Help with K760 saw

[jpcjguy]

Hi Everyone,

New here and have learned a lot! I am a home hobbyist and I picked up this Husqvarna K760 Cut-Off saw off marketplace for $100. Guy said the cord broke on the recoil and he fixed it once with something he had laying around and then that broke and he gave up on it. He said it has been sitting for a few years in his garage and he was cleaning out.
I took a chance and grabbed it, hoping it was just the carb.
I had some universal starter rope from Lowes laying around and I cut it to 45" and put it on. Would not start. Took out the plug and inspected it and cleaned it - looked ok. still no luck. Did not feel a ton of resistance on the cord - got me wondering about the compression. Turns out, after some research (now), concrete dust can eat these machines... great. Oh well. I end up taking the carb off and apart - a little dirty but I have seen worse. Cleaned it up as much as a could - did not mess with welch plugs. Put it all together and tried again. Must have pulled the cord 100 times. On 2 occasions it appeared to start (for as second or two) but then just died. Damn. Tried again (100 times - and probably flooded it - because that is how I roll ) Then I thought, let me check compression (waited to the end to try that because not sure I wanted the answer). Got out my OTC 5605 Deluxe Compression Tester Kit and made sure the decompression valve was pushed in. Pulled it 5 times, 90ish psi. Damn. Did this three more times and averaged between 85 and 95. Uh-oh. Now I did not add any oil into the cylinder. The engine is cold. but I am not optimistic.
I took my inspection camera (harbor freight) and took some pics through the spark plug hole. I will be honest - I am not good (or have the experience) to determine cylinder/piston wall or ring condition by looking at it - unless it is blatantly obvious. I did not see anything blatant but I could be way off here.
In the next post I will include some pics using my iphone through the muffler port that did not come out very well - which I did not realize until I put it back together.

So what do you all think?

I could do cylinder head kit, or just piston and rings or just rings......not sure. OR could it be carb related still? Appears to get fuel fine - pushed the primer bulb and fuel shot out of the hose when it was off the carb (sprayed me nicely! haha!) I did notice that when I pressed the primer bulb, it took a while to come back into shape - not sure what that is about - since the fuel delivery is good I think the tank filter is ok (still need to check), so maybe a check valve?

I don't really need this saw. I could move it along and more than likely get my money back. But I do like a challenge and it would add to my experience. I do have a railroad tie wall that will need to be converted to retaining wall block in the next 2-3 years so technically I could hang on to it and use it.
I can get Stens cylinder head kit for around $100. or cheaper by just doing piston and/or rings. But then a carb on top of it.
I don't want to sink a ton of money into this - thinking if I could get a good running saw for another $100, I would have got a good deal ($200 total). Could get my money back and then some. Or just throw it back up on marketplace and easily get my $100 back and walk away...

Pics of the saw

 

[jpcjguy]

Here a some pics through the spark plug hole

 

[jpcjguy]

video link - hope it works - let me know if I did not share it right in google photos
https://photos.app.goo.gl/z6LCmjakHUzaNNfj8

 

[Agrarian]

You said you pushed the decomp in. I've not worked on a K saw but all the decomps I'm aware of are closed when pulled out.

Barry

 

[jpcjguy]

You said you pushed the decomp in. I've not worked on a K saw but all the decomps I'm aware of are closed when pulled out.

Barry

For the K760 the valve should be pressed in - I looked it up to be sure - because I thought I might be wrong also:
From the owners manual:

Decompression valve: Press in the valve to reduce the pressure in the cylinder, this is to assist starting the power cutter. The decompression valve should always be used when starting. The valve automatically returns to its initial position when the machine starts.

 

[Agrarian]

But you don't press it in when doing compression tests. In addition, when doing compression tests, you should hold the throttle open.

 

[Agrarian]

If you get a low reading (less than 130psi or so) with the decomp closed (pulled out), you should remove it, plug its hole, and test again to ensure the decomp is not faulty. No sense getting into the guts of the thing if your only problem is the valve.

 

[jpcjguy]

But you don't press it in when doing compression tests. In addition, when doing compression tests, you should hold the throttle open.

Sorry - I am a moron - I thought you were talking about starting *head slap*
Yes I had the valve pulled out (got that wrong in my original post - oops!). I am pretty sure I had the throttle open - but I will do another test and ensure the throttle is open fully and pull the valve - I assume I can use any bolt that matches the thread and a washer and some of that thread sealant tape? or do I need to buy the "official" plug?

 

[Agrarian]

The official plug and the valve both have a crush washer to provide a seal. I'm not sure how well a standard bolt would do but it could work. If you're still getting less than 130 psi, then at a minimum you have a ring wear issue. And if you're going in for rings, you should probably plan on a piston and bearing. From the pictures, I couldn't see anything terrible with the cylinder but you may need to remove it to see the kind of wear that silicon dust can do.

 

[jpcjguy]

Ok. So just did another couple tests. Throttle is wide open, decompression valve all the way out. Getting consistent 90 psi.

 

[Agrarian]

Another thing to check is whether your compression tester is designed for measurements on small displacement cylinders. It will give very low readings if it does not have a valve in the piece that screws into the cylinder. It's typically a valve like you would find in a tire stem but the spring is different (lighter action). Are you sure of your tester?

 

[Duce]

Remove muffler and have a look. Those saws have a hard dusty life. Looks like transfer in couple of your pictures.

 

[jpcjguy]

Another thing to check is whether your compression tester is designed for measurements on small displacement cylinders. It will give very low readings if it does not have a valve in the piece that screws into the cylinder. It's typically a valve like you would find in a tire stem but the spring is different (lighter action). Are you sure of your tester?

Did not think of that. Not sure on my tester. This is the one I have: https://www.otctools.com/products/deluxe-compression-tester-kit

 

[Agrarian]

Did not think of that. Not sure on my tester. This is the one I have: https://www.otctools.com/products/deluxe-compression-tester-kit

You have the 5605 kit but the 5604 kit is designed for small engines. If you have to put an adapter on the end of the hose, your readings will be low and with some adapters, very low. It all depends on how much extra volume the adapter's air channel adds to the compressed volume of the compression chamber. You need the tester's Schroeder valve as close to the top of the combustion chamber as possible in order to eliminate that extra volume. So ideally, the Schroeder valve should be in the very end of the hose being screwed in and it should not extend into the chamber any more than a standard sparkplug.

 

[jpcjguy]

I think the tester is ok? The end that screws into the cylinder does have the schroeder valve - but the overall length is pretty long...
I am not opposed to picking up another tool - but only if needed.....

 

[Agrarian]

That's good! I believe the length of the hose doesn't matter so much, it just means that you'll need more revolutions of the motor to get to the final pressure (needle stops climbing). The only other variable on these types of gauges is the spring that is used on the Schroeder valve. It takes a certain amount of pressure to crack the valve open and that pressure is an error to the real pressure measurement. Small engine testers use special Schroeder valves having very light springs in order to minimize this error. That's why you can't use a standard car tire Schroeder valve when the one in your tester goes bad.

 

[Agrarian]

I've used these for my replacement valves: Schroeder Valve

 

[jpcjguy]

Thanks for the information! Never dawned on me the volume on small engines having an impact - makes total sense. Since my valve is at the end, can we can feel fairly certain that the reading is pretty accurate?
So looking at that reading (plus that it wont start) and the fact that this is power cutter saw that was used in the worst environment imaginable (for saws), and the built in water supply is missing a few parts (meaning it probably did some dry cuts in a monster dust cloud) - I think there is a high probability it is due for at least new rings, maybe piston, possibly jug?
I will break it down later today and get some good pics. Learning a lot here!

 

[Agrarian]

Likely your readings are accurate given what you've found about the tester. If you put a little oil in the chamber, retest, and the pressure goes up considerably, that's a good indication that the rings need attention.

 

[jpcjguy]

Likely your readings are accurate given what you've found about the tester. If you put a little oil in the chamber, retest, and the pressure goes up considerably, that's a good indication that the rings need attention.

how much in such a tiny cylinder?