There are several things that can cause this.
1. Then engine has blown and a connecting rod is broken. Then the engien turns over until the crank hits the broken parts.
2.You might have run the engine low on oil and it is locking up. Also it will start toing this if you have been using REGULAR 87 octane gas instead of Primium gas of at least 92 octane.
3.You might have been using gas that is over 30 days old. Ehen old gas is used, it will gum up the inside of the saw and it can cause the engine to get hard to pull. It will also cause the piston and cylinder to score.
One thing that you should do is take the muffler off and look inside the exhaust ports at the pison and cylinder. If there are ANY scoring marks on the pison, it is very likely that the engine is now bad and must be replaced.
If there is black or brown carbon on the pison, this is a sign that old gas has been used. and it probably also has ruined the engine.
Todays gasoline is pretty much "JUNK and Garbage" and it Must be used correctly or the engines will be ruined.
Make sure that you are using the BEST 2-cycle oil as well. It must have an "FD" rating on it. The Best oil to use is "ECHO" chainsaw oil. Most other oils are not certified "FD" and will not protect the saws engine very well.
Also , make sure that you get a GOOD compression Tester and check the compression on your engine. These engines will not start or at least they will not run very well unless thay have AT LEAST 110 LBS of compresson or more.
I hoe this helps.
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