There is a generator on the back side of the flywheel. The magnets are all the way around flywheel, with a stator windings mounted to the crankcase under the flywheel. That is probably what you are feeling. Sometimes concrete and metal bits can make it feel gritty, but a bumpy feeling as it is rotated is normal.
Inertia and airflow will expel most of the dust and debris once it is rev'd up.
At least that's how it's designed, anyway.
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A careful visual inspection of the bearing cages; that the ball bearings are not jumbled up, and there is no up & down play in the bearings at all.
This also goes for the lower rod bearing, checking for cracks in the rod bearing cage.
Testing of the injector.....
Dirt needs to be cleaned off the connections first.
EPA says all constantly wetted hoses should be replaced once disconnected....
With the muffler forward, looking from above. The far left hose is the outlet from the fuel pump/primer. It goes to the left-most nipple on the injector. If your primer is hard to pump, then it may be that the tiny screen just behind the inlet nipple of the injector. ( It pulls out with a pair of common pliers), which this is not in the Stihl service manual for some reason.....
Make sure your fuel filter is replaced first to deduce that as the source of the restriction. Personally, I would only use the OEM magnetic filter. Just because it looks clean does not necessarily mean it is unrestrictive. I've seen some filters that looked new, but you couldn't blow through.
Ok, on to the testing of the injector.
Gently pump to 3 or 4 psi on the inlet to the unpowered unjector until it bypasses.
It should not bleed down below 2.5psi. (note that when performing this test the fuel cap should
not be shut, as pressure will build up, causing a false reading).
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Another test of the injector with it removed from the engine, which you won't find in any Stihl manual is;
Spray a little wd40 on the injector nozzle outlet while it's under pressure, and holding around the 2.5psi range.
If there is any air bubbling from the injector, then the injector will need to be replaced. This indicates the injector's rubber pintle seal, that is not replaceable, is damaged.
If it is not bubbling, and not holding the 2.5psi, then there is a pressure regulator behind the exit nipple, which consists of a small spring, and needle, similar to ones found in a common chainsaw carburetor. The spring may be weak, or the needle faulty. This is a non-serviceable part, but if you are someone like me who has several dozen "dud injectors" lying around.......
I have had luck switching these components with faulty injectors that had other issues besides the needle and spring, which were still good.
Hope that helps.