a "soft" connection between the carb and the cylinder? Is this the part that's new? I will reiterate that a pressure test may well yield different results than a vacuum test.

BigDanT · Feb 23, 2018

T.R.K....I tested carb attaching test line to fuel inlet and submerging carb in water while looking for bubbles. I also tested fuel lines but they are all new so I really didn't think they were a problem anyway.
I haven't tried testing the fuel cap and tank but will try this weekend. Thing is like i said in my first post the saw runs great at full throttle and only acts up when I let off on the trigger. I am running the recommended plug in as well.
I'm with you... I'm leaning towards a pesky carb issue, just don't know how to diagnose it. Thanks
Dan

BigDanT · Feb 23, 2018

Bob Bancroft...No, new part is the rigid intake and clamp, soft bellows is original. Not sure what you mean when you reiterated pressure may yield a different result than a vacuum test, I ran both a vacuum and a pressure test to all components and the results were the same... no leaks indicated.

Bret4207 · Feb 24, 2018

Unless I'm missing something, you proved your rings are well seated. You need to put the vacuum to the CRANKCASE. You run the piston to TDC and leave the plug in. Seal the exhaust and apply vac to the intake either through the intake port, which requires a nipple on intake port block, or through the impulse nipple if your saw has one. Thats the way I do it anyway. And as someone else noted, if it seems to be holding vac, turn the crank a bit and see if it drops. I like to grab the flywheel or clutch and move it up and down and side to side to and see if that makes a difference.

BigDanT · Feb 24, 2018

Bret... are saying that I can't pressurize or pull a vacuum in the crankcase through the spark plug hole? I believe I can. Easy to
tell, pull a vacuum to 7 inHG then released the seal on the intake or muffler and watch the gauge return to zero. Remember fuel air
mixture has to travel through the case to get to the firing chamber this is done at the bottom of the stroke. I realize it can be
done as you suggested but I just found it easier to slip a piece of rubber between the carb and muffler and use the plug hole.

Bret4207 · Feb 25, 2018

I dunno Dan, I was taught at the Jonsered school to do it one way and one way only. I've never seen crank seals tested the way you're doing it, but if you have confidence in the method, more power to ya. It just seems to me that you're adding another link to the chain ( the plug fitting you're using) that might be leaking and giving a false bad reading or having a very tight fitting piston and rings that also give a false good reading.

Either way, if you every get it figured out , let us know what it was.

Chain saw woes update

BigDanT

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Adirondack case guy

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Bob Bancroft

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jim_uny

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guido

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