Stihl FS 94 fuel economy and string length testing

[Buzzsaw]

I purchased a new Stihl FS 94 R this spring and really I like the power and relatively light weight of this trimmer. I don't like running a run a guard on any trimmer and I took this one off when I left the dealer. I prefer Speed Feed trimmer heads and tried a 450 and 375 heads on this unit. I decided to stick with the 375 size over the 450 and stock Autocut heads.

I burned through two tanks of gas before I installed a mini tachometer/hour meter on this unit. I could see that this was a fuel efficient trimmer and I mixed up 1 gallon of fresh gas (90 octane non ethanol) and a bottle of Stihl Ultra (50:1) to see how many hours it could log on this amount of fuel. I run it anywhere between half to WOT during use. It started to run out of the last of the gallon at 10.2 hours. I feel this is pretty good fuel economy.

It should be noted that I'm running Oregon Platinum Supertwist .095" trimmer line an inch to an inch and a half longer than the stock setup will allow. The stock Autocut 25-2 head and the guard allows a max of 6" of line.

At idle the unit runs approx 3200 RPM. Here are the following approx RPMs of this trimmer at wide open throttle at different line lengths:
6" = 6585 RPM
6 1/2" = 6860
7 1/2" = 8760 RPM

To me the trimmer acts like it hits the rev limiter at WOT with anything shorter than 7" of line out and just doesn't sound right. I guess the rev limiter working is why its showing lower RPMs with shorter line length. To me it just flat out cuts stronger and better with 7 to 71/2" of line length. The gearbox and muffler shroud aren't too hot to the touch after an hour or more of trimming so I don't think I'm hurting this trimmer running longer than stock length line.

The only thing I don't like about this trimmer so far is that it has a rotary valve carburetor and there is no low or high speed screws to adjust on it. The manual says that the machine has been adjusted for optimum performance and fuel efficiency in all operating states at the factory. It runs great now but I'm sure it may need adjustment sometime in the future.

I welcome any comments you may have.
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[IndyIan]

Does the longer line do anything for you? Are you mowing an area with it? My dad ran the same experiment with an electric trimmer and it sure would cut better, but the motor burned out quite quickly...
If you aren't getting any 4 stroking at WO with the longer string length, I'd back it off until you do.

 

[Hokiefyd]

Your WOT max RPM readings are counterintuitive. Longer line lengths put more drag on the engine and slow it down. I'm curious what your max RPMs would be as you shortened the line lengths down toward zero. You should see your highest RPM at the shortest (0") line length.

 

[Buzzsaw]

Originally Posted By: IndyIan
Does the longer line do anything for you? Are you mowing an area with it? My dad ran the same experiment with an electric trimmer and it sure would cut better, but the motor burned out quite quickly...
If you aren't getting any 4 stroking at WO with the longer string length, I'd back it off until you do.


I have about 75 yards of ditch to cut that's too steep for a mower to cut so longer line length allows me to cut it faster. Interesting info on the electric trimmer.

 

[Buzzsaw]

Originally Posted By: Hokiefyd
Your WOT max RPM readings are counterintuitive. Longer line lengths put more drag on the engine and slow it down. I'm curious what your max RPMs would be as you shortened the line lengths down toward zero. You should see your highest RPM at the shortest (0") line length.


I know it sounds counter intuitive but that is what my results were. I'll check max RPM with no string and report back when I get a chance.

 

[IndyIan]

I guess the FS94 should stand up to some abuse, and perhaps its jetted kind of rich to begin with as its increasing rpms with load? I have an old FS85 brush cutter and the carb takes very little adjustment to go from blubbering low rpm at WO unloaded, to quite lean and too high rpms for my taste atleast. I should get a tach though as I don't really have any idea what rpm it runs.

I do see a pro guy use a lawnboy on a couple ropes to mow steep ditches like yours. One rope at the back and one on the front and they mow across the slope. I have a steep section on my yard and I find its faster to push my lawnboy up as far as I can from the bottom of the slope to minimize the area I do with the trimmer. Plus its a good upper chest workout.