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Discussion Starter #1
My consumer grade McCulloch 1340b 40cc chain saw all of a sudden lost compression. I had cut thru several large logs (larger than its designed for I would assume) but it went thru them anyway, ran pretty hot. It has been a good saw up till now.

Afterwards hard starting and then backfire and then hardly any compression anymore and of course no more starting. I don't think its a mistake with oil mix, I'm careful about that . Usually too low oil causes seizing anyway, right?

Full disclosure, I have started using avgas (avoiding gasohol) but I would think if anything that would make it run cooler.

I haven't popped the head off yet. Any opinions on what I will find? Would broken reed valves do this? Maybe the compression ring may have broken?

tks
don
 

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The engine may have ingested some debris, or something could have come loose or broken in the engine. I would at least pull the muffler off and have a look to see if anything is evident from that perspective. Depending on replacement costs, it may not be a cost effective repair.

Best of Luck...:thumbsup:
 

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I think you are right, poorly filtered air got it. I had some problems with the air filter clogging and tried replacing it with something homemade that probably let stuff by. My neighbor tells me that he fixed one where something got sucked in and it broke the compression ring.

I'm going to take it apart just to see what happened. I have not checked to see if this MCC1840B has parts available.
 

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I think you are right, poorly filtered air got it. I had some problems with the air filter clogging and tried replacing it with something homemade that probably let stuff by. My neighbor tells me that he fixed one where something got sucked in and it broke the compression ring.

I'm going to take it apart just to see what happened. I have not checked to see if this MCC1840B has parts available.
Here is a website that has old Mac parts. Have a good one. Geo
http://smallenginechainsawparts.com/
 

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Discussion Starter #5
tks for that link, another project I have is to refurb an old MAC 610.

On this original problem, I have met the enemy and it was me. I got it apart and there is a very rough edge in the cyllinder right next to the exhaust port. I suspect my poor excuse of a filter let something in there that got trapped and caused that defect. Of course the rings got abused by this too. Fortunately rings are available and I can smooth out that cyllinder wall with emery cloth.

I did not realize how a small particle can screw up a cylinder so easily. Live and learn. I'm sure that it was a chunk of sawdust as it was flying everywhere.

don
 
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