Introduction
You've cleaned/checked carb, got spark, and compression seems there...but it won't rev up or maybe even run...
Here's a commonly overlooked cause of small two cycle engine problems.
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If you have good compression, and spark, and know you must have carburetor functioning somewhat correctly, then here's the frustrating and rarely identified item that can keep you out of the game...
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Most mufflers on weed eaters and similar motors have a spark arrester screen that clogs with carbon and oil, preventing adequate flow, and making an otherwise healthy engine bog helplessly.
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My example is a contemporary Stihl weed eater, but this may apply to 80% of all brands.
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There's a small screw holding the screen in place over the exhaust outlet hole. Removing this and wiggling out the screen allows a more comprehensive test of your engine performance. It is your option to clean and remount the screen, or leave it out like I did.
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If the open screen didnt help your motors performance any, then there is one more rarely understood problem to check next..
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The engine case cover on back side of crankcase can leak air into crankcase, causing considerable leaning of fuel/air mixture...
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Many of these are simply plastic, others are metal. In any case, a gasket or seal is failing and you can find this by spraying starting fluid or some raw gasoline around edge of cover...if it starts/revs up more, or changes what ever it's doing a little, you need to reseal crankcase cap with quality gasket sealer or o-ring, depending on the model.
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To reassemble your device, follow these instructions in reverse order.
