Need new cylinder??

[upupandaway]

I have a "chopsaw" "cut-off saw" K770 where the compression is too low - 60psi. Spark OK, spray starter straight into dry cyl no go. Looking from carb side, the piston looks perfectly fine if not better than some pistons that work.

My first assumption is it needs a new cylinder\piston but as clean as it looks, can bad rings alone result in this low psi or indeed is it always bad cylinder?

 

[Scrubcadet10]

take off the exhaust and look through the port and take a peek.

 

[Hammermechanicman]

If it has been used to cut concrete they can get "dusted". Fine grit past the air filter. Piston can look fine but piston and cylinder are toast. Very common problem with concrete cutoff saws. Like scrub said, pull the muffler and look at piston for scoring. When you pull the cord how does the compression feel? Concrete saws have a rough life and they see a lot of cylinder kits.

 

[Scrubcadet10]

If it has been used to cut concrete they can get "dusted". Fine grit past the air filter. Piston can look fine but piston and cylinder are toast. Very common problem with concrete cutoff saws. Like scrub said, pull the muffler and look at piston for scoring. When you pull the cord how does the compression feel? Concrete saws have a rough life and they see a lot of cylinder kits.

We need concrete evidence of what's causing the low compression

 

[upupandaway]

Worn from dust - yeah i can see that happening. When i first started looking at it, i noticed the screws holding the air filter housing to the carb was loose and thus a little gap so makes sense it sucked in the dust. After all, there are dust deposits everywhere around the carb.

It feels like it has decent compression but then again -it feels like my weed trimmer which has a much smaller cylinder so this one should be much harder to pull. It actually has an air bleed to make it easier to pull.

It will likely take a few days before i get back and dig in more,, but I'm going to assume the cyl is shot and jjust order the cyl. I'll let u know what i find when i take it apart.

 

[Hammermechanicman]

Don't just change the cylinder. You will need a piston and cylinder kit. Depending on how much grit it sucked in the crank journal bearing might be bad too. Pull the cylinder and check the rod for vertical play, not lateral play. Any perceptible play means you need a crank kit too. Cement dust just eats up the saws. Many folks don't realize you need to clean the air filter every time you add fuel. Quite often the mentally challenged run the saw till the filter plugs to the point the saw won't run and then just remove the filter to finish the job. Of course the saw is finished too.

 

[Scrubcadet10]

Cylinderkit at partstree
piston
They get ya coming and going! $$$

 

[bertsmobile1]

Like everything else.
If it is used to generate income you have to pay a good price for it.

FWIW
I got given a K750 to fix over a year ago
After I had it for 3 months he sheepishly admitted I was the 3rd place he had taken it to
NEw carb, new ignition module , new fuel filter, new breather, new trigger, new throttle linkages and of course new gaskets everywhere .
Still can not get it to run reliably so new rings are sitting here waiting for the roundtuit .
Checked with the dealers and no one will take a concrete saw in for work unless they sold it originally and usually if it is out of warranty they tell them it is uneconomic to repair but offer a discount on a new one.

 

[Hammermechanicman]

Cylinderkit at partstree
piston
They get ya coming and going! $$$

there are lots of AM kits for the saw. I prefer Cross with Caber rings if i can find it but in reality even the cheap kits will work as long as they beveled the ports so it won't hang a ring. Concrete saws just don't live that long if used to dry cut. If water is used they will live a lot longer.

Cylinder Assembly (D) | Husqvarna K770 | 581 47 61-02

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[upupandaway]

I got around to take it apart. The rings are stuck big time. I tried chiseling the rings out with no luck.
There is no play in the connecting rod bearings.
I guess the owner used plain gas and not a dust problem.
Amazingly, the cylinder actually looks perfectly fine. The piston had much better days...

I did order the kit - cyl, piston, rings, etc.