See full version: My Stihl Weed Trimmer Is Dying at Full Throttle
Stihl manufactures trimmers with two-cycle engines as well as ones with four-cycle engines, and both require a mixture of gasoline and oil. One reason why one of these trimmers may die at full throttle is that too much oil is in the gas, which causes poor combustion. Improper operation, including overuse of the choke, can also be responsible. Poor combustion produces carbon deposits that foul the spark plug and exhaust port, making the problem worse.
One of the consequences of operating a Stihl trimmer with the choke on or a fuel mixture that has too much oil is that the engine smokes, which fills the spark arrestor screen with carbon deposits that block air flow. You must clean this screen periodically to prevent the engine from stalling. It's just as important to regularly replace the intake air filter or clean it by spraying it with compressed air -- if it's the felt type -- or washing it with soapy water, if it's the foam type. Always close the choke before removing the air filter to keep dirt out of the carburetor. here
The trimmer can sputter and stall if it isn't getting enough fuel, and that can happen simply because the fuel filter is dirty. Fuel filters are inexpensive, and it's more practical to replace a dirty one than it is to try to clean it. On most models, it's in the gas tank or on the cap. While you're cleaning the filter, try running the trimmer with the cap loosened. If it doesn't stall, the cap vent hole may be plugged; and if so, you should replace the cap as well as the filter. [links]
The rubber diaphragms are getting a little stiff but I put in a rebuild kit a couple of season's ago and it's not yet anywhere near as bad off as it was when I did the rebuild. They're still reasonably pliable.
All that said, my first question is, would you agree that these further tests pretty much unequivocally put the blame on the carburetor? My thinking is that it could not have run at full strength, even briefly, unless both the engine and the coil were performing at least marginally well. Which only leaves the carb [links]
Nobody ever accused me of being the sharpest tool in the shed, and I couldn't be convinced the carburetor had become effectively "dead" so abruptly, or that my rebuild had gone bad after just two seasons when the original parts had lasted fifteen or thereabouts. So I turned to the disused weedeater, the one with the broken recoil mechanism (hereafter referred to as "Weedeater B") and commenced to cannibalizing. more
Using either of them for large jobs can overwork their engines and cause them to shut down.
Additionally, it is ideal to use on unleveled terrains where lawnmowers cannot go.
Over time the muffler is clogged by carbon build-up. This will prevent heat from escaping and stop the engine. [links]
The fuel filter cleans out fuel before it reaches the carburetor. here
Similarly, the air filter cleans the air before it reaches the carburetor. here
The carburetor, air filter, and fuel filter must be regularly cleaned. here
On many/most line trimmers the exhaust port is easily accessible once the muffler is off-you should be able to look directly into it to see the shiny side of the piston. (You may have to change the piston's position by slowly turning the cutting head or pullingthe starter cord slowly with the spark plug out if you can't see it.) For a typical sized trimmer engine, a slot about the width of a nickel and in the center taller than one is thick should be visible. If not, and especially if the edges of the slot look very much narrower than the center does, the odds are strong that the port is partially blocked with burned-on carbon from partly un-burned oil from the fuel mixture. [links]
Was it running well and then one day it suddenly began doing this, or did it gradually get worse? more
If this situation got gradually got worse, after the filter check, try pulling out the clean, re-gapped spark plug (you already did that, right?) and looking at the top of the piston for a carbon buildup there. There's usually some, but there could be a lot-and it might even look crusty. I'd then check the muffler and then finally the exhaust port for possible carbon build up there too. INSIDE many newer mufflers there's a fine metal mesh screen that can easily get carbon blocked (older models often didn't have any screen, then sometimes a coarse one, now a fine one). I've heard that some people remove this screen when it gets blocks, not that I'd recommend doing that, (because it functions as a spark arrestor), but cleaning it would be a good thing even if it's not completely carboned up. If there's any build up, it will limit the amount of air that can flow through the engine, which then limits the amount of fuel that can be pulled through, if the blockage is serious, the problem gets worse. For example if half the exhaust port is crud blocked, the engine may not run at anything above half speed, and may only stay running with difficulty, dies easily under load and runs "best" with the choke half closed.
First thing that I'd check now would be the fuel filter-if you can fish it out of the tank that is. You should be able to blow through it fairly easily via a fuel line. If not, replace it. There's enough fuel in a carb and in the fuel line for an engine to run for a while, even if the filter is partially blocked and can't allow enough fuel through to keep up with the engine by itself, so the engine will then run until the amount that's slowly getting through the filter isn't enough to keep it running, it could be as simple as that. [links]
A hard coating of carbon in the ports' opening can be very difficult to remove without damaging the aluminum of the port or the piston, unless you have a light touch and the right shaped tools-but it's worth a look and if the machine is down due to it, even removing a significant part of the accumulation will make a noticible difference in how the engine runs. Changing the shape and size of the exhaust port can have a large and often negative effect on the engine's performance, so it's best to try to avoid changing it at all by avoiding any very hard or too-aggressive grinding or filing tools, especially any high speed ones. I've used dental-handled type hand tools, (scrapers and mini chisels) and a Dremel tool with a fine wire end brush afterwards to smooth out some of my scratches afterwards. Some people would use an abrasive impregnated rubber bit or a hard felt "bob" and extremely fine grit polishing media to polish the port area. To be sure to get most of your cleaning debris out, have the spark plug out and the piston port fully open, first flush with spray carb cleaner through the spark plug hole and then blow out the piston and cylinder wiith compressed air before closing it up afterwards. more
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