Had my Stihl FS 38 in storage for two years. No gas in tank but some in primer bowl. Added gas and no start up. Thought carb was the issue. Replace carb with OEM Zamba. Tried to start and it began to back fire ( exhaust port and intake port) . Checked coil and tested good. Installed new Stihil Oem coil and the backfires continued and it did not run. Did not look into the shear pin as it ran perfectly before it was stored. Any ideas as were to probe next.
First thought is loosening the muffler and see if it will fire up. Bugs love to build nests inside these mufflers. For mufflers like this, I usually heat them red hot with a torch to burn out the carbon residue and any nests that build up over time. If air cannot get out, air cannot get in.
Next, pull the spark plug out, then remove the crank cover. Rotate the flywheel slowly and verify the magnet is just reaching the coil as the piston reaches the top. If the magnet is out of position, it will need a flywheel because the shear key is part of the casting.
I know you said it was running when stored, but I've seen strange things like this happen before.
The other possibility is just a bad plug.
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