Common Reasons Why a String Trimmer Won't Start

This can be caused by the engine misfiring at the wrong point in the sequence. So instead of the plug sparking and the mixture exploding as the mixture is compressed and the piston is about to change direction, ignition occurs prematurely, and the piston gets pushed back "the other way. Misfiring can be caused by a loose or incorrectly adjusted magnet coil, or a flywheel in the wrong position on the crankshaft due to a damaged flywheel key.

Also, poor fuel and air leaks between carburetor and engine can be an issue. There's very little you can do with the magneto other than making sure the distance from the laminated core to the flywheel is correct and tightening the mounting screws. Loose connections should be checked and screws tightened on ring crimps. Faulty magnetos that don't produce a spark at the plug always try a plug that you know is working and isn't cracked should be replaced with a new unit.

This is likely due to misfiring and the engine turning the "wrong way".


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So it can fire once and pull the rope back. When the piston descends into the crankcase, it pushes mixture previously admitted into the crankcase up along a passageway into the cylinder. This flushes burned gasses out of the cylinder. A clogged exhaust can make it difficult to pull the starter cord. Over time, residue from combustion can clog exhaust ports, spark arrestor screen and muffler. The air filter could be dirty, but it's more likely that something is restricting flow through the carburetor such as a blocked jet.

When starting a trimmer, it can be reluctant to run without choke for the first 20 seconds or so until it warms up and get's going and the carburetor reservoir fills with fuel.

Using the right fuel is essential for the safety and efficacy of your chainsaw

So try alternating running it with full choke and no choke until it can run without stalling on no choke. Also, there could be a hydraulic lock with fuel in the cylinder. If the engine doesn't have a clutch, whatever is down the line from the engine may be sticking and making pull starting difficult. Thee could also be an issue with the recoil starter, causing the rope, spring mechanism or starter pulley to jam. The line that sucks up fuel in the tank is supposed to move around when the trimmer is used at different orientations.

If it doesn't do this, if the fuel is low in the tank when the trimmer is used at an angle, or upside down, air can get sucked into the line and stall the engine. On initial starting, a trimmer can splutter and stall with choke off until it warms up and reaches speed and the carburetor fills properly. Try starting and running with choke back on again or half on for several seconds before turning the choke back off. See the guide for more details. What is the carburetor model shown in your article and can you get a refurbishing kit for it?

McCulloch MAC 120 Tune Up Kit

It's the same as mine. I had a conversation with a reader called Mike about 15 months ago, and he reckoned the carburetor was a Walbro wyl series carb. Search for "Mike" on the page and you should be able to find it. He got a rebuild kit for the trimmer and it worked ok. I don't have a diagram, but if you look below the section in the article "Diaphragm on a Two-Stroke Engine Carburetor", you'll see photos of the arrangement. When I pull the starter rope I get a clanking noise. The engine quit like it was fuel starved. Why does this happen? It sounds like something's rubbing or broken in the engine.

It could be a serious failure like a broken connecting rod which you can check by removing the crankcase cover or possibly a loose magneto coil rubbing against the flywheel, requiring the fixings to be tightened, ensuring the legs of the coil's former are at the correct spacing distance from the flywheel magnets.

McCulloch MAC Chainsaw Tune Up Kit

If the trimmer is 4-stroke, you can follow the procedure outlined in Jacks Small Engines here, or check your manual for the procedure for your specific model. If you can give me the model number, I can check the details. It's fixed to the engine. However, the pickup fuel line moves around in the tank so that it's always under the surface of the fuel, even if the tank isn't full. Sign in or sign up and post using a HubPages Network account. Comments are not for promoting your articles or other sites. It depends on the model of the strimmer and what the screw is actually for.

An idle screw which sets the idle speed of the strimmer will be located close to where the throttle cable attaches onto the carb. It simply acts as a stop, preventing the speed from dropping below a certain rate. There may also be a high and possibly low speed mixture adjustment screw, usually indicated by "H" and "L".


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  6. Stihl recommend the high speed screw is reversed out one turn after screwing it in gently until it will go no further. However it may depend on the strimmer. In any case here's Stihl's manual for a specific model of strimmer giving the adjustment procedure. My 4-stroke Ryobi will not start. There's a strong spark, but if I drop fuel directly into the cylinder it still doesnt run, or even fire briefly. I haven't yet checked the compression as I need to buy a tester. It seems okay though i.

    Is it possible that the engine is worn out already it is 2-years old? Are there other things to try? It will be annoying to throw it out. Not good! Check your lines are the right way around by emptying the tank so the return line without the filter is above the surface of fuel and the other line is below.

    If the lines are correct, when you press the primer, fuel should flow out the line without the filter. It takes about 10 pushes of the button for air to clear out of the carburetor. It is sparking. I used an inline coil tester and it was glowing orange. Is the primer button if it has one sucking up fuel and filling and feeling "spongy" and returning fuel to the tank?

    Hi I have a Stihl MM55 tiller. My magneto gap is set at about 12 thousandths. I looened my fuel gap to see if that made a difference and it did not. The only thing I can think of is timing , but I just guessing.

    Its driving me nuts. I don't know the number of the air filter because the trimmer is a Chinese import, branded by a home store. The store won't tell me who their supplier is. There is a "" printed on it but that doesn't come up in a search. I just got my line trimmer running like a champ, however the air cleaner cover is broken off Mine is a Craftsman, but the part I need is no longer available. It's a Briggs and Stratton part I noticed your line trimmer's air filter also has an integrated choke, and is using the same carb mine is.

    What air filter part number is yours?? Can you find out if it's still available?? It'd sure be a life saver, as I'm sure it'd work just fine on my trimmer. The mechanism is working fine. Is the cable working the throttle arrangement on the carburetor?

    Fuel mix for mcculloch mac 120 chainsaw plz. bought it from…

    If it is, chances are something has gone awry with the jet that lets fuel. Check first though that the cable is pulling the mechanism and causing it to pivot ok. I have a generic strimmer very similar to yours that will not throttle up when i press the trigger. Ive had to remove the saftey latch in front of the saftey cut out switch as it become stuck in position. The throttle cable seems to be working as it should but has no effect on the engine speed.

    Sounds as though you put an awful lot in, so it best to allow it to drain and evaporate. If you try this, you only need to use the equivalent of a teaspoon or so. Don't pour it in directly from the can.

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    Maybe move around the engine at different angles to see if you can drain any more out and then try to pull start with no choke to see if any more flows out. Then follow the starting procedure and troubleshooting tips above. Is it serious? Whether the line attaches to the straight or bent connector may depend on the carb model. The straight connector on mine attaches to the fuel intake line with the filter, but you can check you have the lines the right way around by pulling up the return line above the fuel level in the tank.

    If everything is connected properly, when you press the primer button, fuel should return via the line without the filter. If the lines are the wrong way, bubbles of air will eventually come out of the filter after pressing the button several times. Stihl recommend both L and H screws are opened 1 turn after turning them in very gently to avoid damage to the jet tips and valve seat and idle speed needs to be set also.

    The procedure may be different with your trimmer so ideally you should source the manual and follow it. I take the choke off like I used to, when it would then fire up no problems, but now it won't kick at all with the choke off.