# Stihl Fs90 mods



## Justyn Markland (Jun 9, 2015)

Hi I have a Fs90 4 mix. I have had this trimmer since 2007, and never had any problems from it. But I would like to know if there are any mods for it. Like porting or muffler mods? Any help would be appreciated.


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## catbuster (Jun 9, 2015)

No. Leave it as it is. You can't port a 4 stroke motor. And a muffler mod will do it no good


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## Justyn Markland (Jun 10, 2015)

Ok thanks for the info. I am not doubting you, I just wanted to know why you couldn't open up the intake and exhaust port on a 4 mix? I will more then likely not mess with anything on the engine, but I just wanted to know.


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## catbuster (Jun 10, 2015)

Because a 4 Mix engine doesn't operate with ports. It operates with a valve train. If you were to open the valve openings they would leak and the valves would shatter, which would ruin the entire top end.


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## Justyn Markland (Jun 10, 2015)

No i mean port matching the head for the carb and opening the exhaust port up. Not increasing the valve port size.


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## catbuster (Jun 10, 2015)

But even if you did that the valve will only allow so much flow, so no matter what the size of the port is you're still limited by the valves. Fluid dynamics limits you because you cannot compress a liquid and squeeze more through the valve opening. Increasing flow would be the only way to take your current piston, cylinder and valvetrain, making it impossible enhance the performance.

If you're worried about the performance of an FS 90 you should have ponied up the extra few bucks to buy a 130. These 4 strokers are not easily modifiable like a 2-stroke small engine.


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## Justyn Markland (Jun 11, 2015)

Ok thanks for the info Catbuster. I am happy with the FS90, it just would always be nice to have a little more power. I have less then an acre of land that I have to mow. And I love the handle bars on the frame.


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## Justyn Markland (Jul 9, 2015)

I couldn't help but mod the muffler. I took the muffle off to clean the exhaust port, and since there was not much to clean. I decided to mod the muffler. I drilled a 1/4" hole in it and used a 3" long tube to extend the muffler. I left the other part of the muffler stock. It gave the motor a deeper tone, that is not as ear piercing at full throttle. The only thing I noticed was a little better response from the throttle. I did notice that the top of the piston has a little carbon build up. What is the best way to remove the carbon?


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## catbuster (Jul 10, 2015)

Hydrochloric, also known as Muriatic, acid cleans pistons up rather well. Don't let it soak too long or you'll corrode the piston.


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## Justyn Markland (Jul 10, 2015)

Is there something that is not as corrosive?


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## catbuster (Jul 10, 2015)

Justyn Markland said:


> Is there something that is not as corrosive?



Yeah. Diluted HCl. It doesn't take all that long to do an HCl wash. It's the best way to do it.


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## Justyn Markland (Jul 10, 2015)

Do i do it with a hot motor? And how long do I leave it?


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## catbuster (Jul 10, 2015)

Justyn Markland said:


> Do i do it with a hot motor? And how long do I leave it?



No. Pull the cylinder from a cool motor and just watch it. If there isn't any pitting or holing of the coating of the cylinder, you can pretty much leave it as long as you need. Then rinse with hot water. I would guess a little bit of carbon buildup would come off with some HCl on a cotton ball scrubbed for 2-3 minutes. 

Wear rubber gloves and a respirator. This stuff has the lowest pH on earth of a monoprotic acid. And make sure you buy it marked "muriatic acid." It's less strong than the HCl we used in college chem class.


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## Justyn Markland (Jul 13, 2015)

I really don't want to pull the head. It is a 4 mix motor and I will have to replace the valve cover gasket and head gasket.


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## kirko (Jul 27, 2015)

Cant you just get a larger machine ? the fs 90 is a very under powered machine compared with the majority of the stihl range.
usually plenty of quality second hand machines about for a good price.


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## mels (Aug 5, 2015)

OP, pull the plug and shoot some berrymans B-12 in there. It's available at most any auto parts store, won't attack the aluminum like acid will (or you for that matter) and will do exactly what you're wanting to do.

Make sure that the can you grab has it's spray straw taped to its' side, they're sometimes missing.


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## TRTermite (Mar 3, 2019)

Old post but may help someone with 4-mix engines 
Somewhere on his site I saw Stihl "Ultra" cursed for chainsaw yet praised for 4-mix engines for reasons of carbon buildup and hence valve issues
Not voicing opinions just relaying something I remember reading.


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## ironman_gq (Mar 13, 2019)

TRTermite said:


> Old post but may help someone with 4-mix engines
> Somewhere on his site I saw Stihl "Ultra" cursed for chainsaw yet praised for 4-mix engines for reasons of carbon buildup and hence valve issues
> Not voicing opinions just relaying something I remember reading.



Ultra has been proven to work exceptionally in both 2 stroke and 4 mix engines. Burns clean and doesn't create a lot of carbon build up. Stihl wouldn't offer to double your warranty by using the stuff if it wasn't any good.


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