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Stihl metal brush knife update

Stihl FS90-R brush blade update field notes summary review


I've been having second thoughts on the tri-bladed metal brush knife also known as the "Ninja Blade".

It cuts heavy grass and light scrub, but every now and then I would hit a small tree or the ground of a hillside that would stop it cold.

The FS90-R recovers nicely, but can only handle a limited number of cold stops like that before the flywheel shaft key sheers.
Stihl FS90-R in action with male model
About an hour of research on a weedeater forum revealed some bad news. Turns out you can't just replace the flywheel shaft key. The key is built into the flywheel, which would cost 65 dollars if you could order one, which you cannot unless you're an official Stihl dealer.

Ours is still under warranty, but it's been in the shop for a month, and when I called them last week I got the impression from the guy I talked to that their technician is either slow or buried or both.



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Is the shaft that the flywheel sits on straight or tapered? In the latter case there should be a nut pressing the flywheel on the taper. If that is the case, tap the flywheel with a wooden mallet to that it sits tight on the taper, and tighten the nut well. It might generate enough friction to function normally. But if you manage to dead-stop the engine, the flywheel might slide of the the sfaft so it might cause more wear over time.
Comment by Roland_Smith Thu Jul 12 16:57:06 2012

Roland-From what I read on the forum I think there is no nut to tighten, but it's hard to tell because Stihl does not make their parts catalogs available.

I briefly thought about taking it apart to see if I could figure out the problem, but the warranty should be good for another 11 months.

Comment by mark Thu Jul 12 18:51:15 2012
Roland, On these small engines the flywheel is the harmonic damper, the carrier of momentum to keep the engine running, and the ignition timing device. A shift of only 10 degrees would cause the engine to backfire terribly, this could be catastrophic for a two stroke engine. You would not be able to "tap" the flywheel on tight enough to prevent slipping.
Comment by Andrew Thu Jul 12 21:05:19 2012
Andrew- Thanks for that additional information. The term harmonic damper is new to me. This 2 stroke engine feels especially sensitive, which is why I only use non-ethanol fuel along with a high end synthetic mixing oil. I'll probably stop using the brush blade on hillsides due to this incident. Thinking of trying the saw tooth blade next for heavier stuff.
Comment by mark Fri Jul 13 16:41:02 2012

@Andrew: interesting. But why does a single-cilinder two-stroke need a harmonic damper?

Combining the key with the (die-cast?) flywheel sounds like a cost-cutting measure. Cheaper for the manufacturer, but not so much for the user after the warranty has expired.

My first exposure to two-stroke engines was an old vélosolex that I officially wasn't old enough to drive at that time. Now that was a simple and reliable engine. You could easily see how everything worked. And economical too, around 235 mpg.

Comment by Roland_Smith Fri Jul 13 17:42:48 2012

@ Mark, A harmonic damper is typically the crankshaft pulley on an automobile. Some people call it a harmonic balancer as well, but really it dampens the vibrations.

All small engines (really all engines) will run much better with non-ethanol fuel. The slightest amount of water dissolved into the ethanol in modern gas will cut the performance of a small engine drastically, and will make them hard to start as well. That's not even counting how corrosive ethanol is if it sits... (can you tell I'm a fan?)

@ Roland, All engines need a harmonic damper, especially single cylinders. This piece simply adds mass to the rotating assembly, it reduces the initial spike of the power stroke and helps carry the engine around through the compression stroke. This is crucial for keeping those blades spinning in that tough grass.

I believe these flywheels are die cast, if not then they are little more than pot metal. It's pretty light overall, only a pound or two (but that adds a lot to a small displacement engine). A replaceable key is definitely a much better design for the consumer overall, but I'm not aware of any weed eaters that have one.

Comment by Andrew Fri Jul 13 19:57:15 2012