Piston sometimes gets stuck

[ChrisGaddis]

I'm looking for some advice on what to do next.

Backstory: I have a Farmertec 660 that I built several years ago, ran great, used it for milling, then overheated it (I think?) while milling some Black Locust logs. Piston seemed stuck, couldn't pull the starter cord. I got busy with other things and didn't get around to working on until recently.

Over the weekend I installed a new piston and cylinder head on the saw. The old piston was significantly scored on the exhaust side, and one of the rings was seized up. After reassembly, the saw started right up, but stopped after less than a minute of running.

Upon inspection, I found that the flywheel wouldn't turn normally, but after I turned it in reverse, the piston seemed to free up, and would rotate normally again. I tried spinning it with the pull cord a few dozen times to see if it would get stuck, but it didn't. Started the saw again, but it quit after less than a minute, piston stuck again.

After freeing up the piston by rotating the flywheel in reverse, I tried spinning the flywheel with an electric drill. After a short time, I was able to get the piston to stick again. It's happening on the down stroke, when both rings are visible through the exhaust port.

What could be causing this? Seems like maybe a bad crank bearing? What should I do next?

 

[huskihl]

I'm looking for some advice on what to do next.

Backstory: I have a Farmertec 660 that I built several years ago, ran great, used it for milling, then overheated it (I think?) while milling some Black Locust logs. Piston seemed stuck, couldn't pull the starter cord. I got busy with other things and didn't get around to working on until recently.

Over the weekend I installed a new piston and cylinder head on the saw. The old piston was significantly scored on the exhaust side, and one of the rings was seized up. After reassembly, the saw started right up, but stopped after less than a minute of running.

Upon inspection, I found that the flywheel wouldn't turn normally, but after I turned it in reverse, the piston seemed to free up, and would rotate normally again. I tried spinning it with the pull cord a few dozen times to see if it would get stuck, but it didn't. Started the saw again, but it quit after less than a minute, piston stuck again.

After freeing up the piston by rotating the flywheel in reverse, I tried spinning the flywheel with an electric drill. After a short time, I was able to get the piston to stick again. It's happening on the down stroke, when both rings are visible through the exhaust port.

What could be causing this? Seems like maybe a bad crank bearing? What should I do next?

A loose chunk of metal down in the bottom of the case is hitting the crank lobes probably

 

[Stump Shot]

I'm looking for some advice on what to do next.What should I do next?

Take it back down and see just what is happening. I would think you would want to get that loose material out of there before causing more damage.

 

[ChrisGaddis]

Looks like the underlying issue is that a couple of the rollers inside the rod-crank bearing are busted, so that they usually roll, but sometimes get jammed up.

Time for a new crankshaft.

Is it common on these farmertec saws for that bearing to fail?

 

[Wonkydonkey]

Yes they have been known to go. Although you have to be lucky to get a good one. And some say you need to clean/flush them bearings..

but to a builder they are just more hassle than the savings you make

 

[Sagebrush33]

I'm glad I didn't go Farmertec crank for my 440/460 build.