Craftsman / Husqvarna 33cc trimmer won't start, just pops / backfire. Run w/o muffler

Brickman

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  • / Craftsman / Husqvarna 33cc trimmer won't start, just pops / backfire. Run w/o muffler
My trusty Craftsman 33cc trimmer #358.791170 since 2008. Rebuilt it once in 2016, new bearings, gaskets, carb rebuild, cleaning, etc.

I let a family member borrow it and when I got it back it ran rough and only ran WOT. Found out they used ethanol. It dissolved my fuel filter and put fine gray powder inside the carb. I purchased a new OEM Zama carb. As well as new lines, filter, and spark plug. Worked like new and lots of power for the past couple of yard trimmings then recently halfway through cutting grass it died. Still have gas (I do use 10% ethanol gas, but use sta-bil in my 2 stroke can) in there. Starts up then died twice and since then it will not start, just makes a backfire pop. While I pull the cord the engine does combust the fuel and sends smoke out the exhaust but only as I pull the cord.

Coil is strong (used tester), spark arrester screen is clean, have compression, spark plug gets fuel, has fine coating of gas. Even tried old spark plug, same result. I removed the whole muffler and it started right up, only idles and some higher RPM as its too much air for it to increase RPM and loud.
I tried starting it so often the pull rope is stuck now.

Any idea what can be causing the trimmer not to start / run? I kind of suspect the muffler may be bad? First I would hear of that.
I know the muffler has a roll in the combustion process of 2-stroke engines. I put my tire inflator on one end of the muffler and the same amount of air runs though.
 

bertsmobile1

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  • / Craftsman / Husqvarna 33cc trimmer won't start, just pops / backfire. Run w/o muffler
Running without the muffler & not running with the muffler is either a blocked muffler or bad compression.
So now you take it off and have a good look at the rings & the bore,
Getting accurate compression readings is not particuarly easy with a 40 cube engine let alone a teaspoon.
Then there is compression both above & under the piston and the latter is far more important than the former.
You would not believe how many times I get a hand held in for service tha has just been loaned to son / daugher/ neighbour to find it is totally stuffed.
 

Brickman

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  • / Craftsman / Husqvarna 33cc trimmer won't start, just pops / backfire. Run w/o muffler
Running without the muffler & not running with the muffler is either a blocked muffler or bad compression.
So now you take it off and have a good look at the rings & the bore,
Getting accurate compression readings is not particuarly easy with a 40 cube engine let alone a teaspoon.
Then there is compression both above & under the piston and the latter is far more important than the former.
You would not believe how many times I get a hand held in for service tha has just been loaned to son / daugher/ neighbour to find it is totally stuffed.

I used my thumb for testing compression. Didn't feel it sucking my thumb through the spark plug hole though.
Took the cylinder off to see the piston and inside the cylinder. Looks ok, has enough oil for lube and about a paper thickness of carbon build up on the piston.

The rewind spring for the rope broke, so I'll go ahead and put this one down to rest and get a new trimmer next season.
Wish i could fix it, still need to trim a little bit more. My drill doesn't spin fast enough to turn the engine for combustion. (small inverted bolt head shape on the crank and found a small bolt that fits hole)
 
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